EP4505001A1

EP4505001A1 - Method of manufacturing a platinum complex for plating

Info

Publication number: EP4505001A1
Application number: EP23750692.8A
Authority: EP
Inventors: Alan Boardman; David Ian James KREIGER; Nigel Powell
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2022-08-04
Filing date: 2023-07-21
Publication date: 2025-02-12
Also published as: ZA202408678B; CN119487238A; US20250313981A1; TW202409356A; WO2024028570A1; TWI876427B

Abstract

The specification describes a method of manufacturing a plating solution comprising a platinum (II) complex, comprising the steps of: (i) preparing an acidic aqueous solution comprising a chloride-free platinum (IV) compound and a source of nitrous acid (HNO2) as a reducing agent; (ii) heating the solution from step (i) in order to promote the reduction of platinum (IV) to platinum (II) and decomposition of residual nitrous acid. Also described is a platinum plating solution produced by this process.

Description

Method of manufacturing a platinum complex for plating

Field of the Invention

The present invention relates to a method of manufacturing platinum (II) complexes and their use in plating.

Background

It is well known to use platinum (IV) and (II) salts for plating. A common class of platinum salts contain one or more nitro ligands, and these complexes can be written by the general formula (I)

M_x[Pt(NO₂)aLb] Formula I where M is H, a metal or a non-metal cation, L is a ligand, and x, a and b are integers whose values vary depending on M, L and the oxidation state of platinum.

An example of a complex of Formula I is dihydrogen dinitrosulfatoplatinum (II) H₂[Pt(NO₂)₂(SO₄)] which is also referred to in the literature as dinitrosulfatoplatinous acid, PtDNS or simply DNS. This complex has been described for use in platinum electroplating, see GB2059440A (Johnson Matthey) and is commercially available from Johnson Matthey and others (CAS 12033-81-7).

Dinitrosulfatoplatinous acid (hereafter “H.DNS”) and related complexes such as potassium dinitrosulfatoplatinate (hereafter “K.DNS”) and potassium tetranitroplatinate K₂[Pt(NO₂)4] are used to produce thin platinum films on a variety of substrates. For example, the article “The Electrodeposition of Platinum and Platinum Alloys” (Platinum Metals Rev., 1988, 32, (4), 188-197) describes that these electrolytes can be used to coat platinum onto a wide range of materials including copper, brass, silver, nickel, lead and titanium. CN105132964A (Wuxi Qingyang) describes a platinum plating solution comprising K₂[Pt(NO₂)4], a water- soluble phosphate, dialkyltrimethylammonium bromide and sulfuric acid. The article “Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation" (Corrosion Science 2017, 127, 82-90) describes the use of K.DNS (K₂[Pt(NO₂)₂(SO4)] to electroplate Ni-based superalloys. The manufacture of H.DNS and K.DNS has been reported in the literature. GB897690 (Johnson Matthey) describes that H.DNS can be prepared by the reaction between tetranitroplatinous acid H2[Pt(NC>2)4] and sulfuric acid. The complex H2[Pt(NC>2)4] is prepared in this reference by passing a solution of K2[Pt(NC>2)4] through a cation exchange column with sulfuric acid. The complex K2[Pt(NC>2)4] itself is usually prepared from a platinumchloride salt. For instance the articles “The Stereoisomerism of Complex Inorganic Compounds. XIV. Studies upon the Stereochemistry of Saturated Tervalent Nitrogen Compounds” (J. Am. Chem. Soc. 1952, 74, 14, 3535-3538) and “Molecular Photocrystallography: A Study of Metastable and Transient Species by Non-Ambient Crystallographic Techniques" (Hatcher, L., Doctoral Thesis 2014, University of Bath) describe the preparation of K2[Pt(NC>2)4] by the reaction between K2PtCk and KNO2.

The article Platinum Metals Rev., 1988, 32, (4), 188-197 describes that H.DNS can be prepared from a platinum nitro salt such as K2[Pt(NO2)sCI], K2[Pt(NO2)2Ch] or K₂[Pt(NO₂)2SO₄] (K.DNS). These in turn have to be prepared from a precursor platinum salt which is normally a platinum chloride salt.

The article “Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation" (Corrosion Science 2017, 127, 82-90) describes the use of K.DNS (K2[Pt(NO2)2(SO4)] to electroplate Ni-based superalloys. The K.DNS is prepared by aqueous reaction between K2PtCle and KNO2 at 80 °C to obtain K2[Pt(NC>2)4] crystals, followed by the aqueous reaction between K2[Pt(NC>2)4] and sulfuric acid.

Existing routes to H.DNS, K.DNS and related complexes involve several steps and generally require the use of platinum halide complexes, which are sensitizing. There is a need for a simpler, safer and scalable method to produce platinum (II) complexes such as H.DNS, K.DNS and related complexes. The present invention addresses this need.

