Principle: In strong acidic medium, sodium periodate oxidizes glycerol with three hydroxyl groups into formic acid and formaldehyde. Formic acid neutralized with NaOH, Indicating endpoint with pH meter. Calculating Glycerol Content from Consumption of NaOH Standard Solution
2HCHO+HCOOH+2NalO3+H2O→CH2OH–CHOH–CH2 OH+2NalO4
Reagent
Distilled water: no carbon dioxide.
Ethylene glycol dilution solution: 1 volume of glycerol-free ethylene glycol, neutralized with phenolphthalein as indicator, then diluted with 1 volume of water.
Sulfuric acid solution: about 0.1 mol/L.
Sodium formate solution is about 0.1 mol/L.
Preparation of sodium periodate acidic solution:Weigh 60g(accuracy to 0.1g) sodium periodate, and dissolve it in water of about 500 mL which has been added 120 mL acid solution. Cool it while adding it and transfer it to 1000 mL capacity bottle. Dilute it with water and shake it evenly. Filter with glass filter if necessary. Acidity check of solution: the volume of NaOH solution used in the blank test should be no less than 4.5mL, which is equivalent to the acidity produced by the basic reaction.
NaOH solution: about 0.05 mol/L
NaOH solution: about 0.125 mol/L。
Phenolphthalein indicator: dissolve 0.5g phenolphthalein in 95% (volume ratio) ethanol, dilute to 100 mL.
Instrument
Burette: 50ml.
PH Meter: Calibration of pH meter with Two Kinds of Buffer Solutions
a.Potassium hydrogen phthalate solution: 0.05mol/L (10.12g/L), pH 4.00 at 20℃
b.Disodium tetraborate 10 water (Na2B4O. 10H2O) solution: 0.01mol/L (3.81g/L), pH value is 9.22 at 20℃.
Determination steps
(1) Experimental composition: Samples with glycerol content less than 0.5g (accuracy to 0.0002g). If the approximate content of glycerol is unknown, the sample of 0.5g should be weighed for prediction. Put it in a 500 mL volumetric bottle and dilute it to the scale with water. Shake well and take 50 mL of this solution for determination.
(2) Preparation of test solution: When tar precipitation occurs during acidification of alkaline sample or sample, the test can be divided into flasks equipped with reflux condenser, diluted to 50 mL when needed, added 2 drops of phenolphthalein indicator, and neutralized in sulphuric acid solution to just fade. Add 5ml sulphuric acid solution, boil for 5 minutes, cool, filter if necessary, and wash the filter with water. The filtrate bitch is put into a 600 mL beaker. In the absence of the above, the sample can be directly put into a beaker for determination.
(3) Titration: Dilute the sample with water to 250mL. Stir continuously, add NaOH solution, adjust the pH value to 7.9 + 0.1, add 50mL sodium periodate solution, mix and stir well, cover the surface dish, and place it in the dark place where the temperature does not exceed 35℃ for 3 minutes. Then add 10 mL ethylene glycol dilution solution, mix, and place for 20 minutes under the same conditions. Add 5.0mL sodium formate solution and titrate with NaOH standard solution to pH 7.9 +0.2.
(4) Blank test: Under the same conditions, the same amount of reagent and diluted water, 50 mL water instead of sample, were used as blank test. However, before adding sodium periodate solution, the pH value of blank solution should be adjusted to 6.5. After adding sodium periodate solution, the titration end point to the pH value is also 6.5.
Calculation
Glycerol content (mass,%)=(V1-V2)×c×0.0921×100÷m=9.21c(V1-V2)/m
V1 in the formula – Determine the volume of NaOH standard solution consumed by the sample, mL
V2——The volume of NaOH standard solution used in blank test, mL;
M——Quality of test sample,g;
C——CThe concentration of NaOH standard solution, mol/L
0.0921——Millimolar mass, g/mmol of glycerol
The allowable error of two parallel experiments is 0.2.
