CA2321326A1 - Machine silver-cleaning composition - Google Patents

Abstract

Compositions for the cleaning of tarnished silver surfaces comprise alkaline electrolytes and reducing agents which have a solubility of more than 20 g per liter of water at 20°C.

Description

MACHINE SILVER-CLEANING COMPOSITIONS
Field of the Invention The present invention relates to compositions for the machine cleaning of silver surfaces, and to the use of these compositions in domestic dishwashers, and to a cleaning method using these compositions.
Background of the Invention Cleaning compositions for cleaning soiled dishes in dishwashers are described widely in the prior art. One disadvantage which these compositions often have, however, is the cleaning performance on tarnished silver surfaces. Since machine dishwashing detergents have to satisfy a large number of requirements both with regard to the substrates to be treated and also with regard to the soilings which arise, certain objectives are not achieved to the complete satisfaction of the user. In the case of the cleaning of silver or silver-plated surfaces, therefore, the addition of corrosion inhibitors, which prevent further tarnishing, but which are unable to clean tarnished silver surfaces, has in most cases had to suffice.
Specific cleaning compositions are supplied for the cleaning of silver, some of which can also be used in dishwashers. On the basis of well-known method of placing the silver articles on aluminum foil and covering them with an electrolyte solution, US 3,701,736 (Colgate) proposes machine dishwashing detergents (MDDs) which comprise builder salts and at least 3.25a by weight of aluminum in the form of the pure metal or of aluminum alloys.
A similar product for non-machine cleaning is described in EP 039 193 (Crown & Andrews).

A more recent preparation for the cleaning of silver in dishwashers arises from i~T098/51769 (Procter & Gamble).
According to the teaching of this specification, the aim is to use silver-cleaning compositions which comprise alkaline electrolyte, metal and in each case at least 10 by weight of builders and nonionic surfactant. The metal must have a more negative standard potential than silver.
The compositions can also be incorporated as a component in traditional MDDs. A machine silver-cleaning method is likewise claimed in this specification.
The solutions proposed in the prior art either cannot be used in dishwashers, or lead to results which are not comparably good compared with conventional dipping or polishing methods. There therefore still existed the object of providing silver-cleaning compositions which can be used in dishwashers and which have good cleaning performance which is comparable with cleaners applied manually.
Summary of the Invention We have now found that the solubility of the reducing agent used has a decisive influence on the performance of the composition.
The present invention provides, in a first embodiment, compositions for the cleaning of silver in a dishwasher, which comprise alkaline electrolytes and reducing agents, where, as reducing agents, water-soluble substances having a solubility of more than 20 g per liter of water at 20°C are present in the compositions.
The compositions according to the invention comprise as essential ingredients one or more alkaline electrolyte(s), and one or more reducing agents which have a certain solubility in accordance with the invention. Reducing agents which can be used according to the invention dissolve to give clear solutions at 20°C in amounts of more than 20 grams per liter of deionized water. It is preferred to use more soluble reducing agents, so that preferred compositions according to the invention comprise, as reducing agents, water-soluble substances having a solubility of more than 30 g, preferably of more than 40 g and in particular of more than 50 g, per liter of water at 20°C.
Detailed Description of the Invention Reducing agents which can be used according to the invention may originate from a number of classes of substance. Thus, for example, inorganic or organic salts or covalent compounds can be used. The table below gives an overview of the solubilities of reducing agents which can be used according to the invention. The solubilities given refer to a temperature of 20°C, unless another temperature is given in the first column.

3,4-Dihydroxybenzaldehyde 50 g/1 Ammonium iron(II) sulfate hexahydrate 269 g/1 Ammonium iron(III) citrate 1200 g/1 Ascorbic acid 333 g/1 Pyrocatechol 434 g/1 Citric acid monohydrate 1630 g/1 Cobalt(II) acetate tetrahydrate 380 g/1 Cobalt(II) chloride hexahydrate 76 g/1 Cobalt(II) nitrate hexahydrate 1330 g/1 Cobalt(II) sulfate heptahydrate 260 g/1 D (-) -fructose 3750 g/1 D (+) -galactose (25C) 680 g/1 D (+) -glucose monohydrate (25C) 820 g/1 D(+)-mannose (17C) 2480 g/1 Disodium tartrate dehydrate 290 g/1 DL-malic acid (26C) 1440 g/1 Iron(II) chloride tetrahydrate (10C) 1600 g/1 Iron(II) sulfate heptahydrate 400 g/1 Gluconic acid, sodium salt (25C) 590 g/1 Urea 1080 g/1 Hydroquinone 70 g/1 L-(-)-malic acid 363 g/1 L-(-)-sorbose (17C) 550 g/1 Lactose monohydrate (25C) 216 g/1 Manganese(II) acetate tetrahydrate 330 g/1 Manganese(II) chloride 1400 g/1 Manganese(II) chloride dehydrate 1200 g/1 Manganese(II) chloride tetrahydrate 1980 g/1 Manganese(II) nitrate tetrahydrate 3800 g/1 Manganese(II) sulfate monohydrate 762 g/1 Melibiose monohydrate (25C) 2500 g/1 Sodium dithionite 224 g/1 Sodium hypophosphite monohydrate 1000 g/1 Sodium sulfite 495 g/1 Sodium tetraborate decahydrate 50 g/1 Sodium thiosulfate pentahydrate 680 g/1 Sucrose (15C) 1970 g/1 Depending on the desired dosing of the compositions according to the invention and depending on the reducing agents used, the latter are used in varying amounts.
Preferred compositions according to the invention comprise the reducing agents) in amounts of from 1 to 60% by weight, preferably from 5 to 40% by weight and in particular from 10 to 25% by weight, in each case based on the total composition.
From the group of the abovementioned reducing agents certain subgroups are in turn preferred. Thus, preferred compositions according to the invention comprise one or more reducing agents from the group of reducing sugars, D (-) -fructose, D (+) -galactose, D (+) -glucose monohydrate, D(+)-mannose, L(-)-sorbose, lactose monohydrate, melibiose monohydrate and sucrose being particularly preferred reducing agents.
Particularly preferred compositions according to the invention comprise, as reducing agents, one or more substances from the group of non-salt-like compounds, particularly preferably pyrocatechol, hydroquinone and/or ascorbic acid.
As second constituents, the compositions according to the invention comprise one or more alkaline electrolyte(s).
This electrolyte content effects a higher conductivity of the solution and also provides for an advantageous alkaline pH of the use solution, which is preferably above 9, particularly preferably above 9.5 and in particular above 10 or even 10.5. Preferred alkaline electrolytes are, for example, carbonates, hydrogencarbonates, silicates, metasilicates and mixtures thereof.
Preferred compositions according to the invention comprise, as alkaline electrolyte(s), one or more substances from the group of carbonates, hydrogencarbonates, hydroxides, citrates and/or phosphates, the alkali metal, and in particular the potassium and/or sodium salts, being preferred.
In preferred compositions according to the invention, the content in the compositions of alkaline electrolytes is to 99% by weight, preferably 45 to 95% by weight and in particular 50 to 90% by weight, in each case based on 35 the total composition.