Summary of the Invention

The present inventors have found that platinum (II) complexes can be prepared in a simple process by the aqueous acidic reaction between a chloride-free platinum (IV) compound and a source of nitrous acid (HNO2). The nitrite ions reduce the Pt(IV) to Pt(ll) and are oxidized to nitrate and/or nitrogen oxides, especially nitrogen dioxide. Other nitrite ions may complex with the Pt(ll) centre to produce a nitro complex. A heating step is carried out to promote the reduction of platinum (IV) to platinum (II) and to promote decomposition of residual nitrous acid. An exemplary reaction is:

H₂[Pt(OH)₆] + 3KNO₂ + H₂SO₄ K₂[Pt(NO₂)₂(SO₄)] + KNO₃ + 5H₂O

This process offers several advantages over known methods for producing Pt(ll) complexes, especially H.DNS and K.DNS.

Firstly, the Pt(ll) complex is prepared in a single step from the Pt(IV) compound. This provides a higher yielding route to the desired complex compared with the complex multistep routes described above. The single step method also allows simplification of the manufacturing equipment and requires less space in the plant.

Secondly, nitrites are relatively easy to handle materials compared to reducing agents like H₂, N₂/H₂ or hydrazine which have been used previously for the reduction of Pt(IV) to Pt(ll). In addition, nitrites do not reduce the Pt(ll) further to Pt(O).

Thirdly, the reduction and subsequent processing are quick. This allows increased throughput compared to existing alternative routes.

Fourthly, preferred sources of platinum (IV) used in this route are hexahydroxyplatinic acid and hexahydroxyplatinate salts, which are non-sensitizing unlike the platinum chloride salts which are used in existing processes to make H.DNS I K.DNS and related complexes.

It is known to use Pt(IV) compounds as precursors to Pt(ll) salts. LIS2015/0315224 (Umicore) describes a process in which H₂[Pt(OH)e] is reacted with an uncharged donor ligand L in the presence of a reducing agent and at least one of the hydroxo ligands is replaced. Preferred reducing agents are H₂, N₂/H₂ mixtures, hydrazine, formaldehyde, oxalic acid and formic acid. Typically the ligand L is a monodentate or bidentate amine or phosphine. However, this reference does not describe the use of HNO₂ as a reducing agent.

The combination of a platinum (IV) compound and a nitrite salt is described in US4200626 (Toyo Seiyaku Kasei Co Ltd) which describes a dental composition comprising a water- soluble haloplatinate and a pharmaceutical carrier or diluent. Examples 2 and 3 of this reference describe the combination of sodium hexachloroplatinate hexahydrate and sodium nitrite (Example 2) or sodium hexachloroplatinate hexahydrate, sodium nitrite and sodium chloride (Example 3) at pH 5.5 or 3.5. However, sodium nitrite is not present in sufficient quantity to complete the reduction of Pt(IV) to Pt(ll) and, even if some reduction does take place, a heating step is not carried out.

The book “Handbook of Preparative Inorganic Chemistry” (Volume 1 , Second Edition, Academic Press Inc, 1963) describes on pages 1579-1580 the preparation of cis- dinitrodiammineplatinum(ll), based on a method described in US1779436. In a first step potassium hexachloroplatinate (IV) (K^PtCle) and sodium nitrite are dissolved in water and heated with stirring. NO2 is evolved as fine bubbles. When no further gas evolves the solution now containing K2[Pt(NC>2)4] is cooled and filtered. This solution is then reacted with a stoichiometric quantity of 20% aqueous ammonia to produce the desired complex [Pt(NO₂)2(NH₃)2], which can be recrystallized from hot water. This route uses K2PtCle which contains chloride and is sensitizing.

In a first aspect the invention relates to a method of manufacturing a plating solution comprising a platinum (II) complex, comprising the steps of:

(i) preparing an acidic aqueous solution comprising a chloride-free platinum (IV) compound and a source of nitrous acid (HNO2) as a reducing agent;

(ii) heating the solution from step (i) in order to promote the reduction of platinum (IV) to platinum (II) and decomposition of residual nitrous acid.

In the present invention the Pt(IV) compound can be converted into a diverse range of Pt(ll) nitro complexes depending on the choice of acid and source of nitrous acid.

In a second aspect the invention relates to a plating solution produced by a process described herein. Without wishing to be bound by theory, it is thought that the new method may produce solutions with different speciation compared to those described previously in the literature as H₂[Pt(NO₂)2(SO₄)] or K₂[Pt(NO₂)₂(SO₄)].

Detailed description of the Invention

Any sub-headings are for convenience only and are not intended to limit the invention.

Step (i) The process of the invention uses a Pt(IV) compound as a starting material. The Pt(IV) centre is generally surrounded by ligands and the Pt(IV) compound may also be referred to herein as a complex. The Pt(IV) compound is typically a salt in which the anion contains Pt (IV).

