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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Gotmatt on February 18, 2020, 11:52:51 PM

Title: Sugar content via refractometer
Post by: Gotmatt on February 18, 2020, 11:52:51 PM
I have what I believe is a high-school level chemistry question, although it’s not likely something you’d do in high school chemistry...

I’m brewing kombucha and want to  measure its sugar content each day while it ferments. As I understand it, the byproducts of kombucha fermentation (small amounts of alcohol and acetic acid) will render the readings of a refractometer (measuring Brix (g sugar per 1L)) inaccurate.

My first thought was to boil off the solution, leaving behind sugar crystals. At that point I figured I could reconstitute the simple sugar/water solution with fresh water. I quickly learned how to make caramel. By lowering the heat, I was able to reduce the solution to a thick syrup, but the heat required to boil off the remaining liquid would eventually burn the sugar.

So my question: what is a reasonable method of removing alcohol and acetic acid from an otherwise sugar/water solution? Considering acetic acid’s boiling point (118.1°C) is higher than water’s, it seems unlikely I could boil if off before reaching the thick syrup stage.
Title: Re: Sugar content via refractometer
Post by: Borek on February 19, 2020, 02:58:04 AM
Separation will be always tricky. I wonder if polarimetry is not a better way to go.

If your recipe calls for ending the fermentation at some given Brix value using refractometer can be still possible, you will just need to calibrate it by some other means. Amount of alcohol produced is reasonably stoichiometric, so the the angle of refraction will still consistently change with the sugar concentration (just not necessarily following standard tables).
Title: Re: Sugar content via refractometer
Post by: Enthalpy on February 19, 2020, 01:52:18 PM
Rotation of light polarisation maybe? Water, ethanol and acetic acid don't let light rotate while natural sucrose does.

Viscosity would be simpler, but ethanol and acetic acid differ from water and may influence the reading, impurities too.