Preferred alkaline electrolytes are the citrates, and of these the alkali metal citrates are in turn preferred.
Compositions preferred within the scope of the present invention comprise potassium citrate and/or sodium citrate in amounts of from 5 to 50% by weight, preferably from 10 to 45% by weight, particularly preferably from 15 to 40% by weight and in particular from 20 to 35% by weight, in each case based on the total composition.
The use of carbonates as alkaline electrolyte is also possible and preferred in accordance with the invention.
In the case of the alkali metal carbonates or hydrogencarbonates, the sodium or potassium salts are clearly preferred over the other salts for reasons of cost. Of course, the pure alkali metal carbonates or hydrogencarbonates concerned do not have to be used;
instead, mixtures of different carbonates and hydrogencarbonates may be preferred.
Compositions preferred according to the invention comprise potassium carbonate and/or sodium carbonate in amounts of from 10 to 70% by weight, preferably from 15 to 55% by weight, particularly preferably from 20 to 45%
by weight and in particular from 25 to 40% by weight, in each case based on the total composition.
Other alkaline electrolytes which can be used are hydroxides, silicates and phosphates. In the case of these substances too the alkali metal salts are preferred. Crystalline, sheet sodium silicates suitable as alkaline electrolyte have the formula NaMSiXO2X+1'y H20, where M is sodium or hydrogen, x is a number from 1.9 to 4 and y is a number from 0 to 20, and preferred values for x are 2, 3 or 4. Preferred crystalline phyllosilicates of the formula given are those in which M
is sodium, and x assumes the values 2 or 3. In particular, both (3- and 8-sodium disilicates Na2Si205~y Hz0 are preferred.
It is also possible to use amorphous sodium silicates having an NazO . Si02 modulus of from 1:2 to 1:3.3, preferably from 1:2 to 1:2.8 and in particular from 1:2 to 1:2.6, which have delayed dissolution and secondary detergency properties. The dissolution delay relative to traditional amorphous sodium silicates can have been induced by various means, for example by surface treatment, compounding, compaction/consolidation or by overdrying. Within the scope of this invention the term "amorphous" includes "X-ray amorphous". This means that in X-ray defraction experiments, the silicates do not produce sharp X-ray reflections typical of crystalline substances, but instead, at best, one or more maxima of the scattered X-ray radiation which have a breadth of several degree units of the angle of defraction. However, particularly good builder properties may be the result even if in electron defraction experiments the silicate particles produce poorly defined or even sharp defraction maxima. This is to be interpreted to the effect that the products have microcrystalline regions with a size of from 10 to a few hundred nm, preference being given to values up to at most 50 nm and in particular up to at most 20 nm. Particular preference is given to consolidated/compacted amorphous silicates, compounded amorphous silicates and overdried X-ray amorphous silicates.
It is of course also possible to use the generally known phosphates as alkaline electrolyte. Of the large number of commercially available phosphates, the alkali metal phosphates, particularly preferably pentasodium triphosphate and pentapotassium triphosphate (sodium or potassium tripolyphosphate), are of the greatest importance in the detergents and cleaners industry.
Alkali metal phosphates is the collective term for the alkali metal (in particular sodium and potassium) salts of the various phosphoric acids, for which a distinction can be made between metaphosphoric acids (HP03) and orthophosphoric acid H3P04, as well as higher molecular weight representatives. The phosphates combine a number of advantages: they act as alkali-metal carriers, prevent lime deposits on machine parts or lime incrustations in fabrics and moreover contribute to the cleaning performance.
Sodium dihydrogenphosphate, NaH2P04, exists as the dehydrate (density 1.91 gcm-3, melting point 60°C) and as the monohydrate (density 2.04 gcm-3). Both salts are white powders which are very readily soluble in water, lose the water of crystallization upon heating and at 200°C
convert to the weakly acidic disphospate (disodium hydrogendiphosphate, Na2H2P20~), at a higher temperature to sodium trimetaphosphate (Na2P309) and Maddrell's salt (see below). NaH2P04 is acidic; it forms when phosphoric acid is adjusted to a pH of 4.5 using sodium hydroxide solution and the mash is sprayed. Potassium dihydrogenphosphate (primary or monobasic potassium A
phosphate, potassium biphosphate, KDP), KHZPO4, is a white salt of density 2.33 gcm-3, has a melting point of 253°C
[decomposition with promotion of potassium polyphosphate (KP03)x] and is readily soluble in water.
Disodium hydrogenphosphate (secondary sodium phosphate), NazHP04, is a colorless crystalline salt which is very readily soluble in water. It exists in anhydrous form and with 2 mol of water (density 2.066 gcm-3, water loss at 95°C), 7 mol of water (density 1.68 gcm-3, melting point 48°C with loss of 5 H20) and 12 mol of water (density 1. 52 gcm-3, melting point 35°C with loss of 5 H20) , is anhydrous at 100°C and converts to the diphosphate Na4P20~
upon more intense heating. Disodium hydrogenphosphate is prepared by neutralizing phosphoric acid with a soda solution using phenolphthalein as indicator. Dipotassium hydrogenphosphate (secondary or dibasic potassium phosphate), KZHP04, is an amorphous white salt which is readily soluble in water.
Trisodium phosphate, tertiary sodium phosphate, Na3P04, are colorless crystals which, in the form of the dodecahydrate, have a density of 1.62 gcm-3 and a melting point of 73-76°C (decomposition), in the form of the decahydrate (corresponding to 19-20% of Pz05) have a melting point of 100°C and in anhydrous form (corresponding to 39-40 % of P205) have a density of 2 . 536 gcm-3. Trisodium phosphate is readily soluble in water with an alkaline reaction and is prepared by evaporating a solution of exactly 1 mol of disodium phosphate and 1 mol of NaOH. Tripotassium phosphate (tertiary or tribasic potassium phosphate), K3P04, is a white, deliquescent, granular powder of density 2.56 gcm-3, has a melting point of 1340°C and is readily soluble in water with an alkaline reaction. It is formed, for example, during the heating of Thomas slag with carbon and potassium sulfate.
Despite the higher price, in the detergents industry, the more readily soluble, therefore highly effective, potassium phosphates are often preferred over the corresponding sodium compounds.
Tetrasodium diphosphate (sodium pyrophosphate), Na4Pz0-,, exists in anhydrous form (density 2.534 gcm-3, melting point 988°C, also given as 880°C) and as the decahydrate (density 1.815-1.836 gcm-3, melting point 94°C with loss of water). Both substances are colorless crystals which are soluble in water with an alkaline reaction. Na4P20~
forms during the heating of disodium phosphate to >200°C
or by reacting phosphoric acid with soda in the stoichiometric ratio and dewatering the solution by spraying. The decahydrate complexes heavy metal salts and hardness constituents and therefore reduces the hardness of water. Potassium diphosphate (potassium pyrophosphate) , K4PzO~, exists in the form of the trihydrate and is a colorless hygroscopic powder having a density of 2.33 gcm-3, which is soluble in water, the pH
of the 1% strength solution at 25°C being 10.4.
By condensing NaH2P04 or KHZP04, higher molecular weight sodium and potassium phosphates are formed, amongst which it is possible to differentiate between cyclic representatives, the sodium or potassium metaphosphates, and chain-like types, the sodium or potassium polyphosphates. For the latter in particular, a large number of names are in use: melt or thermal phosphates, Graham's salt, Kurrol's and Maddrell's salt. All higher sodium and potassium phosphates are commonly referred to as condensed phosphates.
The industrially important pentasodium triphosphate, Na5P301o (sodium triphosphate), is a nonhygroscopic, white, water-soluble salt which is anhydrous or crystallizes with 6 H20 and is of the general formula Na0-[P(O)(ONa)-O]n-Na where n=3. In 100 g of water, about 17 g of the salt which is free from water of crystallization dissolve at room temperature, ca. 20 g at 60°C, and about 32 g at 100°C. If the solution is heated for 2 hours at 100°C, about 8% of orthophosphate and 15% of diphosphate form as a ,result of hydrolysis. In the preparation of pentasodium triphosphate, phosphoric acid is reacted with soda solution or sodium hydroxide solution in the stoichiometric ratio, and the solution is dewatered by spraying. Similarly to Graham's salt and sodium diphosphate, pentasodium triphosphate dissolves many insoluble metal compounds (including lime soaps etc.).
Pentapotassium triphosphate, KSP301o (potassium triphosphate), is available commercially, for example, in the form of a 50% strength by weight solution (>23% of PzOs ~ 25 % of K20) . The potassium polyphosphates are used widely in the detergents and cleaners industry.
Furthermore, within the scope of the present invention, it is also possible to use sodium potassium triphosphates. These form, for example, when sodium trimetaphosphate is hydrolyzed with KOH:
(NaP03 ) 3 + 2 KOH -~ Na3KzP301o + Hz0 According to the invention, these can be used exactly like sodium triphosphate, potassium triphosphate or mixtures of the two; mixtures of sodium triphosphate and sodium potassium triphosphate or mixtures of potassium triphosphate and sodium potassium triphosphate or mixtures of sodium triphosphate and potassium triphosphate and sodium potassium triphosphate can also be used according to the invention.
As well as reducing agents and alkaline electrolytes, the compositions according to the invention can comprise further ingredients which improve product performance and/or the appearance of the product. Dispersants and surfactants are particularly suitable here, although dyes and fragrances and also corrosion inhibitors are also suitable ingredients.
Dispersants can be added to the compositions according to the invention in order to disperse detached deposits and constituents thereof, soilings or other foreign substances in a stable manner in the cleaning liquor.