Platinum chloride salts and complexes such as K2PtCle and K2PtCk which have been used previously to prepare platinum (II) complexes are often sensitizing. The Pt(IV) compound used in step (i) is therefore chloride-free. By “chloride free” we mean that the compound does not include chloride complexed to platinum such as K2PtCle or H2PtCle etc. The Pt(IV) compound is preferably halide-free, i.e. the compound does not include any halide complexed to platinum. A particularly preferred Pt(IV) compound is hexahydroxyplatinic (IV) acid, H2Pt(OH)e. This compound is available commercially and is considered to be a nonsensitizing Pt(IV) compound. Salts of hexahydroxyplatinic (IV) acid, containing the hexahydroxyplatinate ion, are also preferred Pt(IV) compounds. The counterion to hexahydroxyplatinate may be a metal ion or a non-metal ion, such as an ammonium or alkylammonium ion.

The role of the source of nitrous acid is to form nitrous acid in situ under the acidic conditions and thereby reduce the Pt(IV) to Pt(ll). It is preferred that the source of nitrous acid is added in an amount sufficient to reduce all of the Pt(IV) to Pt(ll). While an excess of reducing agent is desirable for rapid reaction conversion, too much is undesirable for cost reasons and to avoid excess NOx release.

The reducing agent is referred to herein for convenience as being “nitrous acid” but it will be appreciated that the active reducing agent may be more complicated, and is probably a mixture of nitrous acid (HNO2) and nitrous anhydride (N2O3) which are known to exist in equilibrium in aqueous solution according to the equation:

2HNO₂ — ► N2O3 + H₂O

Under the acidic conditions in steps (i) and (ii) nitrite will exist as a mixture of N2O3, HNO2 and NO2; and possibly other species, depending on the pH of the solution.

In one embodiment the source of nitrous acid is provided as a nitrite salt e.g. of formula [M^a+(NC>2)a)] where M^a+ is a metal or non-metal cation and a is an integer. Nitrous acid is formed from the nitrite salt under acidic conditions. One type of preferred salts are Group I nitrites, preferably lithium nitrite, sodium nitrite or potassium nitrite. Another type of preferred salts are Group II nitrites, preferably magnesium nitrite, calcium nitrite, strontium nitrite or barium nitrite. Another preferred salt is silver nitrite. In a preferred embodiment the source of nitrous acid is a metal nitrite selected from magnesium nitrite, calcium nitrite, strontium nitrite, barium nitrite and silver nitrite. These salts are preferred because the metal ions (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ag⁺) form insoluble salts with many mineral and some organic acids, meaning that they can be precipitated from solution in step (i) or in optional step (iii). For example, additional of phosphoric acid in step (i) or in optional step (iii) will precipitate the residual metal ions as magnesium phosphate, calcium phosphate, strontium phosphate, barium phosphate or silver phosphate. Magnesium nitrite, calcium nitrite and strontium nitrite are preferred over barium nitrite because the latter is toxic, and are preferred over silver nitrite because the latter is expensive. Therefore, a preferred source of nitrous acid is a metal nitrite selected from magnesium nitrite, calcium nitrite and strontium nitrite. Ammonium nitrite is a preferred source of nitrous acid with a non-metal cation.

It is also possible to use alternative sources of nitrous acid. In one embodiment the source of nitrous acid is a M_x[Pt(NO2)4] salt, where x = 2 when M has a charge of +1 and x = 1 when M has a charge of +2, for example potassium tetranitroplatinate (II) (K2[Pt(NO2)4]).

If excess nitrite is present then this is released as brown nitrogen dioxide gas as the reaction warms up. If a metal nitrite salt is used as the source of nitrous acid it may be undesirable to have excessive amounts of countercation remaining in solution after steps (i)/(ii). The molar ratio of Pt(IV) : source of nitrous acid at the beginning of step (i) will depend on the choice of source of nitrous acid. For example one equivalent of KNO2, Ca(NO2)2 or K₂[Pt(NO₂)4] contains 1 , 2 and 4 equivalents of nitrite respectively. Where the source of nitrous acid is a nitrite salt it is preferred that the molar ratio of Pt(IV) : nitrite at the beginning of step (i) is between 1 : 2 to 1 : 10, preferably between 1 : 2 to 1 : 8.

In one embodiment the source of nitrous acid is pure HNO2. By “pure HNO2” we mean that the solution is substantially free of metal ions. Pure nitrous acid can be prepared by treatment of a solution containing a nitrite salt in an organic polar solvent aqueous solution with an ion exchange resin. Examples include the procedures reported in LIS3113837 and the article “Preparation of Solutions of Pure Nitrous Acid” (J. Am. Chem. Soc. 1963, 85, 23, 3888). Pure nitrous acid may also be prepared by electrolysis of nitric acid solutions using a platinum electrode, as described in the article “Alternative Electrode Reactions. Part I. Reactions at a platinum cathode in nitric acid solutions” ( . Chem. Soc. 1932, 1565-1579). Where pure HNO2 is used the solution in step (i) is substantially free of metal ions, other than those from the Pt(IV) compound, optional acid (described later), and optional salt which dissolves to give an acidic solution (described later).

