3C3H5(OH)3 + 7K2Cr2O7 + 28H2SO4 = 9CO2 + 40H2O + 7K2SO4 + 7Cr2(SO4)3.
Now the strong potassium bichromate solution above mentioned is of such a strength that 1 c.c. will oxidise 0.01 gramme glycerine, and 20 c.c. of the ferrous ammonium sulphate solution should require about 10 c.c. of the one-tenth strength bichromate in the blank experiment. If it requires more or less than this, then the amount of ferrous ammonium sulphate solution which would require exactly 10 c.c. (corresponding to 0.01 gramme glycerine) is calculated, and the oxidised glycerine solution run into this until oxidation is complete.
The formula for the calculation of the percentage of glycerol then becomes:—
| Per cent. of glycerol | = | (0.25 -((250 × 0.01)/n)) | × | 500, |
where n equals the number of c.c. of oxidised glycerine solution required to oxidise the ferrous ammonium sulphate solution.
Example:—
In the blank experiment 20 c.c. ferrous ammonium sulphate solution required 9.8 c.c. one-tenth strength bichromate solution, so that 20.4 c.c. ferrous solution would equal 10 c.c. bichromate.
20.4 c.c. ferrous solution required 27.8 c.c. of oxidised glycerine solution before it ceased to give a blue coloration with potassium ferricyanide.
| Therefore, per cent. of glycerol | = (0.25 -((250 × 0.01)/27.8)) | × | 500, |
| = 80.04 per cent. |
Other methods have been suggested for the preliminary purification, e.g., silver oxide, silver carbonate and lead subacetate, and copper sulphate and caustic potash, but the lead subacetate alone with care gives satisfactory results.