From the group of dispersants, polycarboxylic acid salts in particular have proven successful. Also in the case of these salts, the alkali metal salts, and of these in turn the potassium and/or sodium salts, are preferred. In the case of the dispersants, particularly preferred salts are the alkanolammonium salts of polycarboxylic acids, for example the (substituted) mono-, di- and trimethanolammonium salts, the rations of which can be prepared from the corresponding methanaolamines by protonation or quaternization with methylating or ethylating agents. Particular preference is also given to the (substituted) mono-, di- and triethanolammonium salts, where the ration is preferably protonated or methylated or ethylated triethanolamine.
Within the scope of the present invention, polycarboxylic acids whose salts have a dispersing action mean, in particular, those carboxylic acids which carry more than one acid function. These are for example citric acid, adipir acid, succinic acid, glutaric acid, malic acid, tartaric acid, malefic acid, fumaric acid, sugar acids, aminocarboxylic acids, nitrilotriacetic acid (NTA), provided such a use is not objectionable for ecological reasons, and mixtures thereof. Preferred salts are the salts of the polycarboxylic acids, such as citric acid, adipic acid, succinic acid, glutaric acid, tartaric acid, sugar acids and mixtures thereof.
Other suitable dispersants are polymeric polycarboxylates, i.e., for example, the alkali metal salts of polyacrylic acid or of polymethacrylic acid, for example those having a relative molecular mass of from 500 to 70,000 g/mol.
For the purposes of this specification, the molar masses given for polymeric polycarboxylates are weight-average molar masses Mw of the respective acid form which have in principle been determined by means of gel permeation chromatography (GPC), a UV detector being used.
Measurement was made against an external polyacrylic acid standard which, because of its structural similarity to the polymers investigated, gives realistic molecular weight values. This data differs significantly from the molecular weight data where polystyrenesulfonic acids are used as standard. The molar masses measured against polystyrenesulfonic acids are generally significantly higher than the molar masses given in this specification.
Suitable polymers are, in particular, polyacrylates which preferably have a molecular mass of from 2000 to 20,000 g/mol. From this group in turn, because of their superior solubility, the short-chain polyacrylates, which have molar masses of from 2000 to 10,000 g/mol, and particularly preferably from 3000 to 5000 g/mol, may be preferred.
Also suitable are copolymeric polycarboxylates, in particular those of acrylic acid with methacrylic acid and of acrylic acid or methacrylic acid with malefic acid.
Copolymers of acrylic acid with malefic acid which contain 50 to 90% by weight of acrylic acid and 50 to 10% by weight of malefic acid have proven particularly suitable.
Their relative molecular mass, based on free acids, is generally 2000 to 70,000 g/mol, preferably 20,000 to 50,000 g/mol and in particular 30,000 to 40,000 g/mol.
To improve the water solubility, the polymers can also comprise allylsulfonic acids, such as, for example, allyloxybenzenesulfonic acid and methallylsulfonic acid, as monomers.

