After the expiration of four hours 20 c.c. of 10 per cent. solution of pota.s.sium iodide and 150 c.c. water are added to the contents of the bottle, and the excess of iodine t.i.trated with N/10 sodium thiosulphate solution, the whole being well agitated during the t.i.tration, which is finished with starch paste as indicator. The blank experiment is t.i.trated in the same manner, and from the amount of thiosulphate required in the blank experiment is deducted the number of c.c. required by the unabsorbed iodine in the other bottle; this figure multiplied by the iodine equivalent of 1 c.c. of the thiosulphate solution and by 100, dividing the product by the weight of fat taken, gives the "Iodine Number".
_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 Hubl. Wijs" iodine reagent is made by dissolving 13 grammes iodine in 1 litre of glacial acetic acid and pa.s.sing 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 Hubl 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_Bromine absorption_ has now been almost entirely superseded by the iodine absorption, although there are several good methods.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 (_a.n.a.lyst_, 1895, 49) was employed by one of us for many years with very good results, whilst the bromine-thermal test of Hehner and Mitch.e.l.l (_a.n.a.lyst_, 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 Abbe 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 _t.i.tre_ 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.
One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c.
of a 25 per cent. caustic soda solution added, together with 50 c.c. of redistilled methylated spirit. The whole is stirred down on the water bath until a pasty soap is obtained, when another 50 c.c. of methylated spirit is added, which redissolves the soap, and the whole again stirred down to a solid soap. This is then dissolved in distilled water, a slight excess of dilute sulphuric acid added to liberate the fatty acids, and the whole warmed until the fatty acids form a clear liquid on the surface. The water beneath the fatty acids is then syphoned off, more distilled water added to wash out any trace of mineral acid remaining, and again syphoned off, this process being repeated until the washings are no longer acid to litmus paper, when the fatty acids are poured on to a dry filter paper, which is inserted in a funnel resting on a beaker, and the latter placed on the water-bath, where it is left until the clear fatty acids have filtered through.
About 10-15 grammes of the pure fatty acids are now transferred to a test tube, 6" 1", warmed until molten, and the tube introduced through a hole in the cork into a flask or wide-mouthed bottle. A very accurate thermometer, graduated into fifths of a degree Centigrade (previously standardised), is immersed in the fatty acids, so that the bulb is as near the centre as possible, and when the fatty acids just begin to solidify at the bottom of the tube, the thermometer is stirred round slowly. The mercury will descend, and stirring is continued until it ceases to fall further, at which point the thermometer is very carefully observed. It will be found that the temperature will rise rapidly and finally remain stationary for a short time, after which it will again begin to drop until the temperature of the room is reached. The maximum point to which the temperature rises is known as the "t.i.tre" of the sample.
ALKALIES AND ALKALI SALTS.
Care should be bestowed upon the sampling of solid caustic soda or potash as the impurities during the solidification always acc.u.mulate in the centre of the drum, and an excess of that portion must be avoided or the sample will not be sufficiently representative. The sampling should be performed expeditiously to prevent carbonating, and portions placed in a stoppered bottle. The whole should be slightly broken in a mortar, and bright crystalline portions taken for a.n.a.lysis, using a stoppered weighing bottle.
_Caustic Soda and Caustic Potash._--These substances are valued according to the alkali present in the form of caustic (hydrate) and carbonate.
About 2 grammes of the sample are dissolved in 50 c.c. distilled water, and t.i.trated with N/1 sulphuric acid, using phenol-phthalein as indicator, the alkalinity so obtained representing all the caustic alkali and one-half the carbonate, which latter is converted into bicarbonate. One c.c. N/1 acid = 0.031 gramme Na_{2}O or 0.040 gramme NaOH and 0.047 gramme K_{2}O, or 0.056 gramme KOH.
After this first t.i.tration, the second half of the carbonate may be determined in one of two ways, either:--
(1) By adding from 3-5 c.c. of N/10 acid, and well boiling for five minutes to expel carbonic-acid gas, after which the excess of acid is t.i.trated with N/10 soda solution; or
(2) After adding two drops of methyl orange solution, N/10 acid is run in until the solution acquires a faint pink tint.
In the calculation of the caustic alkali, the number of c.c. of acid required in the second t.i.tration, divided by 10, is subtracted from that used in the first, and this difference multiplied by 0.031, or 0.047 gives the amount of Na_{2}O or K_{2}O respectively in the weight of sample taken, whence the percentage may be readily calculated.
The proportion of carbonate is calculated by multiplying the amount of N/10 acid required in the second t.i.tration by 2, and then by either 0.0031 or 0.0047 to give the amount of carbonate present, expressed as Na_{2}O or K_{2}O respectively.
An alternative method is to determine the alkalinity before and after the elimination of carbonate by chloride of barium.