The source of nitrous acid may also exchange with the ligands on the Pt(IV) or Pt(ll) centre to form a nitro complex. As used herein “nitro complex” should be understood in its broadest sense and includes bonding via nitrogen (Pt-NO2) or via oxygen (Pt-ONO).

Some Pt(IV) compounds are not appreciably soluble in water at room temperature, such as hexahydroxyplatinic (IV) acid which is insoluble in water at room temperature. Therefore it is preferred that the mixture is agitated in order to promote dissolution during steps (i) and (ii). Agitation is preferably achieved by moderate stirring. Rapid stirring if needed should be limited to short bursts as it promotes nitrogen dioxide loss. The present inventors have found that an aqueous suspension of hexahydroxyplatinic (IV) acid solubilised rapidly in the presence of nitrous acid, presumably due to the reduction of Pt(IV) to Pt(ll) by nitrite.

The concentration of Pt in step (i) should be chosen with consideration of process economy (dilute solutions require more energy to heat). Typically the Pt concentration is 5 to 100 g/L, such as 10 to 100 g/L. The upper limit is determined by the solubility of the Pt(IV) compound. When manufacturing at commercial scale a Pt concentration of 30 to 80 g/L may be appropriate.

The reaction is carried out under acidic conditions, i.e. pH < 7, preferably pH < 6, more preferably pH < 5, more preferably pH < 4. The minimum pH is not especially limited but is preferably not less than -1 , preferably not less than 0. Typically the reaction is carried out at pH < 4, such as pH 0-4. The acidic conditions promote formation of nitrous acid (HNO2) which is believed to be a major species responsible for reducing the Pt(IV).

Depending on the Pt(IV) compound and source of nitrous acid used it may not be necessary to add any additional acid. However, in some embodiments an acid is added in step (i) to promote the formation of nitrous acid. The acid is distinct from the Pt(IV) compound or source of nitrous acid. The skilled person will appreciate that if the conjugate base of the acid is strongly coordinating towards platinum (II) then it is possible for it to be incorporated into the platinum (II) complex. For example, if sulfuric acid is added as the acid then it is possible that a platinum (II) sulfato complex will be formed. The choice of acid will therefore be guided by the desired platinum (II) complex. The acid may be a mineral acid or an organic acid. Preferred mineral acids include phosphoric acid, sulfuric acid and nitric acid. Preferred organic acids include carboxylic acids, and aldehydes that often contain carboxylic acid impurities on ageing, such as glyoxal.

The addition of phosphoric acid or sulfuric acid as the acid in step (i) has the additional advantage that they form insoluble salts with a variety of metal ions. Therefore, if the source of nitrous acid is a metal nitrite salt in which the metal forms an insoluble salt with either phosphate or sulfate then it may be possible to acidify the solution and precipitate a metal phosphate or sulfate at the same time in step (i).

In one embodiment the source of nitrous acid is a metal nitrite, the acid is phosphoric acid and a metal phosphate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is phosphoric acid and magnesium phosphate, calcium phosphate or strontium phosphate is precipitated in step (i).

In one embodiment the source of nitrous acid is a metal nitrite, the acid is sulfuric acid and a metal sulfate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is sulfuric acid and magnesium sulfate, calcium sulfate or strontium sulfate is precipitated in step (i). The use of calcium nitrite or strontium nitrite with sulfuric acid is preferred over the use of magnesium nitrite because of the much greater insolubility of calcium sulfate and strontium sulfate compared to magnesium sulfate.

Preferred organic acids include acetic acid, oxamic acid and oxalic acid. These acids dissolve in aqueous solution to provide the acidic conditions necessary to form HNO2 and reduce the Pt(IV) to Pt(ll). These acids, particularly oxalic acid, also form precipitates with some metal ions and are particularly advantageous where a metal nitrite is used as the source of nitrous acid. Oxalic acid is a particularly preferred organic acid. In one embodiment the source of nitrous acid is a metal nitrite, the acid is oxalic acid and a metal oxalate is precipitated. In a particularly preferred process the source of nitrous acid is magnesium nitrite, calcium nitrite, or strontium nitrite, the organic acid is oxalic acid and magnesium oxalate, calcium oxalate or strontium oxalate is precipitated.

Using an excess of oxalic acid tends to lead to a blackening and matt finish when the product from step (ii) is used for electroplating. While there are some instances where a dark and matt finish is desired, avoiding excess oxalic acid and/or adding a mixture of oxalic acid and sulfuric acid mitigates this and gives bright platinum plates.