Also particularly preferred are biodegradable polymers of more than two different monomer units, for example those which contain, as monomers, salts of acrylic acid and of malefic acid, and of vinyl alcohol or vinyl alcohol derivatives, or which contain, as monomers, salts of acrylic acid and of 2-alkylallylsulfonic acid and sugar derivatives.
Further preferred builder substances which may likewise be mentioned are polymeric aminodicarboxylic acids, salts thereof or precursor substances thereof. Particular preference is given to polyaspartic acids and to salts and derivatives thereof which, in addition to cobuilder properties, also have a bleach-stabilizing action.
Further suitable dispersants are polyacetals, which can be obtained by reacting dialdehydes with polyol carboxylic acids which have 5 to 7 carbon atoms and at least 3 hydroxyl groups. Preferred polyacetals are obtained from dialdehydes such as glyoxal, glutaraldehyde, terephthalaldehyde and mixtures thereof, and from polyol carboxylic acids such as gluconic acid and/or glucoheptonic acid.
Further suitable organic dispersants are dextrins, for example oligomers or polymers of carbohydrates, which may be obtained by partial hydrolysis of starches. The hydrolysis can be carried out by customary processes, for example acid-catalyzed or enzyme-catalyzed processes. The hydrolysis products preferably have average molar masses in the range from 400 to 500,000 g/mol. Preference is given here to a polysaccharide with a dextrose equivalent (DE) in the range from 0.5 to 40, in particular from 2 to 30, where DE is a customary measure of the reducing effect of a polysaccharide compared with dextrose, which has a DE of 100. It is also possible to use maltodextrins with a DE between 3 and 20 and dry glucose syrups with a DE between 20 and 37, and also so-called yellow dextrins and white dextrins having higher molar masses in the range from 2000 to 30,000 g/mol.
The oxidized derivatives of such dextrins are the reaction products thereof with oxidizing agents which are able to oxidize at least one alcohol function of the saccharide ring to the carboxylic acid function. A
product oxidized on C6 of the saccharide ring may be particularly advantageous.
Oxydisuccinates and other derivatives of disuccinates, preferably ethylene diamine disuccinate, are further suitable dispersants. Here, ethylene diamine N,N'-disuccinate (EDDS) is preferably used in the form of its sodium or magnesium salts. Also preferred in this connection are glycerol disuccinate and glycerol trisuccinate.
Examples of further organic dispersants which can be used are acetylated hydroxycarboxylic acids and salts thereof, which may also be present in lactone form and which contain at least 4 carbon atoms and at least one hydroxyl group and at most two acid groups.
Preferred compositions according to the invention additionally comprise dispersants, preferably from the group of polycarboxylic acid salts, particularly preferably the alkanolamine salts of polycarboxylic acids, in amounts of from 0.1 to 10% by weight, preferably from 0.25 to 5% by weight and in particular from 0.5 to 2.5% by weight, in each case based on the total composition.

Further preferred ingredients of the compositions according to the invention are surfactants, nonionic surfactants being clearly preferred over anionic and/or cationic surfactants.
The nonionic surfactants used are preferably alkoxylated, advantageously ethoxylated, in particular primary alcohols having, preferably, 8 to 18 carbon atoms and on average 1 to 12 mol of ethylene oxide (EO) per mole of alcohol, in which the alcohol radical can be linear or, preferably, methyl-branched in the 2-position, or can contain linear and methyl-branched radicals as a mixture, as is usually present in oxo alcohol radicals. In particular, however, alcohol ethoxylates having linear radicals from alcohols of native origin having 12 to 18 carbon atoms, e.g. from coconut, palm, tallow fatty or oleyl alcohol, and on average 2 to 8 EO per mole of alcohol are preferred. Preferred ethoxylated alcohols include, for example, Clz-i4-alcohols having 3 EO or 4 EO, C9_11-alcohol having 7 EO, Cls-is-alcohols having 3 EO, 5 EO, 7 EO or 8 EO, Clz-ia-alcohols having 3 EO, 5 EO or 7 EO
and mixtures of these, such as mixtures of Clz-i4-alcohol having 3 EO and Clz-la-alcohol having 5 EO. The degrees of ethoxylation given are statistical averages which may be an integer or a fraction for a specific product.
Preferred alcohol ethoxylates have a narrowed homolog distribution (narrow range ethoxylates, NRE). In addition to these nonionic surfactants, fatty alcohols having more than 12 EO can also be used. Examples thereof are tallow fatty alcohol having 14 EO, 25 EO, 30 EO or 40 EO.
In addition, further nonionic surfactants which may be used are also alkyl glycosides of the general formula RO(G)X, in which R is a primary straight-chain or methyl-branched, in particular methyl-branched in the 2-position, aliphatic radical having 8 to 22, preferably 12 _ CA 02321326 2000-09-28 to 18, carbon atoms, and G is the symbol which stands for a glycose unit having 5 or 6 carbon atoms, preferably glucose. The degree of oligomerization x, which indicates the distribution of monoglycosides and oligoglycosides, is any number between 1 and 10; preferably, x is 1.2 to 1.4.
A further class of nonionic surfactants used with preference, which are used either as the sole nonionic surfactant or in combination with other nonionic surfactants, are alkoxylated, preferably ethoxylated or ethoxylated and propoxylated fatty acid alkyl esters, preferably having 1 to 4 carbon atoms in the alkyl chain, in particular fatty acid methyl esters.
Nonionic surfactants of the amine oxide type, for example N-cocoalkyl-N,N-dimethylamine oxide and N-tallowalkyl-N,N-dihydroxyethylamine oxide, and of the fatty acid alkanolamide type, may also be suitable. The amount of these nonionic surfactants is preferably no more than that of the ethoxylated fatty alcohols, in particular no more than half thereof.
Further suitable surfactants are polyhydroxy fatty acid amides of the formula (I), R-CO-N- [Z] (I) in which RCO is an aliphatic acyl radical having 6 to 22 carbon atoms, R1 is hydrogen, an alkyl or hydroxyalkyl radical having 1 to 4 carbon atoms, and [Z] is a linear or branched polyhydroxyalkyl radical having 3 to 10 carbon atoms and 3 to 10 hydroxyl groups. The polyhydroxy fatty acid amides are known substances which are _ CA 02321326 2000-09-28 customarily obtainable by reductive amination of a reducing sugar with ammonia, an alkylamine or an alkanolamine and subsequent acylation with a fatty acid, a fatty acid alkyl ester or a fatty acid chloride.
The group of polyhydroxy fatty acid amides also includes compounds of the formula (II), Ri-O-Rz R-CO-N- [Z] (II) in which R is a linear or branched alkyl or alkenyl radical having 7 to 12 carbon atoms, R1 is a linear, branched or cyclic alkyl radical or an aryl radical having 2 to 8 carbon atoms, and Rz is a linear, branched or cyclic alkyl radical or an aryl radical or an oxyalkyl radical having 1 to 8 carbon atoms, where C1_4-alkyl or phenyl radicals are preferred, and [Z] is a linear polyhydroxylalkyl radical whose alkyl chain is substituted by at least two hydroxyl groups, or alkoxylated, preferably ethoxylated or propoxylated, derivatives of said radical.
[Z] is preferably obtained by reductive amination of a reducing sugar, for example glucose, fructose, maltose, lactose, galactose, mannose or xylose. The N-alkoxy- or N-aryloxy-substituted compounds can be converted to the desired polyhydroxy fatty acid amides by reaction with fatty acid methyl esters in the presence of an alkoxide as catalyst.
Preferred surfactants are weakly foaming nonionic surfactants. Particularly preferably, the compositions according to the invention for the machine cleaning of silver comprise nonionic surfactants, in particular nonionic surfactants from the group of alkoxylated alcohols. The nonionic surfactants used are preferably alkoxylated, advantageously ethoxylated, in particular primary alcohols having, preferably, 8 to 18 carbon atoms and on average 1 to 12 mol of ethylene oxide (EO) per mole of alcohol, in which the alcohol radical can be linear or, preferably, methyl-branched in the 2-position, or can contain linear and methyl-branched radicals as a mixture, as is usually present in oxo alcohol radicals.
In particular, however, alcohol ethoxylates having linear radicals from alcohols of native origin having 12 to 18 carbon atoms, e.g. from coconut, palm, tallow fatty or oleyl alcohol, and on average 2 to 8 EO per mole of alcohol are preferred. Preferred ethoxylated alcohols include, for example, Clz-i4-alcohols having 3 EO or 4 EO, Cs-11-alcohol having 7 E0, C13-ls-alcohols having 3 EO, 5 EO, 7 EO or 8 EO, Clz-ls-alcohols having 3 EO, 5 EO or 7 EO
and mixtures of these, such as mixtures of Clz-14-alcohol having 3 EO and Clz-is-alcohol having 5 EO. The degrees of ethoxylation given are statistical averages which may be an integer or a fraction for a specific product.
Preferred alcohol ethoxylates have a narrowed homolog distribution (narrow range ethoxylates, NRE). In addition to these nonionic surfactants, fatty alcohols having more than 12 EO can also be used. Examples thereof are tallow fatty alcohol having 14 EO, 25 EO, 30 EO or 40 EO.
Particular preference is given to compositions according to the invention which comprise a nonionic surfactant which has a melting point above room temperature.
Accordingly, preferred compositions comprise nonionic surfactants) with a melting point above 20°C, preferably above 25°C, particularly preferably between 25 and 60°C
and in particular between 26.6 and 43.3°C.

