Understanding boric acid titrations
Q. Hi .... I'm trying to better understand/improve the boric acid determinations we perform on our baths. Currently we follow a procedure very similar to that specified in the Metal Finishing Guidebook. In essence: add mannitol to convert the boric to a titratable form and use bromocresol purple to indicate the endpoint. The problem I have is that the endpoint isn't as clear as I would like it to be. So I've looked in a few texts discussing plating bath analysis, "The Chemical Analysis of Electroplating Solutions" [linked by editor to info about book at Amazon], Irvine, 1970 and "Analysis of Electroplating and Related Solutions" [linked by editor to info about book at AbeBooks] and I've found that both references call for phenolphthalein as the indicator. They also call for the addition of sodium citrate to prevent formation of Ni(OH)2. Does anyone have experience with these two indicators and which would work better? I didn't try it because I don't currently have any sodium citrate. The bromocresol purple method doesn't call for sodium citrate I presume because the bromocresol purple changes color before Ni(OH)2 precipitation becomes a problem. If I've done my calculations correctly, Ni(OH)2 would start forming around pH 7.6 for a nickel bath with Ni conc at 10 oz/gal. At this pH bromocresol purple has already been converted from its acid form to its base form and phenolphthalein hasn't even begun to change.
Also if anyone is aware of other plating analysis texts I'd be interested in the references.
A. The endpoint is a very good one.
The manitol is added to a thin paste, just a little thinner than Elmers, because the dilute NaOH is going to add quite a bit of liquid.
Double the amount of BCP indicator that you are using. If that does not help, make up a new solution.
I normally used a glass stirring rod in a 25 ml beaker. A friend of mine used a slightly thinner solution and used a flea and a magnetic stirrer for the actual titration.
It is an excellent and very reproduceable analytical procedure within plus or minus 0.1 ml of titrant.
Once you see it done , you will agree that it is a very good procedure.
PS: the amount of boric acid in your nickel solution is very dependent on temperature of the sample. Hot solution holds a lot more than a room temp one. Normal concentrations will salt out at room temp and give you a low and very erratic reading.James Watts
- Navarre, Florida
A. Mr. Watts's P.S. is an excellent lead-in into a less orthodox way of determining where you stand with boric acid concentration. Since the actual concentration of boric acid in the solution is not as important as that it is above a certain limit (usually above its solubility at room temperature), consider this "test" I have been using for years:
Fill a small (1 cc) test tube with your solution and let it cool down to room temperature. The amount of boric acid crystals formed is a pretty good indication of whether you need to make an add.
Once you use this method for a while, you will agree that it is the only procedure you need for boric acid analysis :-)
Good luck from PlaterB
"PlaterB" Berl Stein
Rochester, New York
Sounds good, but does not work until some homework is done. It takes quite a bit of volume to cover the pH probe. The normal way is to use a thin paste, which does not give much quantity. It would be more of a pain than it is worth. The indicator method works very well, is quick, easy and generates a lot less waste.
I am not sure that a large dilution will give the same answer.James Watts
- Navarre, Florida
A. Potentiometric titration with pH electrode provides very accurate determination of end point. Most probably, you already have necessary equipment: pH-meter and glass electrode.Gene Chalyt
- E. Rutherford, New Jersey
A. I USED TO USE BROMOCRESOL PURPLE,UNTIL I FOUND A BETTER WAY. MY PROCEDURE TO CHECK BORIC CONTENT IN NICKEL BATHS IS 5 ML SAMPLE IN ERLENMEYER FLASK,ADD 5 DROPS OF BORIC ACID INDICATOR, SWIRL WHILE TITRATING WITH 1.N NaOH TILL BLUE RECORD THIS AS A. NEXT ADD A HEAPING TEASPOON OF MANITOL.(THIS WILL TURN BACK TO GREEN) SWIRL WHILE CONTINUING TITRATING WITH 1.N NaOH TILL SLURRY TURNS BLUE AGAIN.RECORD THIS NUMBER AS B. CALCULATION IS B-A X 1.65 GIVES YOU OZ/GAL BORIC.DO NOT REZERO BURET AFTER FIRST TITRATION. THIS HAS SEEMED TO BE VERY ACCURATE AND CONSISTENT FOR ME.JOHN ROLLINS
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