Why is a sugar solution pre-treated with ammonium hydroxide before analyzing? And why is it specifically ammonium hydroxide instead of sodium hydroxide or calcium hydroxide?
2 Answers
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Do you see a potential corresponding to the sugars, or to their Schiff bases, or to said imines' trimers? Perhaps ammonia gives you a nice pH buffer, as opposed to alkali hydroxides that will wildly swing pH and allow Cannizzaro disproportionation of reducing sugars' aldehyde groups at high pH. Ammonia does not reduce at the cathode.
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If I'm not wrong, this is about polarimetry, not polarography. (No dripping mercury, but optical rotation).
If your sugar is a hexose, the following species will exist in aqueous solution:
- open form (a bit)
- furanose (traces)
- pyranoses (major amount)
The pyranoses will adopt the $\alpha$- and the $\beta$-form. Both rotate polarized light by a different angle.
The interconversion between these forms (mutarotation) is facilitated by addition of acid or base. Thus, an equilibrium, resulting in a constant angle in polarimetry, is reached faster.
answered Mar 28, 2014 at 16:42