Suitable nonionic surfactants which have melting or softening points in said temperature range are, for example, weakly foaming nonionic surfactants, which may be solid or of high viscosity at room temperature. If nonionic surfactants are used which are of high viscosity at room temperature, then it is preferable for these to have a viscosity above 20 Pas, preferably above 35 Pas and in particular above 40 Pas. Nonionic surfactants which have a wax-like consistency at room temperature are also preferred.
Preferred nonionic surfactants which are solid at room temperature and are to be used originate from the groups of alkoxylated nonionic surfactants, in particular ethoxylated primary alcohols and mixtures of these surfactants with structurally complex surfactants, such as polyoxyproplylene/polyoxyethylene/polyoxypropylene (PO/EO/PO) surfactants. Such (PO/EO/PO) nonionic surfactants are, moreover, notable for good foam control.
In a preferred embodiment of the present invention, the nonionic surfactant having a melting point above room temperature is an ethoxylated nonionic surfactant resulting from the reaction of a monohydroxyalkanol or alkylphenol having 6 to 20 carbon atoms with, preferably, at least 12 mol, particularly preferably at least 15 mol, in particular at least 20 mol, of ethylene oxide per mole of alcohol or alkylphenol.
A particularly preferred nonionic surfactant which is solid at room temperature and is to be used is obtained from a straight-chain fatty alcohol having 16 to 20 carbon atoms (Cls-ao-alcohol) , preferably a C18-alcohol, and at least 12 mol, preferably at least 15 mol and in particular at least 20 mol, of ethylene oxide. Of these, the so-called "narrow range ethoxylates" (see above) are particularly preferred.
Accordingly, particularly preferred compositions according to the invention comprise ethoxylated nonionic surfactant (s) which has/have been obtained from C6-zo monohydroxyalkanols or C6_zo-alkylphenols or Cls-zo-fatty alcohols and more than 12 mol, preferably more than 15 mol and in particular more than 20 mol, of ethylene oxide per mole of alcohol.
The nonionic surfactant which is solid at room temperature preferably additionally has propylene oxide units in the molecule. Such PO units preferably constitute up to 25% by weight, particularly preferably up to 20% by weight and in particular up to 15% by weight of the total molar mass of the nonionic surfactant.
Particularly preferred nonionic surfactants are ethoxylated monohydroxyalkanols or alkylphenols which additionally have polyoxyethylene/polyoxypropylene block copolymer units. The alcohol or alkylphenol moiety of such nonionic surfactant molecules preferably constitutes more than 30% by weight, particularly preferably more than 50% by weight and in particular more than 70% by weight, of the total molar mass of such nonionic surfactants. Preferred cleaning composition components comprise, as ingredient a), ethoxylated and propoxylated nonionic surfactants in which the propylene oxide units in the molecule constitute up to 25% by weight, preferably up to 20% by weight and in particular up to 15% by weight of the total molar mass of the nonionic surfactant.
Further particularly preferred nonionic surfactants to be used having melting points above room temperature comprise 40 to 70% of a polyoxypropylene/polyoxy-_ CA 02321326 2000-09-28 ethylene/polyoxypropylene block polymer blend which comprises 75% by weight of an inverted block copolymer of polyoxyethylene and polyoxypropylene containing 17 mol of ethylene oxide and 44 mol of propylene oxide, and 25% by weight of a block copolymer of polyoxyethylene and polyoxypropylene initiated with trimethylolpropane and comprising 24 mol of ethylene oxide and 99 mol of propylene oxide per mole of trimethylolpropane.
Nonionic surfactants which can be used with particular preference are, for example, those available under the name Poly Tergent~ SLF-18 from Olin Chemicals.
Further preferred compositions according to the invention comprise nonionic surfactants of the formula R10 [CHzCH (CH3) O] X [CHZCH20] y [CH2CH (OH) Rz] , in which R1 is a linear or branched aliphatic hydrocarbon 2 0 radical having 4 to 18 carbon atoms or mixtures thereof , RZ is a linear or branched hydrocarbon radical having 2 to 26 carbon atoms or mixtures thereof, and x is a value between 0.5 and 1.5, and y is a value of at least 15.
Further preferred nonionic surfactants which can be used are the terminally-capped poly(oxyalkylated) nonionic surfactants of the formula R10 [CH2CH (R3) O] X [CHZ] kCH (OH) [CH2] ~OR2 in which R1 and Rz are linear or branched, saturated or unsaturated, aliphatic or aromatic hydrocarbon radicals having 1 to 3 0 carbon atoms , R3 is H or a methyl , ethyl , n-propyl, isopropyl, n-butyl, 2-butyl or 2-methyl-2-butyl radical, x is a value between 1 and 30, k and j are values between 1 and 12, preferably between 1 and 5. If x is >_ 2, each R3 in the above formula may be different. R1 and RZ are preferably linear or branched, saturated or unsaturated, aliphatic or aromatic hydrocarbon radicals having 6 to 22 carbon atoms, radicals having 8 to 18 carbon atoms being particularly preferred. For the radical R3, H, -CH3 or -CHZCH3 are particularly preferred.
Particularly preferred values for x are in the range from 1 to 20, in particular from 6 to 15.
As described above, each R3 in the above formula can be different if x is >_ 2. As a result, the alkylene oxide unit in the square brackets can be varied. If, for example, x is 3, the radical R3 can be chosen to form ethylene oxide (R3 - H) or propylene oxide (R3 - CH3) units which can be joined to one another in any order, for example (EO) (PO) (EO) , (EO) (EO) (PO) , (EO) (EO) (EO) , (PO) (EO) (PO) , (PO) (PO) (EO) and (PO) (PO) (PO) . The value 3 is chosen here for x by way of example and can of course be greater, the scope for variation increasing with increasing x values and embracing, for example, a large number of (EO) groups, combined with a small number of (PO) groups, or vice versa.
Particularly preferred terminally-capped poly(oxy-alkylated) alcohols of the above formula have values of k - 1 and j - 1, thereby simplifying the above formula to R10 [CHzCH (R3) O] XCHZCH (OH) CHzOR2 .
In the last-mentioned formula, R1, R2 and R3 are as defined above and x is a number from 1 to 30, preferably from 1 to 20 and in particular from 6 to 18. Particular preference is given to surfactants in which the radicals R1 and Rz have 9 to 14 carbon atoms, R3 is H, and x adopts values from 6 to 15.

