The Purpose of pH Testing

[color="Navy"]This article was published in the July 2005 issue of Southern Metal Finishing. If you would like register to receive our free newsletter and review our online archives please visit www.southernmetalfinishing.com[/color]

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Shane Moore of Scovill Fasteners, Clarksville, GA

pH testing is a process of analyzing the acidity or alkalinity of a solution. Chemically, the test is an analysis of the concentration of hydrogen ions (H+), in comparison to the number of hydroxyl ions (OH-) per liter of solution. The general pH scale is from 0-14; 0 being the most acidic, 14 being the most alkaline, and 7 being neutral. The pH scale is logarithmic by a base of 10, therefore a solution with a pH of 4 is 10 times more acidic than a pH of 5. A solution with a pH of 4 is 100 times more acidic than a pH of 6 (10 X 10). A solution with a pH of 4 is 1000 times more acidic than a pH of 7 (10 X 10 X 10).

The number assigned to a pH concentration is actually a short-hand term. A solution with a pH of 6 means the solution has 1 X 10-6 gram equivalents per liter of H+ ions (0.000006). As stated earlier, the pH correlates the concentration of H+ ions to OH- ions. These are inversely proportional to one another, according to the pH scale. For example, a solution with a pH of 6 has 1 X 10-6 H+ ions, and 1 X 10-8 OH- ions. The table to the right may help with this correlation.

Since the concentration of hydrogen ions exceeds that of the hydroxyl ions, the solution is considered acidic. If the opposite were true, the solution would be considered alkaline.

Many plating solutions are sensitive to pH and must be routinely adjusted. During the electroplating process, water molecules in most solutions are broken into their primary elements (hydrogen and oxygen). These two elements are liberated at the site of the anode and cathode as side reactions – hydrogen at the cathode, and oxygen at the anode. Thus, a plating process that is more efficient at the anode than the cathode will evolve more hydrogen than oxygen, creating a decrease in the pH. This solution pH would need to be adjusted with a hydroxyl ionizing solution, such as a hydroxide. Conversely, a solution that is more cathodically efficient than anodically, will evolve more oxygen than hydrogen, creating an increase in pH. This solution pH would need to be adjusted with a hydrogen ionizing solution, such as an acid. Specific acids and bases are recommended for pH adjustment, based on the compatibility of the acid/base with the process solution. You can contact your local supplier for these recommendations.

Many plating solutions contain a pH buffer. The role of the buffer is to reduce the amount of sudden fluctuations in the pH. One example is the presence of boric acid in a watts nickel solution. This type of nickel plating solution is about 98% efficient, thus a larger amount of hydroxyl ions are liberated at the anode than hydrogen at the cathode. Boric acid can be considered a “bag” of hydrogen ions, in which the plating solution can draw from when the concentration of hydrogen ions begins to deplete during the plating process. The pH still needs periodic adjustment with sulfuric acid, but the boric acid helps to maintain the proper pH without constant adjustments having to be made.