US3245886A

US3245886A - Electroplating process and self-regulating electroplating baths therefor

Info

Publication number: US3245886A
Application number: US298424A
Authority: US
Inventors: Michael Gregor
Original assignee: DEHYDAG GmbH
Current assignee: DEHYDAG DEUTSCHE HYDRIERWERKE G M B H; DEHYDAG GmbH
Priority date: 1961-08-10
Filing date: 1963-07-29
Publication date: 1966-04-12
Anticipated expiration: 1983-04-12
Also published as: CH429356A; DE1421977A1; NL281922A; GB941823A; DE1421977B2; DE1224111B; US3257294A; BE621297A; GB1055243A

Description

United States Patent Office 3,245,886 Patented Apr. 12, 1966 D 39,52 7 Claims. (Cl. 204-45) This invention relates to an electroplating method and bath composition. The invention more particularly relates to electroplating baths containing additive compounds which are difficultly soluble and which are deposited on a solid carrier material.

In my copending application Serial No. 214,490, filed August 3, 1962, is described the employment of difficultly soluble organic additives with electroplating activity, which form a solid, non-colloidal phase in the electrolyte, in acid metal electroplating baths for self-regulation of the brightening effect and other bath properties which can be influenced by the addition of organic agents. The difficultly soluble agents may, for instance, be provided in a customary solution filter which is inserted in the bath recycling line after the impurities trap filter.

In the prior art, electroplating-active additives, such as brightening agents, leveling agents, grain-reducing agents, porosity-prevention agents, equalizing agents and wetting agents, have been added to the electroplating baths in the form of aqueous solutions having a certain concentration. During the operation of the electroplating bathsa more or less pronounced consumption of these additives occurs. Therefore, if the consumed agents were not continuously replenished, the critical concentration, that is the concentration below which acceptable electrodeposi ts are no longer achieved, would soon be reached in the bath. The satisfactory operation of such electroplating bath, therefore, requires continuous analytical surveillance in order to maintain the concentration of the additives within the range necessary for achievement of satisfactory electrodeposits. In practice, however, this addition is in most cases accomplished on a purely empirical basis, that is after the lapse of certain time intervals and often only when the undesirable effects become visible, which leads to products that must be rejected. Moreover, the periodic replenishment of electroplating additive agents may also lead to excessive concentrations in the cathode space because of insufficient admixture, and this excessive concentration has an unfavorable effect upon the metal electrodeposit. In addition, there exists quite generally the danger of inadequate or excessive concentration of additives in the electrolyte solution, which may give rise to troubles of manifold types.

In' contrast thereto, electroplating-active organic additives are used in my copending application Serial No. 214,490, in accordance with the present invention which are diflicultly soluble in the bath and form a heterogeneous phase. The electroplating bath is thereby continuously and in a self-adjusting manner maintained near the saturation concentration of the additive component. It the concentration of additives in the bath is reduced by normal consumption, the saturation concentration is again restored by the difficultly soluble substance lying onthe bottom, so that further additions for the purpose of correcting the bath composition are not required during operation. Since the concentration of additives in the electrolyte remains constant, the bath does not require a continuous analytical surveillance. Consequently, in the baths compounded in accordance with the present invention, the danger of an insuflicient or of an excessive concentration of additives does not exist. Furthermore, the constant additive concentration of the electrolyte assures a completely uniform consumption, whereby not only an improvement in the quality of the electrodeposits is achieved, but also the economy of the process is improved. Of course, if the electrodeposits are deposited from an electrolyte which has always a uniform composition, their quality will remain completely constant, which is of considerable practical importance for the deposition of thick metal deposits such as those that are required in the production of rolled copper coatings and in the production of phonograph record matrices. The diflicultly soluble additives of the heterogeneous bath phase may be liquid or solid and in the latter case ordinarily form a bottom deposit. The solid agents may also be employed in electroplating baths which are continuously or periodically cycled with the aid of a pump. Under these circumstances, for example, they may be introduced by means of solution filters into the cycling pump line.