A further group of preferred surfactants are so-called "fluorine surfactants" which carry a perfluoroalkyl radical as hydrophobic group. Compared with nonfluorinated surfactants, fluorine surfactants are notable for lower critical micelle concentration values and therefore, even in extremely small concentrations, effect a significant lowering of the surface tension of water. They have high chemical and thermal stability, meaning that they can also be used in aggressive media and at high temperatures.
In summary, preferred compositions according to the invention additionally comprise nonionic surfactant(s), preferably alkoxylated, particularly preferably ethoxylated and/or propoxylated nonionic surfactants having 8 to 24, preferably 10 to 20 and in particular 12 to 18, carbon atoms, particularly preferably nonionic surfactants containing fluoroalkyl groups ("fluorine surfactants"), in amounts of from 0.1 to 10% by weight, preferably from 0.25 to 5% by weight and in particular from 0.5 to 2.5% by weight, in each case based on the total composition.
Dyes and fragrances can be added to the machine silver-cleaning compositions according to the invention in order to improve the esthetic impression of the resulting products and to provide the consumer with not only the performance but also a visually and sensorially "typical and unmistakable" product. Perfume oils and fragrances which can be used are individual scent compounds, e.g.
the synthetic products of the ester, ether, aldehyde, ketone, alcohol and hydrocarbon type. Scent compounds of the ester type are, for example, benzyl acetate, phenoxyethyl isobutyrate, p-tert-butylcyclohexyl acetate, linalyl acetate, dimethylbenzylcarbinyl acetate, phenylethyl acetate, linalyl benzoate, benzyl formate, _ CA 02321326 2000-09-28 ethyl methylphenylglycinate, allyl cyclohexylpropionate, styrallyl propionate and benzyl salicylate. The ethers include, for example, benzylethyl ether; the aldehydes include, for example, the linear alkanals having 8-18 carbon atoms, citral, citronellal, citronellyloxy-acetaldehyde, cyclamen aldehyde, hydroxycitronellal, filial and bourgeonal; the ketones include, for example, the ionones, a-isomethylionone and methyl cedryl ketone;
the alcohols include anethole, citronellol, eugenol, geraniol, linalool, phenylethyl alcohol and terpineol;
and the hydrocarbons include mainly the terpenes, such as limonene and pinene. Preference is given, however, to using mixtures of different scents which together produce an appealing fragrance note. Such perfume oils can also comprise natural scent mixtures, as are obtainable from vegetable sources, e.g. pine oil, citrus oil, jasmine oil, patchouli oil, rose oil or ylang-ylang oil. Also suitable are clary sage oil, camomile oil, oil of cloves, melissa oil, mint oil, cinammon leaf oil, lime blossom oil, juniperberry oil, vetiver oil, olibanum oil, galbanum oil and labdanum oil, and also orange blossom oil, neroliol, orange peel oil and sandalwood oil.
The fragrances can be incorporated directly into the cleaning compositions according to the invention, although it may also be advantageous to apply the fragrances to carriers. Such carriers which have proven successful are, for example, cyclodextrins, it being possible in addition for the cyclodextrin-perfume complexes to be additionally coated with further auxiliaries.
In order to improve the esthetic impression of the compositions according to the invention, it (or parts thereof) can be colored using suitable dyes. Preferred dyes, the choice of which does not present any problems to the person skilled in the art, have high storage stability and insensitivity toward the other ingredients of the composition and toward light, and also no marked substantivity toward the substrates to be treated with the compositions, such as glass, ceramic or plastic dishes, in order not to color these.
The cleaning compositions according to the invention can, to protect the ware or the machine, comprise corrosion inhibitors, so-called silver protectants in the area of machine dishwashing, in particular, being of particular importance . The known substances of the prior art can be used. Silver protectants which can be used are, in general, particularly those chosen from the group of triazoles, benzotriazoles, bisbenzotriazoles, aminotriazoles, alkylaminotriazoles and transition metal salts or complexes. Particular preference is given to using benzotriazole and/or alkylaminotriazole. Moreover, cleaning formulations frequently comprise agents which contain active chlorine, which are able to significantly reduce corrosion of the silver surface. In chlorine-free cleaners, use is made in particular of oxygen-containing and nitrogen-containing organic redox-active compounds, such as divalent and trivalent phenols, e.g.
hydroquinone, pyrocatechol, hydroxyhydroquinone, gallic acid, phloroglucinol, pyrogallol, and derivatives of these classes of compound. Inorganic compounds in the form of salts and complexes, such as salts of the metals Mn, Ti, Zr, Hf, V, Co and Ce, are also frequently used.
Preference is given here to the transition metal salts chosen from the group of manganese and/or cobalt salts and/or complexes, particularly preferably cobalt (ammine) complexes, cobalt (acetate) complexes, cobalt (carbonyl) complexes, chlorides of cobalt or manganese, and manganese sulfate. Zinc compounds can likewise be used for preventing corrosion on the ware.

