(2) Determine the total residue at 160° C.
(3) Determine the acetin value of the residue at (2) in terms of glycerol.
(4) Deduct the result found at (3) from the percentage obtained at (1) and report this corrected figure as glycerol. If volatile acetylizable impurities are present these are included in this figure.
Trimethyleneglycol is more volatile than glycerine and can therefore be concentrated by fractional distillation. An approximation to the quantity can be obtained from the spread between the acetin and bichromate results on such distillates. The spread multiplied by 1.736 will give the glycol.
BICHROMATE PROCESS FOR GLYCEROL DETERMINATION. REAGENTS REQUIRED.
(_A_) _Pure potassium bichromate_ powdered and dried in air free from dust or organic vapors, at 110° to 120° C. This is taken as the standard.
(_B_) _Dilute Bichromate Solution._--7.4564 grams of the above bichromate are dissolved in distilled water and the solution made up to one liter at 15.5° C.
(_C_) _Ferrous Ammonium Sulphate._--It is never safe to assume this salt to be constant in composition and it must be standardized against the bichromate as follows: dissolve 3.7282 grams of bichromate (_A_) in 50 cc. of water. Add 50 cc. of 50 per cent. sulphuric acid (by volume), and to the cold undiluted solution add from a weighing bottle a moderate excess of the ferrous ammonium sulphate, and titrate back with the dilute bichromate (_B_). Calculate the value of the ferrous salt in terms of bichromate.
(_D_) _Silver Carbonate._--This is prepared as required for each test from 140 cc. of 0.5 per cent. silver sulphate solution by precipitation, with about 4.9 cc. _N_/1 sodium carbonate solution (a little less than the calculated quantity of _N_/1 sodium carbonate should be used as an excess to prevent rapid settling). Settle, decant and wash one by decantation.
(_E_) _Subacetate of Lead._--Boil a 10 per cent. solution of pure lead acetate with an excess of litharge for one hour, keeping the volume constant, and filter while hot. Disregard any precipitate which subsequently forms. Preserve out of contact with carbon dioxide.
(_F_) _Potassium Ferricyanide._--A very dilute, freshly prepared solution containing about 0.1 per cent.