These electroplating additives are difiicultly soluble organic compounds which are characterized by known groupings that bring about brightening, leveling, grainimproving or porosity-preventing effects. Such groups are, for example, nitrogen and/or sulfur-containing groups, such as thiourea groups, dithiocarbamic acid groups, thiozanthogenic acid groups, trithiocarbonic acid groups, xanthogenic acid groups, thioimidazole or thiobenzimidazole groups, mercaptothiazole or mercaptobenzthiazole groups and other groups which contain a carbon atom attached only to hetero atoms, as well as azido groups, alkylene-diamine groups, polyamide groups and the like. These compounds mayalso contain watersolubilizing radicals, such as carboxyl groups and sulfonic acid groups, which form diflicultly soluble metal salts with the heavy metal ions of the baths. Furthermore, the additives may contain the activating groups several times and in combination with each other, such as diflicultly soluble compounds which contain simultaneously one or more thiourea groups and one or more dithiocarbamic, acid groups.

Therefore, quite generally, difficultly soluble compounds with known electroplating-active groups may be employed for the baths according to the present invention. In many instances, therefore, it is possible to start from known brightening, leveling and other such substances which have been made diflicultly soluble by introduction I of suitable substituents, such as aryl radicals.

Since the consumption of brightening agent in selfregulating electrolytes is compensated by dissolution from the solid phase, at least 0.5 bath volume per hour must' be pumped through the solution filter.

In the operation of such baths it has been shown that despite the insertion of an impurities trap filter, the solution filter may become clogged. Moreover, the permeability of the solution filter depends to a large extent upon the consistency and the grain size of the solid,

diflicultly soluble agent with electroplating activity and i also upon the degree to which the difiicultly soluble additive sinters together. The solution surface of the difiicultly soluble electroplating additive-stands in close relationship to this degree of sintering. If the filter cake sinters 'in the course of the operation, fissures and-channels are formed through which the electrolyte flows with Also, many solid, difiicultly soluble brightening agentscannot be placed into the solution filter as continuous replenishment substances because of their soap-like or gel-like consistency, since it is practically impossible to pump the electrolyte through such precipitates.

. '3 a It has now been found that substantial advantages are derived by not using the difficultly soluble additives in the form of a powder in the solution filters, but rather to introduce them into the bath in the form of a surface layer applied to a carrier substance. For this purpose the difficultly soluble organic com-pounds are admixed with solvents, swelling agents, thickening agents, and the like, to form a viscous-to-pasty mass which can be spread with a brush or applied by immersion or spraying, and this mass is then applied to the carrier by immersion, spreading or spraying. Examples of suitable carriers are spheres, Raschig rings, perforated discs, or the like, which may be made of glass, ceramic material, porcelain, filter paper or plastic resistant to chemicals. The carrier material may either be inserted into the previously employed solution filters, or also may be provided in any other manner, for example in the form of a column inserted into the recycling line or in the bath itself.

The process according to the present invention has the following substantial advantages over the location of the difi'icultly soluble electroplating additives in a solution filter: Clogging of the solution filter, such as may occur in connection with the powder agents is excluded. The

permeability of the solution filter always remains constant and the flow resistance is much lower in comparison to the filter cake. Furthermore, the difiicultly soluble substance retains a constant solution surface during the entire period of operation. An aged, inactive surface of the electroplating additive may, if necessary, be roughened up and thereby regenerated by simple treatment with a brush. The process is very economical with respect to the active substance consumption because the --difficultly soluble additive can be applied to large surfaces in thin layers and can be practically completely utilized.

In the preparation of the spreadable, pasty brightening agent constant replenishment compositions, two possibilities can be considered. If difficultly soluble organic compounds form paintlike products with organic solvents or swelling agents, they can be employed as such. A prerequisite for this is a good spreadability and a good adherability of such masses on the carrier substrate (see Example VI below). However, since in most cases these prerequisites are not present, the difficultly soluble brightening agent replenishment substances must be compounded with solvents and with a thickening agent which is insoluble in the electrolyte to form a pasty mass. This mass must not only have good spreadability and good adhering properties with respect to the carrier surface, but it must also furnish films of sufficient porosity in order to enable the difiicultly soluble brightening agent to go into solution. Thus, according to the present process, difficultly soluble brightening agent constant replenishment substances are incorporated into porous, spreadable films.