Taking all of the information given hitherto regarding ingredients and amounts thereof, then particularly preferred compositions according to the invention for the cleaning of silver in a dishwasher comprise, in addition to further optional constituents, a) 10 to 70% by weight of potassium carbonate and/or sodium carbonate, b) 5 to 50% by weight of potassium citrate and/or sodium citrate, c) 1 to 60% by weight of one or more reducing agents having a solubility of more than 20 g per liter of water at 20°C, d) 0 to 10% by weight of one or more dispersants, and e) 0 to 10% by weight of one or more nonionic surfactants.
In particularly preferred compositions, the sum of components a) and b) constitutes 40 to 99% by weight, preferably 45 to 95% by weight and in particular 50 to 90% by weight, of the total composition.
As well as the carbonates and citrates, the other alkaline electrolytes described above can also be present in the compositions, preferred compositions additionally comprising hydrogencarbonates, hydroxides and/or phosphates in amounts of from 1 to 30% by weight, preferably from 2 to 25% by weight and in particular from 5 to 15% by weight, in each case based on the total composition.
The reducing agents which can be used are in turn the substances described above. Advantageously, the compositions comprise, as component c), pyrocatechol, hydroquinone, ascorbic acid or mixtures thereof in amounts of from 2.5 to 50% by weight, preferably from 5 _ CA 02321326 2000-09-28 to 45% by weight and in particular from 10 to 25% by weight, in each case based on the total composition.
The compositions according to the invention can be formulated in different ways, meaning that they can be used, for example, as powder, granulate or tablet. By dissolving or suspending the solid ingredients, it is also possible to provide liquid formulations.
The present invention further provides a method of cleaning silver in which articles made of silver or silver-plated articles or alloys of silver are treated with a cleaning composition according to the invention in a dishwasher.
For this, the discolored or tarnished silver articles are placed into a dishwasher, the composition according to the invention is introduced into the dishwasher, and a wash program is left to run. The compositions according to the invention can be added via the dosing chamber of the machine, although it is also possible to introduce them directly into the machine, in which case a dosing aid can be used if required. An analogous method for removing oxide and/or sulfide deposits from tarnished silver, which comprises treating articles made of tarnished silver or alloys thereof with a cleaning composition according to the invention in a dishwasher, is further provided by the present invention.
To increase the shine, the articles can be afterpolished with a soft cloth after the cleaning program.
Corresponding methods according to the invention in which, after the cleaning in the dishwasher, an aftertreatment step, in particular afterpolishing, is carried out are preferred embodiments of the present invention.

The dosing of the cleaning composition in the method according to the invention should preferably be chosen such that the cleaning liquor in the dishwasher comprises the reducing agents) in amounts of from 1 to 100 mmol/1, preferably from 2.5 to 75 mmol/1 and in particular from 5 to 50 mmol/1.
Analogous statements can also be made for ingredients a) and b) of the preferred compositions according to the invention. In preferred methods, on the one hand, the cleaning liquor in the dishwasher comprises carbonate(s), in particular potassium carbonate and/or sodium carbonate, in amounts of from 1 to 500 mmol/1, preferably from 10 to 250 mmol/1 and in particular from 50 to 150 mmol/l, and on the other hand the cleaning liquor in the dishwasher, in a further preferred method, comprises citrate(s), in particular potassium citrate and/or sodium citrate, in amounts of from 1 to 150 mmol/l, preferably from 2.5 to 100 mmol/1 and in particular from 5 to 75 mmol/1.
The present invention further provides for the use of cleaning compositions which comprise alkaline electrolytes and reducing agents having a solubility of more than 20 g per liter of water at 20°C, for the cleaning of silver.
The above statements regarding preferred embodiments are entirely valid here by analogy. Particular preference is given to using cleaning compositions according to the invention comprising a) 10 to 70% by weight of potassium carbonate and/or sodium carbonate, b) 5 to 50% by weight of potassium citrate and/or sodium citrate, c) 1 to 60% by weight of one or more reducing agents having a solubility of more than 20 g per liter of water at 20°C, d) 0 to 10% by weight of one or more dispersants, and e) 0 to 10% by weight of one or more nonionic surfactants, for the removal of oxide and/or sulfide deposits on tarnished silver.
In the case of the use according to the invention too it is preferred that the compositions are used in a dishwasher and that, after the cleaning in the dishwasher, an aftertreatment step, in particular afterpolishing, is carried out.

Examples:
Two cleaning compositions according to the invention having the compositions below were prepared (stated in by weight, based on total composition):

Sodium citrate 29.0 27.0 Sodium carbonate 58.1 54.0 Hydroquinone 12.9 -Ascorbic acid - 19.0 To produce tarnished silver surfaces, sheets of silver measuring 12 x 12 cm were placed into an oxidizing solution at 20°C and heated to 65°C. When this temperature was reached, 5 g of sodium percarbonate were added to the solution, which was maintained at 65°C. The plates were left in the solution for a total of 20 minutes (including heating-up time).
Composition of the oxidizing solution:
2.5 liters of water [16° German hardness]
23 g of sodium citrate 16 g of sodium hydrogencarbonate 5 g of sodium carbonate 2 g of TAED
0.5 ml of ammonium sulfide solution [20% strength]*
5 g of sodium percarbonate*
* (added only at 65°C) The tarnished silver sheets were then removed, rinsed with distilled water and dried in the air. The powdery deposit then present on the plate was rubbed with a soft cloth, so that an anthracite-colored lustrous surface remained.
Tarnished sheets were then placed into a dishwasher (Miele Turbothermic G 590) and washed using a 65°C
universal program [water hardness: 16° German hardness].
In each of the cleaning cycles, an amount of formulation 1 according to the invention or of formulation 2 according to the invention was used such that the amount of sodium carbonate in the wash liquor was 95 mmol/1, the amount of sodium citrate was 20 mmol/1 and the amount of reducing agent was 20 mmol/1.
When the wash program had finished, the silver surfaces had been completely freed from dark deposits. The shine on the surfaces could be produced or increased by rubbing with a soft cloth.

Claims (65)

1. A composition for cleaning silver in a dishwasher, comprising alkaline electrolytes and reducing agents, which comprises, as reducing agents, water-soluble substances having a solubility of more than 20 g per liter of water at 20°C.

2. The composition as claimed in claim 1, which comprises, as reducing agents, water-soluble substances having a solubility of more than 30 g, per liter of water at 20°C.

3. The composition as claimed in claim 2, wherein the water-soluble substances having a solubility of more than 40 g per liter of water at 20°C.

4. The composition as claimed in claim 2, wherein the water-soluble substances having a solubility of more than 50 g per liter of water at 20°C.

5. The composition as claimed in any of claims 1 to 4, which comprises the reducing agent(s) in amounts of from 1 to 60% by weight, based on the total composition.

6. The composition as claimed in claim 5, wherein from 5 to 40% by weight of the reducing agent is present, based on the total composition.

7. The composition as claimed in claim 5, wherein from 10 to 25% by weight of the reducing agent is present, based on the total composition.

8. The composition as claimed in any of claims 1 to 7, which comprises, as reducing agents, one or more substances from the group of non-salt-like compounds.

9. The composition as claimed in claim 8, wherein the reducing agents are selected from pyrocatechol, hydroquinone and/or ascorbic acid.

10. The composition as claimed in any of claims 1 to 9, which comprises, as alkaline electrolyte(s), one or more substances from the group of carbonates, hydrogencarbonates, hydroxides, citrates and/or phosphates and the alkali metal.

11. The composition as claimed in claim 9, wherein the alkaline electrolyte is selected from potassium and/or sodium salts.

12. The composition as claimed in any of claims 1 to 11, which comprises the alkaline electrolyte(s) in amounts of from 40 to 99% by weight, based on the total composition.

13. The composition as claimed in claim 12, wherein from 45 to 95% by weight, based on the total composition of the alkaline electrolyte is present.

14. The composition as claimed in claim 12, wherein from 50 to 90% by weight, based on the total composition of the alkaline electrolyte is present.

15. The composition as claimed in any of claims 1 to 14, which comprises potassium citrate and/or sodium citrate in amounts of from 5 to 50% by weight, based on the total composition.

