Letter 25111

Boric Acid in Nickel Sulfamate Solutions  

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Hello,

I am trying to determine Boric Acid in Nickel Sulfamate solutions. The method I am using calls for 2 mL of sample, 3-5 drops of bromothymol blue and bromocresol purple mixed indicator, and about 5 grams of mannitol until a thick paste is formed. The paste is titrated with 0.1 N sodium hydroxide until a purple endpoint. The problem that I am having is a lack of accuracy. My results are low for a given sample that was tested by another laboratory. My value was 32 g/L, and the other laboratory was 37 g/L. Any ideas on what may be happening?

Thank you,

Lyle Cline
- North Vancouver, BC, Canada

As a first shot, try increasing the sample size to 5-10 ml and reworking the calculation. Of course, you should also check the decinormal caustic standardization. Finally, did you get anything along the lines of QA/QC from the outside lab, and did you do any yourself (repetitions, spikes, etc.)

James Totter, CEF - Tallahassee, Florida

The first probability is that some of the boric acid has precipitated out of the solution that you are testing. The other guy may be taking it to the tank temperature, mixing well and titrating. The end point is not perceived exactly the same by any two chemists or technicians. You may be getting it exactly at a faint color change and the other shop is taking it to a very positive color change. Make up a small amount of plating solution as a standard with a know amount of reagent quality boric acid added. Mix well, sample and test. This will let you know who is the most correct as it is a standard.

James Watts
- Navarre, Florida

As they said above, take a larger sample, be sure to take it at operating temperature, then After putting in the Bromcresol Purple,and before adding the manitol neutralize to an aqua color. Then add manitol, re-zero the buret. That is to say, you must neutralize the free acid first (a pH 3.9 nickel solution will contain more free acid than a 4.5 pH nickel solution), then after neutralizing the free acid, you add manitol and re-zero the buret, then go to light blue , then calculate based on the sample size and factor thereof.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

The pretitrate was a given to me, so I totally overlooked it. Since his values are "low", there is a possibility that his outside lab is not pretitrating.

James Watts
- Navarre, Florida

The boric acid end point is sometimes very difficult to see. This problem has been discussed before on submission 15586 where there is quite an extensive discussion. You may get help from that page.

Good luck.

Trevor Crichton R&D practical scientist - UK

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