About 7-8 grammes of the sample are dissolved in water, and made up to 100 c.c., and the total alkalinity determined by t.i.trating 20 c.c. with N/1 acid, using methyl orange as indicator. To another 20 c.c. is added barium chloride solution (10 per cent.) until it ceases to give a precipitate, the precipitate allowed to settle, and the clear supernatant liquid decanted off, the precipitate transferred to a filter paper and well washed, and the filtrate t.i.trated with N/1 acid, using phenol-phthalein as indicator. The second t.i.tration gives the amount of caustic alkali present, and the difference between the two the proportion of carbonate.
When methyl orange solution is used as indicator, t.i.trations must be carried out cold.
Reference has already been made (p. 39) to the manner in which the alkali percentage is expressed in English degrees in the case of caustic soda.
_Chlorides_ are estimated by t.i.trating the neutral solution with N/10 silver nitrate solution, pota.s.sium chromate being used as indicator. One c.c. N/10 AgNO_{3} solution = 0.00585 gramme sodium chloride.
The amount of acid necessary for exact neutralisation having already been ascertained, it is recommended to use the equivalent quant.i.ty of N/10 nitric acid to produce the neutral solution.
_Sulphides_ may be tested for, qualitatively, with lead acetate solution.
_Aluminates_ are determined gravimetrically in the usual manner; 2 grammes are dissolved in water, rendered acid with HCl, excess of ammonia added, and the gelatinous precipitate of aluminium hydrate collected on a filter paper, washed, burnt, and weighed.
_Carbonated Alkali (Soda Ash)._--The total or available alkali is, of course, the chief factor to be ascertained, and for this purpose it is convenient to weigh out 3.1 grammes of the sample, dissolve in 50 c.c.
water, and t.i.trate with N/1 sulphuric or hydrochloric acid, using methyl orange as indicator. Each c.c. of N/1 acid required represents 1 per cent. Na_{2}O in the sample under examination.
A more complete a.n.a.lysis of soda ash would comprise:--
_Insoluble matter_, remaining after 10 grammes are dissolved in warm water. This is washed on to a filter-paper, dried, ignited, and weighed.
The filtrate is made up to 200 c.c., and in it may be determined:--
_Caustic soda_, by t.i.trating with N/1 acid the filtrate resulting from the treatment of 20 c.c. (equal to 1 gramme) with barium chloride solution.
_Carbonate._--t.i.trate 20 c.c. with N/1 acid, and deduct the amount of acid required for the Caustic.
_Chlorides._--Twenty c.c. are exactly neutralised with nitric acid, t.i.trated with N/10 AgNO_{3} solution, using pota.s.sium chromate as indicator.
_Sulphates._--Twenty c.c. are acidulated with HCl, and the sulphates precipitated with barium chloride; the precipitate is collected on a filter paper, washed, dried, ignited, and weighed, the result being calculated to Na_{2}SO_{4}.
_Sulphides and Sulphites._--The presence of these compounds is denoted by the evolution of sulphuretted hydrogen and sulphurous acid respectively when the sample is acidulated. Sulphides may also be tested for, qualitatively, with lead acetate solution, or test-paper of sodium nitro-prusside.
The total quant.i.ty of these compounds may be ascertained by acidulating with acetic acid, and t.i.trating with N/10 iodine solution, using starch paste as indicator. One c.c. N/10 iodine solution = 0.0063 gramme Na_{2}SO_{3}.
The amount of sulphides may be estimated by t.i.trating the hot soda solution, to which ammonia has been added, with an ammoniacal silver nitrate solution, 1 c.c. of which corresponds to 0.005 gramme Na_{2}S.
As the t.i.tration proceeds, the precipitate is filtered off, and the addition of ammoniacal silver solution to the filtrate continued until a drop produces only a slight opacity. The presence of chloride, sulphate, hydrate, or carbonate does not interfere with the accuracy of this method. The ammoniacal silver nitrate solution is prepared by dissolving 13.345 grammes of pure silver in pure nitric acid, adding 250 c.c.
liquor ammoniae fortis, and diluting to 1 litre.
_Carbonate of Potash (Pearl Ash)._--The total or available alkali may be estimated by taking 6.9 grammes of the sample, and t.i.trating with N/1 acid directly, or adding 100 c.c. N/1 sulphuric acid, boiling for a few minutes, and t.i.trating the excess of acid with N/1 caustic soda solution, using litmus as indicator. In this case each c.c. N/1 acid required, is equivalent, in the absence of Na_{2}CO_{3}, to 1 per cent.
K_{2}CO_{3}.
Carbonate of potash may be further examined for the following:--
_Moisture._--From 2-3 grammes are heated for thirty minutes in a crucible over a gas flame, and weighed when cold, the loss in weight representing the moisture.
_Insoluble residue_, remaining after solution in water, filtering and well washing.