16. The composition as claimed in claim 15, wherein the citrate comprises 10 to 45% by weight, based on the total composition.

17. The composition as claimed in claim 15, wherein the citrate comprises 15 to 40 % by weight, based on the total composition.

18. The composition as claimed in claim 15, wherein the citrate comprises 20 to 35% by weight, based on the total composition.

19. The composition as claimed in any of claims 1 to 18, which comprises potassium carbonate and/or sodium carbonate in amounts of from 10 to 70% by weight, based on the total composition.

20. The composition as claimed in claim 19, wherein the carbonate comprises 15 to 55% by weight, based on the total composition.

21. The composition as claimed in claim 19, wherein the carbonate comprises 20 to 45% by weight, based on the total composition.

22. The composition as claimed in claim 19, wherein the carbonate comprises 25 to 40% by weight, based on the total composition.

23. The composition as claimed in any of claims 1 to 22, which additionally comprises dispersants, in amounts of from 0.1 to 10% by weight, based on the total composition.

24. The composition as claimed in claim 23, wherein the dispersants are selected from polycarboxylic acid salts.

25. The composition as claimed in claim 23, wherein the dispersants are selected from alkanolamine salts of polycarboxylic acids.

26. The composition as claimed in any of claims 23 to 25, wherein the amounts are from 0.25 to 5% by weight.

27. The composition as claimed in any of claims 23 to 25, wherein the amounts are from 0.5 to 2.5% by weight.

28. The composition as claimed in any of claims 1 to 27, which additionally comprises nonionic surfactant (s) , in amounts of from 0.1 to 10% by weight, based on the total composition.

29. The composition as claimed in claim 28, wherein alkoxylated nonionic surfactants are present.

30. The composition as claimed in claim 28, wherein ethoxylated and/or propoxylated nonionic surfactants having 8 to 24 carbon atoms are present.

31. The composition as claimed in claim 28, wherein ethoxylated and/or propoxylated nonionic surfactants having 10 to 20 carbon atoms are present.

32. The composition as claimed in claim 28, wherein ethoxylated and/or propoxylated nonionic surfactants having 12 to 18 carbon atoms are present.

33. The composition as claimed in claim 28, wherein nonionic surfactants containing fluoroalkyl groups are present.

34. The composition as claimed in any of claims 28 to 33, wherein the surfactants are present in amounts of from 0.25% to 5% by weight, based on the total composition.

35. The composition as claimed in any of claims 28 to 33, wherein the surfactants are present in amounts of from 0.5% to 2.5% by weight, based on the total composition.

36. A composition for cleaning silver in a dishwasher, comprising, in addition to further optional constituents, a) 10 to 70% by weight of potassium carbonate and/or sodium carbonate, b) 5 to 50% by weight of potassium citrate and/or sodium citrate, c) 1 to 60% by weight of one or more reducing agents having a solubility of more than 20 g per liter of water at 20°C, d) 0 to 10% by weight of one or more dispersants, and e) 0 to 10% by weight of one or more nonionic surfactants.

37. The composition as claimed in claim 36, wherein the sum of components a) and b) is 40 to 99% by weight.

38. The composition as claimed in claim 37, wherein the sum is 45 to 95% by weight.

39. The composition as claimed in claim 37, wherein the sum is 50 to 90% by weight.

40. The composition as claimed in any of claims 36 to 39, which additionally comprises hydrogencarbonates, hydroxides and/or phosphates in amounts of from 1 to 30% by weight, based on the total composition.

41. The composition as claimed in claim 40, wherein the amounts are from 2 to 25% by weight, based on the total composition.

42. The composition as claimed in claim 40, wherein the amounts are from 5 to 15% by weight, based on the total composition.

43. The composition as claimed in any of claims 36 to 43, which comprises, as component c), pyrocatechol, hydroquinone, ascorbic acid or mixtures thereof in amounts of from 2.5 to 50% by weight, based on the total composition.

44. The composition as claimed in claim 43, wherein the amounts are from 5 to 45% by weight, based on the total composition.

45. The composition as claimed in claim 43, wherein the amounts are from 10 to 25% by weight, based on the total composition.

46. A method of cleaning silver, which comprises treating articles made of silver or silver-plated articles or alloys of silver with a cleaning composition as claimed in any of claims 1 to 45 in a dishwasher.

47. A method of removing oxide and/or sulfide deposits from tarnished silver, which comprises treating articles made of tarnished silver or alloys thereof with a cleaning composition as claimed in any of claims 1 to 45 in a dishwasher.

48. The method as claimed in either of claims 46 or 47, wherein, after the cleaning in the dishwasher, an aftertreatment step is carried out.

49. The method as claimed in claim 48, wherein after-polishing is carried out.

50. The method as claimed in any of claims 46 to 49, wherein the cleaning liquor in the dishwasher comprises the reducing agent(s) in amounts of from 1 to 100 mmol/1.

51. The method as claimed in claim 50, wherein the amounts are from 2.5 to 75 mmol/l.

52. The method as claimed in claim 50, wherein the amounts are from 5 to 50 mmol/l.

53. The method as claimed in any of claims 46 to 52, wherein the cleaning liquor in the dishwasher comprises carbonate(s), in amounts of from 1 to 500 mmol/1.

54. The method as claimed in claim 53, wherein potassium carbonate and/or sodium carbonate are present.

55. The method as claimed in claim 53 or 54, wherein the amounts are from 10 to 250 mmol/1.

56. The method as claimed in claim 53 or 54, wherein the amounts are from 50 to 150 mmol/l.

57. The method as claimed in any of claims 46 to 56, wherein the cleaning liquor in the dishwasher comprises citrate(s), in amounts of from 1 to 150 mmol/l.

58. The method as claimed in claim 57, wherein the citrate is potassium citrate and/or sodium citrate.

59. The method as claimed in claim 57 or 58, wherein the amounts are from 2.5 to 100 mmol/l.

60. The method as claimed in claim 57 or 58, wherein the amounts are from 5 to 75 mmol/l.

61. The use of cleaning compositions which comprise alkaline electrolytes and reducing agents having a solubility of more than 20 g per liter of water at 20°C, for the cleaning of silver.

62. The use of cleaning compositions comprising a) 10 to 70% by weight of potassium carbonate and/or sodium carbonate, b) 5 to 50% by weight of potassium citrate and/or sodium citrate, c) 1 to 60% by weight of one or more reducing agents having a solubility of more than 20 g per liter of water at 20°C, d) 0 to 10% by weight of one or more dispersants, and e) 0 to 10% by weight of one or more nonionic surfactants, for the removal of oxide and/or sulfide deposits on tarnished silver.

63. The use as claimed in either of claims 61 or 62, wherein the composition is used in a dishwasher.

64. The use as claimed in claim 63, wherein, after the cleaning in the dishwasher, an aftertreatment step, is carried out.

65. The use as claimed in claim 64, wherein an after-polishing step is carried out.