Essentially, all difficultly soluble organic additives with an electroplating effect, and especially the agent described in my copend'ing application Serial No. 214,490, and set forth above, may be used for the preparation of the coating composition. Suitable solvents or swelling agents are primarily readily volatizable compounds such as methanol, ethanol, propanol, isopropanol, acetone, diethyl ether, methylacetate, ethylacetate, butylacetate, xylene, pyridine, methylene chloride, carbon tetrachloride, alone or in combination with each other, and in certain instances also water, while products such as methyl cellulose, carboxymethylcellulose, polyvinylacetate, polyvinylpropionate, and the like, are useful as thickening agents which have a certain capability to swell in the water.

Normally, -30% solvent or swelling agent and l- 10% of thickening agent may be present in the brightening agent constant replenishment paste. Depending on the type of bath, about 10-100 mgm. of difficultly soluble brightening agent are applied per cm. of solution surface. About 4-20 cm. of solution surface per liter of bath volume are required. The ratio of solution surface to bath volume is very strongly dependent upon the bath load (amp./liter) and upon the recycling rate of the electrolyte through the solution filter.

It should further be mentioned that the present process also makes it possible to use diflicultly filterable gel-like brightening agent constant replenishment substances.

The process according to the present invention may be applied in conjunction with acid electroplating baths for copper, zinc, nickel, lead, tin and cadmium. The baths may be modified with customary wetting agents, such as the known ethylene oxide addition products to high molecular organic compounds with exchangeable hydrogen atoms, alkyl sulfates and the like. These wetting agents are soluble, but they are effective over a broad range of concentrations and therefore do not require close surveillance. If the difficultly soluble additive is a brightening agent, whose leveling effect" is. insufficient, the leveling effect may be improved by"simultaneously employing a leveling additive which is either in the form of a difficultly soluble compound or also in soluble form. Of course, in the last-mentioned instance, the content of leveling agent in the, bath requires surveillance. The' agent must from time to time be replenished in accordance with its consum tion. Accordingly, particularly advantageous are di cultly soluble additives which perform several functions, that is, which simultaneously produce full brightness, a leveling effect and also possess a temperature tolerance which is sufficient for practical requirements. However, there are an entire series of technical electroplating processes in which a full brightening effect is not required and merely an increase in the fineness of the grain meets the technical requirements.

In those cases where the bath additive consists solely of the difiicultly soluble additives according to the present invention and possibly also wetting agents, these baths can be operated completely without surveillance provided care is taken that the circulating pump filtration operates satisfactorily and the additives in the solution filter are replenished when necessary. Thus, it is not only possible to operate the baths in a considerably simpler manner but also in a more economical manner. By virtue of the self-regulation of the baths the production of imperfect products which must be rejected is practically completely eliminated.

The following specific examples are presented to illustrate the invention and to enablepersons skilled in the art to better understand and practice the invention and are not intended to be limitative.

Example I 10 gm. of N,N" dibenzylthiocarbaminyl diethylenetriamine-N'-dithiocarbonyl-S-propane-w-sodium sulfonate which acts as a difficultly soluble brightening agent, were admixed with 40 gm. of water and 1.8 gm. of carboxymethyl cellulose and the mixture was worked into a paste. Perforated plastic discs were coated by means of a brush with this pasty mass, about 20 mgm. of the above indicated brightening agent being applied per cm. of solution surface. About 6 cm. of solution surface per liter of bath volume were required. After drying of the coating thus applied, the discs were separated from each other by plastic rings, and this ensemble was inserted into a solution filter located downstream from the contaminntion trap filter in the recycling line of a self-regulating bright copper electroplating bath. The electroplating bath had the following basic composition: 210 gm./l. of copper sulfate; 120 gm./l. of sulfonic acid; and 3 gm./l. of the addition product of 8 mols ethylene oxide to 1 mol of a coconut fatty alcohol mixtureC -C At a recycling rate of 1-4 bath volumes per hour the bath produced bright, smooth copper electroplates free from buds and pores in a current density range of 0.5- 8 amp./dm. at a temperature tolerance of 35 C. The consumption of brightening agent per ampere-hour was 0.8 mgm. From this value and from the total amount of substance applied to the carrier, it can readily be determined when the plastic discs are spent and must be replaced by freshly coated discs; the utilization of the constant replenishment brightening agent can be assumed to be virtually complete.

