Evaluating Acid Concentration by Titration

Q. We have installed a vibratory clean/deburr system that uses stainless media and sulfuric acid to clean copper and brass brazed parts. When performing a titration to evaluate the acid concentration we get a coalescing of black material in the solution as the end point is approached. This makes it difficult to observe a pink color shift. It also seems to slow the reaction. Does anyone have an idea what this substance may be? Any suggestions?. D.B.

A. The question and comments above were posted on the Cleaning and Pretreatment Forum on PRODUCTS FINISHING ONLINE.

I think I have an idea what the problem is with this titration method. I am assuming that you are using phenolphthalein as the titration indicator for this concentration check since you mention that you are trying to observe a pink color shift. Phenolphthalein is an acid-base indicator. What that means is that it changes color with pH, so it can be used in titrations to signify when a solution has reached a certain pH. The phenolphthalein indicator changes from colorless to pink in the pH range of 8.2 to 10.0.

The solution described above is acidic (contains sulfuric acid) and is used to clean copper and brass parts following brazing. What will happen in this process is some amount of dissolved copper and zinc will be contained in the deburring cleaner/chemical after reacting with the base metals. As you increase the pH of this solution during the titration, the solubility of the copper and the zinc go way down. What you are seeing is very likely the precipitation of copper as copper hydroxide as the pH approaches the range of the indicator. The copper will have a very high solubility in the acid cleaner (likely in the hundreds or thousands as ppm or mg/L copper), but as the pH rises, that solubility drops way off and reaches about 0.05 ppm (mg/L) at a pH about 8. The zinc also exhibits this behavior, but not until a higher pH.

The best solution to this problem would be to ask your supplier if they can recommend a new indicator for your sulfuric acid cleaner. The indicator should be chosen so it ideally falls in the “inflection point” of the titration curve (a point with a rapid pH change with relatively small corresponding additions of titrant). This makes the titration more sensitive. My experience has been to use methyl orange as an indicator for sulfuric acid. It changes from red to yellow in the pH range of 3.2-4.4 and would avoid the precipitation problem you describe.

Although this seems to be a fairly low pH, the sulfuric acid is a very strong acid and even at low concentrations will still have a very low pH. Also, even though sulfuric acid is a diprotic acid (two hydrogens), it releases its second hydrogen at a low enough pH that the choice of the methyl orange will still capture the second hydrogen in the
titration.

I am suggesting you contact your chemical supplier to ensure that they go along with this method and check to verify that there are not other chemicals in the cleaner that would interfere with this choice of indicator. They should also supply you with a new titration factor for the method.

If you want to try this on you own first, though, mix up a known concentration of the cleaner as precisely as possible. Sample the same volume as you normally would and titrate following the same procedure, this time using the methyl orange as an indicator (the methyl orange would be available from any regent chemical sales company). Record the volume until you see the endpoint change. Then divide the known concentration you made up by the milliliters used to get to the endpoint. That will be you new titration factor.

Hope this helps.