In some embodiments the acid is provided in step (i) by a salt which dissolves in aqueous solution to give an acidic solution. This may be in place of or in addition to a mineral acid or organic acid. It is preferred that the salt dissolves in solution to produce a pH below 6. This helps to ensure that the N2Os/HNO2/NO2^_ equilibrium is in favour of N2O3/HNO2 which are believed to be the active species for reducing Pt(IV) to Pt(ll). Examples of suitable salts include Group I metal sulfates, Group I metal hydrogensulfates, Group I metal dihydrogenphosphates and Group I metal hydrogenoxalates, in each case the sodium or potassium salts are preferred for their commercial availability . Preferred salts are those which dissolve in solution to produce a pH below 4.

The addition of the Pt(IV) compound, source of nitrous acid, and any acid, should be controlled so as to avoid exotherms. If necessary the solution may be cooled to a temperature of 0-10 °C such as 0-5 °C.

The order of addition of Pt(IV) compound, source of nitrous acid and any additional acid in step (i) is not particularly important, unless the acid is capable of reducing Pt(IV) to Pt(O), in which case the acid should be added after combining the Pt(IV) compound and source of nitrous acid or at the same time as combining the Pt(IV) compound and source of nitrous acid. This is particularly the case for oxalic acid which is capable of fully reducing Pt(IV) to Pt(O).

Step (ii) In step (ii) the reaction mixture is heated. The role of step (ii) is to promote the reduction of Pt(IV) to Pt(ll) and to promote decomposition of any residual nitrous acid to nitrogen oxides.

In some embodiments heating is only commenced once the chloride-free platinum (IV) compound and source of nitrous acid are both present in solution.

In alternative embodiments a solution of the chloride-free platinum (IV) compound is preheated and the source of nitrous acid is added to the pre-heated solution. In this embodiment steps (i) and (ii) take place simultaneously. The source of nitrous acid may be added as a solid or as a solution.

Excess nitrous acid was seen to escape as brown nitrogen dioxide gas in the 50-80 °C range in the present invention. Therefore it is preferred that the reaction is heated to a temperature of at least 50 °C during step (ii), preferably at least 80 °C. Typically the reaction is heated to a temperature of at least 90 °C to ensure any conversion based on final solution color change, e.g. reddening. However, in many cases solutions held for an extended duration at lesser temperatures e.g. 60 °C will plate successfully. If the source of nitrous acid is added to a pre-heated solution of the chloride-free platinum (IV) compound then it is preferred that the pre-heated solution is at a temperature of at least 50 °C.

Step (Hi)

Step (iii) is an optional step. If excess metal nitrite salt is included in step (i) then it will be appreciated that the resulting solution will contain residual metal ions unless these have been precipitated in step (i). In order to limit the possibility of the metal ions interfering in subsequent plating e.g. by contaminating the coating, it may be desirable to remove the metal ions. Therefore, in some embodiments the source of nitrous acid is chosen so that a salt MX can be precipitated in a subsequent step following steps (i) and (ii). The process involves a step (iii) of adding an acid or salt in order to precipitate the countercation of the metal nitrite as the corresponding metal salt. Any acid or salt which forms an insoluble salt with [M^a+] ions may be used.

It was described above that step (i) may involve the addition of a mineral acid or organic acid to acidify the solution and at the same time precipitate metal ions (e.g. from the source of nitrous acid) as the corresponding metal salt. It will be appreciated that if the metal ions are precipitated in step (i) then step (iii) will usually not be necessary.

In one embodiment oxalic acid is added in step (iii) and a metal oxalate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite the acid is oxalic acid and magnesium oxalate, calcium oxalate or strontium oxalate is precipitated in step (iii).

In one embodiment phosphoric acid is added in step (iii) and a metal phosphate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is phosphoric acid and magnesium phosphate, calcium phosphate or strontium phosphate is precipitated in step (iii).

In one embodiment sulfuric acid is added in step (iii) and a metal sulfate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is sulfuric acid and magnesium sulfate, calcium sulfate or strontium sulfate is precipitated in step (iii). The use of calcium nitrite or strontium nitrite with sulfuric acid is preferred over the use of magnesium nitrite because of the much greater insolubility of calcium sulfate and strontium sulfate compared to magnesium sulfate.

To ensure maximum removal of the metal salt the solution may be concentrated, chilled and/or the pH may be adjusted before acid addition in order to maximise metal salt removal.

The solution produced after step (ii), or after step (iii) where step (iii) is carried out, is suitable for use in plating e.g. electroplating. In some embodiments the concentration may be adjusted (concentrated or diluted) depending on the required [Pt] for electroplating. Typically, reactions (i)-(iii) are carried out at a relatively high [Pt] and then diluted, e.g. with deionized water. The concentration of Pt in a “ready for use” solution for electroplating is typically from 0.5 to 30 g/L. Therefore, in some embodiments the concentration of Pt is adjusted to 0.5 to 30 g/L after step (ii) (or after step (iii) if applicable).

