Bedlasky
International Hazard
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Registered: 15-4-2019
Location: Period 5, group 6
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Boric acid complexes with cis-1,2-diols
Boric acids forms 1:2 complexes with cis-1,2-diols. These complexes are colourless, so you cannot see them with eye. However you can observe
their formation by meassuring pH of boric acid solution before and after addition of diol. Another option is observing change in colour of methyl
orange indicator.
These complexes are much more acidic than boric acid itself. They dissociate 10 000-100 000 times more than H3BO3. This property is used in
alkalimetric determination of boric acid. Boric acid itself doesn't provide sharp endpoint, so some sugar alcohol (usually mannitol) is added before
titration to improve boric acid dissociation.
There are many easily available polyols with two OH groups next to each other which you can use for this experiment. Monosaccharides (glucose,
fructose, mannose), sugar alcohols (glycerol, xylitol, sorbitol, mannitol) or even simple 1,2-diols (ethylene glycol, propylene glycol).
https://colourchem.wordpress.com/2021/12/12/komplexy-kyselin...
https://colourchem.wordpress.com/2021/12/12/complexes-of-bor...
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Fery
International Hazard
Posts: 1000
Registered: 27-8-2019
Location: Czechoslovakia
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Bedlasky well done!!! I remember how weak is H3BO3... so adding diols change pKa substantially from more than 9 for pure acid to less than 5 with
diol, interesting.
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