Boric acid purity

[MEC]

I had a request to determine the purity of high purity boric acid powder. I
googled the topic and came up with two procedures that our lab can do
without purchasing new equipment.

One procedure uses a coulometric titration of 0.2 to 1.0 gram samples,
dissolved in 1M KCl and 0.75M mannitol. Endpoint is achieved at maximum
inflection of the potentiametric curve. The rest of the details are left
outj.

The second procedure is done by titration and colour change, using 0.1N
NaOH, D-Mannitol, Bromocresol Purple 0.04% aq.

What procedure would be best to determine purity in the 99%+ range?

Any thoughts would be greatly appreciated.

[Marvin]

Post by MEC
I had a request to determine the purity of high purity boric acid powder. I
googled the topic and came up with two procedures that our lab can do
without purchasing new equipment.
One procedure uses a coulometric titration of 0.2 to 1.0 gram samples,
dissolved in 1M KCl and 0.75M mannitol. Endpoint is achieved at maximum
inflection of the potentiametric curve. The rest of the details are left
outj.
The second procedure is done by titration and colour change, using 0.1N
NaOH, D-Mannitol, Bromocresol Purple 0.04% aq.
What procedure would be best to determine purity in the 99%+ range?
Any thoughts would be greatly appreciated.

I don't have experience with this assay, but, as a
generality, i think the potentiometric end point will be
more reproducible than the visual detection of a color change.

[Bob M]

Titration with standard NaOH in the presence of mannitol or glycerol
can be done very accurately indicator detection end point by indicator
or pH shoul give comparable results. A 25 ml titration should give
reproducable results to easily one part in 500 in other words plus or
minus 0.2 % on your sample. A good chemist should be able to get 1
part per thousand without much trouble. This is assuming that you are
familiar with the half drop technique. A bigger titration say 40 mls
from burrette look at 1 part per two thousand or 0.05% as a
possibility.

Bob M

www.molab.co.nz