If the constant replenishment brightening agent is used in powder form instead of in the form of a coating applied to plastic discs, at least 0.6 gm. of the above powder must be used per liter of bath volume. After a short period of time the filter cake becomes impermeable to the circulation due to swelling of the powder.

Example If 0.5 gm. of methyl cellulose, 40 gm. of methanol and 40 gm. of water was used for coating the perforated plastic discs. After installation of the dry discs in the solution filter of the previously described copper electroplating bath, this bath furnished bright smooth pore-free copper deposits free from buds at a recycling rate of 1 bath volume per hour in a current density range of 0.5-8 amp./ drn. and at a temperature tolerance of l7-30 C. Since the consumption of brightening agent of about 1.5 mgm. per ampere-hour and the amount of brightening agent applied to the carrier are known, it is easy to calculate when the plastic discs are spent and must be replaced by fresh ones.

Example III QCHr-N are worked into a paste of 0.4 gm. of carboxymethyl cellulose in gm. of methanol and 15 gm. of water, and the paste was applied with a brush to perforated plastic discs. After drying of the coating, the discs, separated by plastic rings, were inserted into the solution filter mounted downstream from the contamination trap filter of a bright copper plating bath which had the following basic composition:

210 gm./l. copper sulfate CUSO -5H O 60 gm./l. sulfuric acid and i 3 gm./l. of the addition product of 8 mols ethylene oxide to 1 mol of a coconut fatty alcohol mixture C -C At a recycling rate of one bath volume per hour this bath produced uniformly bright, satisfactorily leveled copper electroplates in a current density range of 0.5-8 ampJdm. at a temperature tolerance of 17-.30 C.

Knowing the consumption of brightening agent mixture of about 1.2 mgm. per ampere-hour, it is easy to calcnlate when the carrier discs must be renewed.

Example IV 1 gm. of blood albumin and 0.05 gm. of carboxymethyl cellulose were stirred into a slurry with 5 gm. of concentrated ammonia solution, the paste was applied onto perforated plastic discs as carrier surfaces and the coating was dried at -80 C. After installation of the discs into the solution filter of an acid copper electroplating bath having the basic composition 210 gm./l. of copper sulfate and 60 gm./l. of sulfuric acid, this bath produced, even without wetting agents or other additives, at a recycling rate of one bath volume per hour, virtually blank,

smooth, pore-free and bud-free, very fine-grained light a copper electrodeposits in a current density range of 1-8 amp/dm. at a temperature tolerance, of 17-.35" C. Knowing the consumption of 2 mgm./ampere-hour, it is easy to determine when the carrier discs must again be renewed.

Example V Perforated plastic discs coated with N,N"-dibenzylthiocarbaminyl-diethylenetriamine-N'-dithiocarbonyl S propane-arsodium sulfonate as a brightening agent were inserted into the solution filter of an acid zinc electroplating bath with the basic composition:

240 gm./l. of zinc sulfate 20 gm./l. of boric acid 10 gm./l. of aluminum sulfate 5 gm./l. of sodium chloride and 1 gm./l. of the sodium salt of the sulfate of an addition product of 4 mols ethylene oxide to 1 mol octyl-alcohol.

This bath produced at a recycling rate of one bath volume per hour, blank, pore-free, soft zinc electrodedeposits in a current density range of 1-8 amp./dm. at a temperature tolerance of 17-30 C. and a pH value of the bath of 1.5-3.0. Knowing the consumption of brightening agent of about 1.2 mgm./ampere-hour, the time for replenishing the carrier discs may readily be calculated.

Example VI bright copper electroplating bath having the same basic composition as that in Example III. At a recycling rate of one bath volume per hour, this bath produced smooth, bright budand pore-free copper electrodeposi-ts in a current density range of 2-8 amp/rim. at a temperature tolerance of 17-35 C. Knowing the brightening agent consumption of about 2.0 mgm./ampere-hour, the required amount of brightening agent for a desired period of time may be readily calculated.

