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Titration of a Sulfiuric and Phosphoric bath

 

I am doing a titration for sulfuric acid and phosphoric acid in a etching bath. The bath only contains sulfuric phosphoric and water. I get two inflections points but do not know which one is which, or how to quantate them. Could someone please help.

Thanks,

Thomas Rohrer
- Columbus, Ohio


 

Hi Thomas,

I came up with a great way of analyzing that same solution using volumetric titration. Unfortunately, my lab notes are in my office and I am at home, so I will tell you how to accomplish the results using a known standard. First prepare a one liter solution of your sulfuric/phosphoric bath. It should be an "ideal" solution; a miniature copy of the one you are using on the process floor. Using a volumetric flask will give you the most accurate standard. Next, analyze that standard using the following procedure:

1. Pipette [pipettes on eBay or Amazon [affil links] a 5 mL allocate of the known and blended standard into a 250 mL Erlenmeyer flask.
2. Add 50-75 mL DI or distilled water.
3. Add 5-7 drops bromocresol purple ⇦this on eBay or Amazon [affil links] Indicator Solution.
4. Slowly, titrate the sample against 1.0 Normal Sodium Hydroxide Solution, to achieve the first endpoint. Color change will be clear or milky white to blueish purple. DO NOT TITRATE RAPIDLY, OR YOU WILL OVERSHOOT YOUR ENDPOINT.
5. Record volume of titrant consumed as T1.
6. Add 10 drops of phenolphthalein ⇦this on eBay & Amazon [affil links] Indicator Solution to the flask.
7. Continue titration, dropwise, until you achieve a color change to pink-red. Record the volume of titer consumed for this endpoint as T2.
8. Calculate for the individual acids, using the following equations:
H2SO4 Factor = mL T1 / %H2SO4 in Standard
H3PO4 Factor = (mL T2 - mL T1) / %H3PO4 in Standard

I'll get out my lab notes tomorrow and find my original study, then enter it in this thread tomorrow evening. This will get you going for now.

Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA

Actually sulfuric has two inflection points and phosphoric has three (i.e., H2SO4 & H3PO4). Unfortunately they partially overlap. If memory serves me, you can take the inflection at ~pH4 as a combination of the sulfuric's second proton and the phosphoric's first proton. The inflection at ~ pH6 (or is it pH8?) is due to the phosphoric's second proton. A minor inflection at ~pH12 is due to the phosphoric's third proton. Do your fundamental calculations based on: NaoH consumption at pH4 inflection equivalent to conc sulfuric & phosphoric, capacity at pH6 (pH8?) inflection equivalent to conc phosphoric only. Randy's titration is indicating these points with bromocresol purple ⇦this on eBay or Amazon [affil links] and phenolphthalein ⇦this on eBay & Amazon [affil links] indicators). It's a good opportunity to bone up on fundamental mass balance calculations. RANDY, HOW AM I DOING HERE?

John Tuohy
- Ireland