Example 1 - H₂[Pt(OH)₆] + K₂[Pt(NO₂)₄] (1.04 equiv) + H₂SO₄ A suspension of hexahydroxyplatinic acid (0.75 g, 2.51 mmol) and potassium tetranitroplatinate (1.2 g, 2.62 mmol) in 30 mL water was stirred. Concentrated sulfuric acid (1 .0 mL) was added dropwise causing some yellowing of the color. The stirred mixture was heated to boiling causing the color to change through yellow and orange until a clear cherry- red solution was produced. Some foaming occurred around 70-80 °C due to residual nitrogen dioxide release, but it soon subsided. After a few minutes of boiling to ensure no residual nitrogen oxides remained, the beaker was removed from the hotplate and cooled to room temperature without further color change. The final cherry-red liquid was diluted to 200 mL with distilled water, giving a concentration of 5.0 g/L Pt and the resultant solution was used to plate a shaped stainless steel test piece at 9.3 ASF at 90 °C for 90 mins, producing a silvery bright Pt coat at a cathodic current efficiency of 20% for the first plate through the bath. The red plating solution was continuously stirred during plating. A circular Pt/Ti mesh anode surrounded the cathodic test piece. The red liquid slightly darkened to red brown during early plating.

Example 2 - H₂[Pt(OH)₆] + K₂[Pt(NO₂)₄] (1.5 equiv) + H₂SO₄

Hexahydroxyplatinic acid pellets (1.5 g, 5.0 mmol) in a 150 ml glass beaker were crushed to a fine powder with a spatula, potassium tetranitroplatinate (3.4 g, 7.4 mmol) and 40 mL of water were added and the mixture magnetically stirred. Concentrated sulfuric acid (2.5 mL) was rapidly added dropwise at room temperature to the stirred suspension causing it to turn at first yellow, then orange with much dissolution. The orange suspension was then heated with stirring, resulting in a brown suspension at 60 °C and an almost clear red solution at 85 °C with a small amount of brown nitrogen dioxide noticed inside the beaker under the watchglass placed on top. The solution was boiled for several minutes forming a clear red solution, which was used for electroplating, turning brown during use. The bath showed cathodic current efficiencies of 15-25% using 8-10 ASF at 55 or 80 °C and produced continuous bright plates.

Example 3 - H₂[Pt(OH)₆] + Ca(NO₂)₂ (3.5 equivs) + HNO₃ A suspension of hexahydroxyplatinic acid H₂[Pt(OH)e] (3.0 g, 10.0 mmol), calcium nitrite solution (4.005 g of calcium nitrite, 30.3 mmol) and 50 mL water was stirred with cooling to below 5 °C using an ice bath. Diluted nitric acid (approx. 20%, 20 mL, 6.3 mmol) was added in portions with stirring. The suspension dissolved as the final portion was added to give a lemon-yellow solution. Additional diluted calcium nitrite solution (0.667 g of calcium nitrite, 5.1 mmol) and concentrated nitric acid (0.3 g) were added but there was no further change in the solution. The yellow solution was removed from the ice bath, made up to 120 mL and left to stir overnight. The clear yellow solution was then heated with stirring to drive off any residual nitrogen dioxide gas. Above 90 °C the yellow solution turned clear orange. Heating was stopped and the solution allowed to cool below 5 °C using an ice bath. Oxalic acid dihydrate (3.7 g, 29.3 mmol) dissolved in 50 mL of warm water was added slowly to the chilled solution with rapid stirring, producing a dense heavy white precipitate of calcium oxalate which was filtered off leaving a clear orange solution. The oxalic acid added was slightly less than 1 equivalent relative to the content of calcium, in order to avoid the presence of excess oxalic acid which could reduce platinum (II) ions. The orange solution was used for electroplating steel parts at 8-10 ASF producing bright plates at c. 55 °C or 80 °C with cathodic current efficiencies of 10-20%.

Example 4 - H₂[Pt(OH)₆] + Ca(NO₂)₂ (3 equivs) + HNO₃

A suspension of hexahydroxyplatinic acid (3.0 g, 10.0 mmol), calcium nitrite solution (4.005 g of calcium nitrite, 30.3 mmol) and 50 mL water was stirred in a 250 mL glass beaker cooled in an ice bath below 5 °C. A 70% nitric acid solution (6 g, diluted to 20 mL) was added in four 4 x 5 mL aliquots to the stirred cold white suspension over 40 mins, resulting in a yellow suspension that formed a hazy yellow solution soon after the third nitrite addition. The beaker with its contents was removed from the ice bath and allowed to warm to room temperature, creating a very slightly hazy yellow solution, which on stirring at room temperature for 5 mins became a clear yellow solution. This solution was left stirring overnight at room temperature without change. It was then heated with stirring to 90 °C to drive off any residual nitrogen oxides (hardly any), turning into an orange solution. The stirred solution was then cooled, chilled in an ice bath and oxalic acid dihydrate (3.7g, 29 mmol) dissolved in water (50 mL) was added to precipitate the calcium as insoluble white calcium oxalate. The calcium oxalate was removed by vacuum filtration, washed, dried and weighted 3.97g (in agreement with the weight of calcium oxalate hydrate expected, c.3.9g). The clear orange filtrate of 125 mL was diluted to 200mL and boiled for four hours with water replacement to prevent liquid loss. This resultant clear red-brown solution was used for electroplating steel and titanium parts at 8-10 ASF producing bright plates at 55 °C or 80 °C, with cathodic current efficiencies in the 10-20% range.