Example VII 10 gm. of the copper salt of the condensation product of 1 mol dipropylenetriamine, 2.6 mols epichlorohydrin,

3 mols copper disulfide, 1.5 mols propanesultone, and 1.5 mols benzylchloride were compounded with 30 gm. of an aqueous 3% carboxymethyl cellulose solution into a paste, and this paste was applied to customary filter paper discs. After drying the discs, they were inserted into a customary discs filtering device whose plastic discs were additionally provided with bore holes. The solution filter was inserted into a continuous recycling line of a bath having the same basic composition as in Example III. At a recycling rate of 2-3 bath volumes per 2 hour and a bath load of 0.5-1.5 amp./l. very smooth, bright adequately level, budand pore-free copper electroplates were furnished by this bath in a current density range of 2-8 amp./dm. at a temperature tolerance of 25-30 C. This bath was eminently useful for the production of Ballard films. Since the brightening agent consumption is about 3 mgmJampere-hour, the required amount of brightening agent for a predetermined amount of deposited copper can be calculated.

Example VIII For coating of filter paper discs a paste of 10 gm. of N-phenylthiocarbaminylpiperazine-N-dithiocarbonyl S propane-w-copper sulfonate, 0.8 gm. of polyvinylacetate and 50 gm. ethanol was used. The solution filter and the basic composition of the copper bath were analogous to that of Example VII. At a recycling rate of 2-3 bath volumes per hour and a bath load of 1 amp./l., copper electrodeposits with good brightening and leveling effects were obtained at current densities of l-8 amp./dm. at temperatures of 17-30 C. The brightening agent consumption was about 0.6 mgm. per ampere-hour. Since the above-mentioned brightening agent was very difficulty soluble, a solution surface of about 15-20 cmF/l. of bath volume must be provided.

Example IX 10 gm. of calcium caseinate were stirred with 20 gm. of the condensation product of 1 mol diethylenetriamine, 1.5 mols epichlorohydrin, 3 mols carbondisulfide, 2 mols propane-sultone and 1 mol benzylchloride and gm. of

acetone into a lacquer-like spreadable paste and the paste was applied in this form to filter paper discs. After drying, the discs were inserted into a disc filtering device (as described in Example VII) which had been inserted into the recycling line of a tin electroplating bath having the following composition:

100 gm./l. tin-II-sulfate, 30 gm./l. concentrated sulfuric acid and 30 gm./l. boric acid.

At a recycling rate of 1-2 bath volumes per hour, this bath furnished smooth, semi-bright, pore frce tin electroplates in a current density range of l-4 amp./dm. at temperatures of 20-25 C. The consumption of brightening agent paste was about 5 mgm./ampere-hour.

While certain specific examples and preferred modes of practice of the invention have been set forth it will be understood that this is solely for the purpose of illustration and that various changes and modifications may be made without departing from the spirit of the disclosure and the scope of the appended claims.

I claim:

1. A self-regulating electroplating bath for the electrodeposition of metals capable of being electrodeposited which comprises an acid solution of an inorganic salt of the metal to be electrodeposited and ditficultly soluble auxiliary electroplating-active organic addition agents which are diflicult to dissolve, whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to about half of said saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, whereby the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath.

2. A self-regulating electroplating bath for the electrodeposition of metals capable of being electrodeposited which comprises an acid solution of an inorganic salt of the metal to be electrodeposited and diflicultly soluble auxiliary electroplating-active organic addition agent, which are difficult to dissolve, whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to half of said saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, so that the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath, whereby the concentration of said bath is continuously replenished and said bath may be operated for a prolonged period of time.

3. A self-regulating electroplating bath for the electrodeposition of metals capable of being electrodeposited which comprises an acid solution of an inorganic salt of the metal to be electrodeposited and ditficnltly soluble auxiliary electroplating-active organic addition agents which are difiicult to dissolve, whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to half of this saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, whereby the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath, wherein said bath contains in addition readily soluble electroplating addition agents.

