Example.—1 c.c. of the N/10 sodium thiosulphate solution is found equal to 0.0126 gramme iodine.
0.3187 gramme of fat taken. Blank requires 48.5 c.c. thiosulphate.
Bottle containing oil requires 40.0 c.c. thiosulphate.
48.5 - 40.0 = 8.5, and the iodine absorption of the fat is—
| 8.5 × 0.0126 × 100 | ||
| ————————— | = | 33.6. |
| 0.3187 |
Wijs showed that by the employment of a solution of iodine monochloride in glacial acetic acid reliable iodine figures are obtained in a much shorter time, thirty minutes being sufficient, and this method is now in much more general use than the Hübl. Wijs' iodine reagent is made by dissolving 13 grammes iodine in 1 litre of glacial acetic acid and passing chlorine into the solution until the iodine is all converted into iodine monochloride. The process is carried out in exactly the same way as with the Hübl solution except that the fat is preferably dissolved in carbon tetrachloride instead of in chloroform.
Bromine absorption has now been almost entirely superseded by the iodine absorption, although there are several good methods. The gravimetric method of Hehner (Analyst, 1895, 49) was employed by one of us for many years with very good results, whilst the bromine-thermal test of Hehner and Mitchell (Analyst, 1895, 146) gives rapid and satisfactory results. More recently MacIlhiney (Jour. Amer. Chem. Soc., 1899, 1084-1089) drew attention to bromine absorption methods and tried to rewaken interest in them.
The Refractive index is sometimes useful for discriminating between various oils and fats, and, in conjunction with other physical and chemical data, affords another means of detecting adulteration.
Where a great number of samples have to be tested expeditiously, the Abbé refractometer or the Zeiss butyro-refractometer may be recommended on account of the ease with which they are manipulated. The most usual temperature of observations is 60° C.
The Titre or setting point of the fatty acids was devised by Dalican, and is generally accepted in the commercial valuation of solid fats as a gauge of firmness, and in the case of tallow has a considerable bearing on the market value.