Test for Carbonate Concentration in Caustic Solutions

Test for Carbonate Concentration in Caustic Solutions?

Letter 10083

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Does anybody know of a way to test carbonate concentration in caustic solutions?

Brian Lenkowski
- Baltimore, Maryland

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The standard method is to precipitate the carbonate as barium carbonate, then filter, dry, and weigh. But, assuming that your caustic solution is a hydroxide/carbonate mix (non-cyanide), a much simpler and quicker method is to pipette equal amounts into two separate flasks. To one flask add a stoichiometric excess of barium chloride which will precipitate a white powder of barium carbonate. Then titrate both with acid to a phenolphthalein endpoint. From the flask that has the precipitate, you can calculate the amount of hydroxide in the solution. From the flask without the precipitate, you have total alkalinity. Using a little math, you can calculate the carbonate concentration from the difference between the total alkalinity and the hydroxide concentration. I have found this method to be just as accurate as the time consuming gravimetric methods.

If your alkaline bath is a mixture of hydroxide, cyanide, and carbonate, then I recommend a different method that is still much quicker than the full gravimetric method. To your pipetted sample, add a stoichiometric excess of barium chloride to precipitate barium carbonate. Filter the precipitate through a fine filter paper. Wash the precipitate with DI water until the filtrate is a neutral pH (at this point all of the hydroxide and cyanide have been washed out of the precipitate). Transfer the filter paper (with all of the barium carbonate) to a clean flask. Add hot DI water to the flask and use a stirring rod to mash the filter paper to a pulp. Titrate (pulp and all) with acid to a phenolphthalein (or other) endpoint that persists for at least a minute with stirring. It is important to wait for the final endpoint as the color will keep changing back until all of the barium carbonate has dissolved. Then just do the math.

Jon Barrows
aerospace jobshop
Tulsa, Oklahoma

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Detailed procedures for analyzing the carbonate content of alkaline plating solutions can be found in the Metal Finishing Guidebook, published yearly by Metal Finishing Magazine. Also refer to the Electroplating Engineering Handbook, edited by Lawrence Durney.

Gene Packman
process supplier - Great Neck, New York

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