4. A method for self-regulation of the concentration tolerance range of electroplating-active, organic addition agents in an acid electroplating bath which comprises adding difiicultly soluble auxiliary electroplating-active organic addition agents, which are difiicult to dimolve, whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to half of this saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, so that the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath, whereby said excess undissolved addition agent continuously dissolves in said bath as previously dissolved addition agent is consumed, thereby maintaining said bath liquid within the concentration tolerance range.

5. A method for self-regulation of the concentration tolerance range of electroplating-active, organic addition agents in an acid electroplating bath which comprises adding difiicultly soluble auxiliary electroplating-active organic addition agents, which are difficult to dissolve, whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to half of this saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, so that the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath, whereby said excess undissolved addition agent continuously dissolves in said bath as previously dissolved addition agent is consumed, thereby maintaining said bath liquid within the concentration tolerance range for a prolonged period of time.

6. A method for self-regulation of the concentration tolerance range of electroplating-active, organic addition agents in an acid electroplating bath which comprises adding difficultly soluble auxiliary electroplating-active organic addition agents, which are difiicult to dissolve, whose saturation concentration amounts to 0.5 to 500 mg./1. of bath liquid and whose critical concentration amounts to half of this saturation concentration, said addition agents being coated on a solid carrier, said addition agents further being added to said bath in multiple quantities of said saturation concentration, so that the portion of the addition agent in excess of the saturation concentration always remains in undissolved form in the bath, whereby said excess undissolved addition agent continuously dissolves in said bath as previously dissolved addition agent is consumed, thereby maintaining said bath liquid within the concentration tolerance range for a prolonged period of time, wherein readily soluble electroplating addition agents are also added to said bath.

7. A self-regulating process for the electrodeposition of metals capable of being electrodeposited, in the presence of electroplating-active organic addition agents, which comprises, conducting said electrodeposition in a volume of an aqueous acid bath solution of an inorganic salt of the metal to be electrodeposited, continuously circulating said solution from the bath volume through a confined body of difficultly soluble electroplating-active organic addition agents coated on a solid carrier and back to said bath volume. said addition agent 'being one whose saturation concentration amounts to 0.5 to 500 mg./l. of bath liquid and whose critical concentration amounts to half of this saturation concentration, said addition agents being present in multiple quantities of said saturation coneentration whereby the portion of the addition agent in excess of the saturation concentration always remains in undissolved form, whereby the amount of said additive in said bath is maintained within the concentartion tolerance range.

References Cited by the Examiner UNITED STATES PATENTS 1,003,092 9/1911 1,371,414 3/ 1921 Edison 204-49 2,248,092 I 7/ 1941 Korpiun 204-49 2,449,422 9/1948 Smith 204-49- 3/1959 Kardos et al. 20449 Dow et al. 204-49

Claims

1. A SELF-REGULATING ELECTROPLATING BATH FOR THE ELECTRODEPOSITION OF METALS CAPABLE OF BEING ELECTRODEPOSITED WHICH COMPRISES AN ACID SOLUTION OF AN INORGANIC SALT OF THE METAL TO BE ELECTRODEPOSITED AND DIFFICULTLY SOLUBLE AUXILIARY ELECTROPLATING-ACTIVE ORGANIC ADDITION AGENTS WHICH ARE DIFFICULT TO DISSOLVE, WHOSE SATURATION CONCENTRATION AMOUNTS TO 0.5 TO 500 MG./L. OF BATH LIQUID AND WHOSE CRITICAL CONCENTRATION AMOUNTS TO ABOUT HALF OF SAID SATURATION CONCENTRATION, SAID ADDITION AGENTS BEING COATED ON A SOLID CARRIER, SAID ADDITION AGENTS FURTHER BEING ADDED TO SAID BATH IN MULTIPLE QUANTITIES OF SAID SATURATION CONCENTRATION, WHEREBY THE PORTION OF THE ADDITION AGENT IN EXCESS OF THE SATURATION CONCENTRATION ALWAYS REMAINS IN UNDISSOLVED FORM IN THE BATH.