Example 5 - H₂[Pt(OH)₆] + KNO₂ (3.4 equivs) + H₂SO₄

A suspension of hexahydroxyplatinic acid H₂Pt(OH)e (6.06 g, 20.3 mmol), 3.5 g concentrated sulfuric acid and 160 mL water was stirred in a 400 mL glass beaker cooled in an ice bath below 5 °C. Potassium nitrite (5.77 g, 67.6 mmol) dissolved in 40 mL water at room temperature was added in portions to the stirred iced suspension over 30 mins. During this addition the white suspension in the beaker turned yellow and then began to dissolve. After the addition of potassium nitrite the beaker with its contents was removed from the ice bath and allowed to warm to room temperature naturally. At 16 °C the suspension had nearly all dissolved leaving a slightly hazy yellow orange solution. This solution was heated with stirring to 52 °C, when the first indication of a weak intensity brown vapour was noted inside the beaker, which was topped with a watchglass. Heating was continued and the yellow orange solution converted to a bright orange solution above 75 °C. No brown gas presence or foaming was noted above c. 80 °C. The bright orange solution was finally boiled for a few minutes producing no further color change. The solution was vacuum filtered warm through a combined paper and glass fibre filters to remove a tiny amount of likely, orange, insoluble, hydrous platinum oxide that was formed in the process. The clear orange filtrate of 200 mL was diluted to 600 mL and electroplated, initially two plates at 80 °C, then 13 plates at c.55 °C over the next 10 days, with replenishment from another identical orange colored bath. During prolonged electroplating without replenishment the orange color fades to form an almost colorless solution. All plates were bright with efficiencies in the 10-20% range for mainly 8-10 ASF, the highest efficiencies obtained at the higher temperature.

Example 6 - H₂[Pt(OH)₆] + KNO₂ (4 equivs) + H₂SO₄

A white suspension of hexahydroxyplatinic acid (6.03 g, 20.0 mmol) and potassium nitrite (5.75 g, 67.6 mmol) in 100 mL of water in a 400 mL glass beaker was stirred then cooled in an ice bath below 5 °C. Concentrated sulfuric acid (3.6 g in 20 mL water) at room temperature was added over 10 minutes in portions, creating a yellow suspension. After the addition of acid the beaker with its contents was removed from the ice bath and allowed to warm up to room temperature naturally with stirring. This resulted in most of the yellow suspension dissolving to form a yellow translucent solution. The beaker and its contents were cooled again with an ice bath below 5 °C and treated with potassium nitrite (1.0 g, 12.0 mmol dissolved in 10 mL of water) and 0.75 g of concentrated sulfuric acid added dropwise to the stirred cold translucent solution. This addition caused the translucence to rapidly vanish resulting in a clear yellow-orange solution. The beaker with its contents was removed from the ice bath and heated with stirring to 50 °C, producing an almost clear light orange solution of volume c.130 mL. Heating between 50-80 °C resulted in the excess nitrous acid being lost as brown nitrogen dioxide gas, as seen inside the beaker topped by a watch glass. Around 80 °C slight translucence developed in the clear orange solution due to the formation of insoluble orange hydrated platinum oxide, which was filtered off. Boiling the bright orange solution for 15 mins produced no further change. Boiling the orange solution for several hours (with cycles of water replenishment to replace evaporated water) causes it to redden. Further heating of the red solution without electroplating, but more rapidly with electroplating at 80 °C or above, resulted in a deep red brown solution, finally a brown solution, very similarly in color to current commercially made H.DNS solution. The new brown solution slowly fades to almost colorless on prolonged plating bath use without replenishment. Multiple plates from this red brown bath gave cathodic current efficiencies in the 10-20% range at 8-10 ASF, 55 or 80 °C.

Example 7 - H2[Pt(OH)e] + Ca(NC>2)2 (3 equivs) + oxalic acid

Hexahydroxyplatinic acid (2 g as a crushed powder, 6.6 mmol), calcium nitrite (30% solution, 2.75 g of calcium nitrite, 20.8 mmol) and 50 mL water were stirred together in a 250 mL beaker in an ice bath below 5 °C. Oxalic acid dihydrate (2.64g in 40 mL of water, 21.0 mmol) was added cautiously in one portion, but this resulted in some brown nitrogen dioxide loss from the cold stirred suspension, which rapidly turned yellow. After 30 mins the beaker and its contents was removed from the ice bath and heated to 40 °C, causing a small amount of brown nitrogen dioxide to be released. Further slight releases were noticed up to 85 °C. The heating and stirring was then stopped and the white suspension settled from a bright yellow solution, which was were separated by vacuum filtration. The yellow solution on prolonged heating above 80 °C turned orange. The orange solution was used for electroplating steel parts at 8-10 ASF producing bright plates at c. 55 °C or 80 °C with cathodic current efficiencies of 10-20%

