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is kept for some time in a solution of salt. Theproportion of water is also sometimes increased bysubmitting the bars of soap to the action of high-pressure steam in tight chests or bins; also byboiling the soap in high-pressure coppers beforecleansing.
The proportion of water contained in a sample ofsoap may be readily ascertained by treating a por-tion in a water or oil bath, until it ceases to lose inweight: 2 ozs. or more of the sample to be examinedshould be taken and reduced to small thin shavings.Of these, after carefully mixing together, 100 grainsare taken, placed in a small porcelain or platinumcrucible, and dried by exposure in a water bath fora sufficient length of time. When the weight re-mains constant, the operation is at an end. Thedifference between the first and last weighing indi-cates the percentage of water present.
To ascertain the amount of fatty matter, 100grains of the soap are dissolved in 4 or 5 ozs. ofboiling water, and the solution is filtered. A suf-ficient quantity of dilute acid is added to effectneutralization, and afterwards a weighed portion ofpure and dry white wax. The whole is then heateduntil the wax is liquefied, and has combined with ortaken up the oil or fat liberated from the soap bythe stronger acids. After allowing it to cool thecake of wax obtained is removed and weighed; theincrease above the original weight of the wax indi-cates the proportion of fat, oil, or grease containedin the soap.
If the fatty matter of the soap when liberated bythe acid itself concretes on cooling, the addition ofwax is unnecessary, as the fat may be weighed per se.
The liquid decanted from the fat, or solidifiedwax, may afterwards be submitted to examination,to ascertain the nature and state of purity of thealkali. If this is intended, hydrochloric acid shouldbe used for the liberation of the fat. To ascertainthe nature of the fatty material, Dumas directs theaqueous solution of the soap to be saturated withtartaric acid, the fatty acids to be collected, andtheir point of fusion carefully observed. It is amatter of great difficulty to determine the particularkind of fat or oil used in the production of the soapunder examination; yet by the above method, it ispossible at least to prove the identity of the fattycomponent; as, for instance, whether the soap ismade from tallow or from oil, or whether it is ofthe same kind as the sample from which it may havebeen purchased. The odour evolved by the fattyacids at the moment of decomposition of the soapby a stronger acid, or that developed when the fattymatter so liberated is treated in a small porcelaindish, will often indicate the nature of the fatty sub-stance employed in its production, or that at leastwhich is present in the greatest proportion.
According to Normandy , another test as to thequality of the soap is, that the fatty or oily acidshould be readily and completely dissolved by hotalcohol.
The estimation of the proportion of alkali con-tained in a sample of soap is frequently a point ofVOL. II.
great difficulty. When only one alkali is present,the determination is most easily effected by alkali-metry.
It not unfrequently happens that a soap containsmore or less uncombined or non-saponified fattymatters; and in this case, when the soap is dissolvedin water a film of oily matter is seen to float on thesurface. The aqueous solution is. decomposed bymeans of hydrochloric acid, when the fatty acids aswell as the unaltered fatty matter collect togetheron the surface of the liquid. These are separatedby filtration, thoroughly washed, and then treatedwith caustic baryta. The precipitate produced isthrown on a. filter, washed with boiling water, andthen acted upon by hot alcohol. By this meansthe non-saponified grease dissolved is from the soapof baryta; and after separation of the alcohol byevaporation, its weight may be ascertained. Whentested by litmus paper, it should not show an acidreaction.
The following is a resume of the method of analysisgiven by Richardson and Watts :—
The soap is dried over sulphuric acid, or in awater bath at 100° C., and is then dissolved inalcohol (100 grains require 3 ozs. of alcohol), andheated to boiling in a water bath. The soap, rosin,and free fat enter into solution, leaving the mineralconstituents, glue, starch, dextrin, &c., undissolved.The liquid is filtered, and the residue washed withalcohol.
The alcohol is eliminated from the filtrate bythe heat of a water bath. Addition of water thensets free any rosin or uncombined fat; these arecollected on a filter, dried, and weighed.
The filtrate now contains only the fat soap androsin soap, if any, and must be treated with a gradu-ated solution of sulphuric acid, to determine theamount of potash or soda in combination with thefat acids. At the same time, the soap solution isdecomposed; the fat and rosin acids rise to thesurface, are collected on a weighed filter, washedwith hot water, dried in vacuo, and weighed. Theweight expresses the joint amount of fat and rosinacids in the soap. Cold alcohol will dissolve out allthe fat acid, together with a small proportion of therosin from the filter; and the filter, dried in vacuoand weighed as before, gives approximately theamount of rosin in the soap.
To determine whether the base of the soap issoda or potash, the solution of the sulphates filteredfrom the fat acids is concentrated, and treated withtartaric acid or dichloride of platinum in the usualway.
The filter containing the matter insoluble in alco-hol is dried and weighed, after being thoroughlywashed with alcohol. In genuine soap this insolublematter is of very small amount, not exceeding 1 percent, for mottled, and being even less for yellow soap.
Cailletet has proposed a method of analysis inwhich only one weighing is required. In soap 10grams, in thin shavings is decomposed by a measuredvolume of sulphuric acid of known strength, in pres-ence of a measured volume of oil of turpentine.
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