Electroplating requires strict chemical control of the plating baths. Concentrations of chemicals that are too high or too low will cause undesired plating results such as surface blistering, poor adhesion, burning, and discoloration of the parts to be plated. This page contains explanations of some of the various testing methods we use to keep these levels in check.


Metal Ions in Solution:

A good method of determining the amount of metal ions in solution is to titrate with EDTA (ethylenediaminetetraacetic acid), using an organic dye as an indicator. The idicator is usually complexometric, meaning that it binds to the metal ions in solution, causing a color change. The EDTA, however, binds more strongly to the metal ions, and will displace the indicator, reverting it back to its non-complexing color. At the point of the color change, the EDTA has bound up all of the metal ions. This is the endpoint of the reaction, and can be used to determine the concentration of metal in the tank solution. Be mindful of the possible presence of other metal ions in solution. If other metals are allowed to interfere, a false high reading could be measured. In order to cut down on this effect, a dilute solution of triethanolamine can be added. In situations where the cadmium content is being titrated, triethanolamine can be used to mask the presence of other metals, such as zinc. The following link explains more in-depth the chemistry of EDTA titrations:Complexometric Titrations

Left: Eriochrome Black "T" complexed with metal ions.  Right: EDTA has displaced the dye, reverting the eriochrome to its non-complexed color
Left: Eriochrome Black "T" complexed with metal ions. Right: EDTA has displaced the dye, reverting the eriochrome to its non-complexed color




Chromate and Dichromate Concentrations:

Checking the level of chromates in a plating bath usually requires the use of Redox Titrations. In most cases, this will occur through a reaction with Sodium Dithionite (Na2S2O4), Sodium Thiosulfate (Na2S2O3), or Sodium Hydrogen Sulfite (NaHSO3).

The reaction for the thiosulfate ion with the chromate in the plating solution goes as follows:

reduction reaction:
8H+ + (CrO4)2- +3e- Cr3+ + 4H2O

oxidation reaction:
6(S2O3)2- 3(S4O6)2- + 6e-

combined reaction:
16H + 2(CrO4)2- + 6(S2O3)2- 2Cr3+ + 3(S4O6)2- + 8H2O


Finding an endpoint for this reaction can be accomplished using an iodine indicator. Dissolving I2 in a potassium iodine solution works well for this purpose. The iodine and thiosulfate ions react through redox as well, producing the following equations:

reduction reaction:
I2 + 2e- 2I-

oxidation reaction:2(S2O3)2- (S4O6)2- + 2e-

combined reaction:

I2+ 2(S2O3)2- 2I- + (S4O6)2-

The Iodine will form an intense blue compound in the solution. This will persist until all of the Iodine has been reduced. The endpoint of the reaction involves a sudden color change from deep blue to clear. One warning about the starch indicator: it should be made fresh at least once a month. Otherwise, mold will begin to grow in the solution, and the endpoint of the reaction will be very sluggish!



Acid/Base Concentrations:

Some of the simplest and most important analyses a plating lab performs are acid/base titrations. Many times, metal parts will undergo cleaning in an alkaline degreaser solution, then allowed to soak in an acid solution to develop a good plating surface on the part. Thus, control of the concentrations of the cleaners and the acids is critical to keep the plating process going smoothly. The lab work is simplifed, however, by the knowledge that most of the acid/base reactions encountered through titration will be easy to work with. For example, if a tank containing nitric acid and the balance water is to be analyzed:

Titrating with standard normal base (for example, 1 Molar NaOH):

H+ + NO3- + Na+ + OH- H2O + Na+ + NO3- (spectator ions included)
H+ + OH- H2O (only active species included)

And that's it! You have a reagent of known volume and concentration neutralizing a reagent of known volume and unknown concentration. Simple reaction stoichiometry will allow you to solve for the unknown quantity. Note: The reaction presented above is for strong, monoprotic acids. Weaker acids and acids which donate more than one proton during neutralization can become more complex to deal with. Be sure to check the individual acid's dissociation constant (or constants, if polyprotic). Keep in mind the rules about strong acids and strong bases here. For a quick refresher on the subject, visit Purdue's Chemistry Education website. For a list and explanation of strong acids and bases, try the ChemTeam's website.

One final note on the subject of acid/base titrations: be sure to choose an appropriate indicator. Different combinations of acids and bases (weak acid vs. strong base, strong acid vs. strong base, etc.) will reach an equivalence point at different pH values. The indicator chosen must have a distinct and rapid color change at (or very close to) the pH of the equivalence point. Because many acid/base titrations find an equivalence point around pH 7, the indicator phenolphthalein is a popular one to use. For a good explanation and list of common indicators, check out the wikipedia article on indicators.




A quick note on Boric Acid:

Though used in nickel solutions to buffer pH and provide electrolytic action, control of the concentrations can be tricky at times. One suggested route for checking the boric acid level is to titrate against .1N hydroxide, using a mixture of phenolphthalein dissolved in warm ethanol, and sodium citrate dissolved in glycerin. The citrate prevents the formation of insoluble nickel hydroxide compounds, while the phenolphthalein indicates the endpoint of the reaction. The color of the solution is usually a green-blue color at the start. This turns to a weak blue, then voilet/pink at the endpoint. The following video demonstrates this type of determination:



Cleaners and Degreasers:

One of the most important things to remember about plating: Good adherence is only possible from good cleaning processes. If a part has any oil or dust on it, the metal plating will blister off, resulting in a very poor quality product! Thus, it's important to keep the cleaning solutions at the correct concentrations. Most cleaners on the market are alkaline in nature, and the usual determination involves titration with standard, normal acid. The degreaser and cleaner formulae change from product to product, so the information on the product should be given by the manufacturer. This will explain assay procedures for the individual cleaner. The following is an example of a titration of a cleaner with 1N sulfuric acid:



Free Cyanide Concentration:

Cyanide (CN-) is used in several plating baths such as cadmium, copper and silver. Free cyanide, or cyanide dissociated in solution aids in anode corrosion, helps to maintain a constant metal ion level and to a lesser extent, increases the conductivity of the solution thus giving it enhanced "throwing power" at optimal concentrations. In less ideal concentrations of cyanide the metal coating will be uneven, at low concentrations, or burned, at high concentrations. Although the concentration of free cyanide in a plating bath is rather important for the plating process, the analytical test method is actually quite easy. To test for free cyanide, Silver Nitrate is used as the titrant with a potassium iodide indicator. When titrating the silver metal will complex with the cyanide consuming it from solution. When no free cyanide remains in solution the silver metal added to the solution will instead complex with the potassium iodide producing a turbid result. This is the end point of the reaction, and can be used to determine the concentration of free cyanide in the tank solution.

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