Example 8 - H2[Pt(OH)e] + Ca(NC>2)2 (3 equivs) + oxalic acid

Oxalic acid dihydrate (2.53 g dissolved in 40 mL water with slight warming, 20.1 mmol) was stirred into hexahydroxyplatinic acid (2 g as a crushed power, 6.6 mmol) in a 250 mL beaker contained in an ice bath. Below 5 °C, calcium nitrite solution (2.63 g of calcium nitrite, 19.9 mmol) in 40 mL of water was added in portions over several minutes to minimise any loss of brown nitrogen dioxide. The stirred white suspension rapidly turned yellow. After 45 mins the suspension was removed from the ice bath and warmed to 35 °C, causing slight brown nitrogen dioxide loss, and rapid separation of a yellow solution from the settled precipitate on standing. After four days of standing the yellow solution had turned orange. The stirred orange solution was treated with calcium nitrite solution (0.26 g, 2.0 mmol), then oxalic acid dihydrate (0.26 g dissolved in 20 mL of water at room temperature, 2.06 mmol). Some brown nitrogen dioxide was released at 80 °C, but color of the solution remained unchanged. The solution and a white suspension were separated by vacuum filtration. The orange solution was used for electroplating steel parts at 8-10 ASF producing bright plates at c. 55 °C or 80 °C with cathodic current efficiencies of 10-20%.

Claims

1. A method of manufacturing a plating solution comprising a platinum (II) complex, comprising the steps of:

2. A method according to claim 1 , wherein the source of nitrous acid in step (i) is a metal nitrite salt.

3. A method according to claim 2, wherein the molar ratio of Pt(IV) : nitrite at the beginning of step (i) is between 1 : 2 to 1 : 10.

4. A method according to any of claims 1 to 3, wherein the source of nitrous acid is a Group I or Group II nitrite salt.

5. A method according to any of claims 1 to 4, wherein the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite.

6. A method according to claim 1 , wherein the source of nitrous acid is a M_x[Pt(NO2)4] salt, where x = 2 when M has a charge of +1 and x = 1 when M has a charge of +2.

7. A method according to claim 6, wherein the source of nitrous acid is potassium tetranitroplatinate (II) (K2[Pt(NO2)4]).

8. A method according to any of claims 1 to 7, wherein the platinum (IV) compound is hexahydroxyplatinic (IV) acid (H2Pt(OH)e) or a salt thereof.

9. A method according to any of claims 1 to 8, wherein the pH of the solution in step (i) is < 6.

10. A method according to any of claims 1 to 9, wherein the pH of the solution in step (i) is < 4.

11. A method according to any of claims 1 to 10, wherein an acid is added in step (i).

12. A method according to claim 11 , wherein the acid is selected from the group consisting of phosphoric acid, sulfuric acid and nitric acid.

13. A method according to claim 11 , wherein the acid is oxalic acid.

14. A method according to claim 11 , wherein the acid is a salt which dissolves in aqueous solution to give an acidic solution with a pH below 6.

15. A method according to any of claims 1 to 14, wherein the concentration of Pt in step (i) is between 5 to 100 g/L.

16. A method according to any of claims 1 to 15, wherein the mixture from step (i) is heated to a temperature of at least 50 °C in step (ii).

17. A method according to any of claims 1 to 16, comprising a subsequent step of using the product of step (ii) for plating.

18. A method according to any of claims 1 to 16, wherein the source of nitrous acid is a metal nitrite and comprising a step (iii) of adding an acid or salt and precipitating the metal countercation of the source of nitrous acid as a salt.

19. A method according to claim 18, wherein the acid is oxalic acid and a metal oxalate is precipitated.

20. A method according to claim 18, wherein the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is oxalic acid and magnesium oxalate, calcium oxalate or strontium oxalate is precipitated in step (iii).

21 . A method according to claim 18, wherein the acid is phosphoric acid and a metal phosphate is precipitated.

22. A method according to claim 18, wherein the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is phosphoric acid and magnesium phosphate, calcium phosphate or strontium phosphate is precipitated in step (iii).

23. A method according to claim 18, wherein the acid is sulfuric acid and a metal sulfate is precipitated.

24. A method according to claim 18, wherein the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is sulfuric acid and magnesium sulfate, calcium sulfate or strontium sulfate is precipitated in step (iii).

25. A method according to claim 18, wherein the source of nitrous acid is selected from calcium nitrite or strontium nitrite, the acid is sulfuric acid and calcium sulfate or strontium sulfate is precipitated in step (iii).

26. A method according to any of claims 18 to 25, comprising a subsequent step of using the product of step (iii) for plating.

27. A platinum plating solution produced by a process according to any of claims 1 to