The Handbook of Soap Manufacture Part 20

Cocoa-nut oil 12 "

Palm-kernel oil 18 "

Olive oil 10 "

Palm oil 6 "

Greases (Bone fats) 6-8 "

The materials vary in glycerol content with the methods of preparation; especially is this the case with tallows and greases.

Every care should be taken that the raw materials are fresh and they should be carefully examined to ascertain if any decomposition has taken place in the glycerides--this would be denoted by the presence of an excess of free acidity, and the amount of glycerol obtainable from such a fat would be correspondingly reduced.

CHAPTER X.

a.n.a.lYSIS OF RAW MATERIALS, SOAP, AND GLYCERINE.

_Fats and Oils--Alkalies and Alkali Salts--Essential Oils--Soap--Lyes--Crude Glycerine._

_Raw Materials._--Average figures have already been given in Chapters III. and VIII. for the more important physical and chemical characteristics of fats and oils, also of essential oils; the following is an outline of the processes usually adopted in their determination.

For fuller details, text-books dealing exhaustively with the respective subjects should be consulted.

FATS AND OILS.

It is very undesirable that any of these materials should be allowed to enter the soap pan without an a.n.a.lysis having first been made, as the oil may not only have become partially hydrolysed, involving a loss of glycerine, or contain alb.u.minous matter rendering the soap liable to develop rancidity, but actual sophistication may have taken place. Thus a sample of tallow recently examined by the authors contained as much as 40 per cent. of an unsaponifiable wax, which would have led to disaster in the soap pan, had the bulk been used without examination. After observing the appearance, colour, and odour of the sample, noting any characteristic feature, the following physical and chemical data should be determined.

_Specific Gravity at 15 C._ This may be taken by means of a Westphal balance, or by using a picnometer of either the ordinary gravity bottle shape, with perforated stopper, or the Sprengel U-tube. The picnometer should be calibrated with distilled water at 15 C. The specific gravity of solid fats may be taken at an elevated temperature, preferably that of a boiling water bath.

_Free acidity_ is estimated by weighing out from 2 to 5 grammes of the fat or oil, dissolving in neutral alcohol (purified methylated spirit) with gentle heat, and t.i.trating with a standard aqueous or alcoholic solution of caustic soda or potash, using phenol-phthalein as indicator.

The contents of the flask are well shaken after each addition of alkali, and the reaction is complete when the slight excess of alkali causes a permanent pink coloration with the indicator. The standard alkali may be N/2, N/5, or N/10.

It is usual to calculate the result in terms of oleic acid (1 c.c. N/10 alkali = 0.0282 gramme oleic acid), and express in percentage on the fat or oil.

_Example._--1.8976 grammes were taken, and required 5.2 c.c. of N/10 KOH solution for neutralisation.

5.2 0.0282 100 ------------------ = 7.72 per cent. free fatty acids, 1.8976 expressed as oleic acid.

The free acidity is sometimes expressed as _acid value_, which is the amount of KOH in milligrammes necessary to neutralise the free acid in 1 gramme of fat or oil.

In the above example:--

5.2 5.61 ---------- = 15.3 acid value.

1.8976

The _saponification equivalent_ is determined by weighing 2-4 grammes of fat or oil into a wide-necked flask (about 250 c.c. capacity), adding 30 c.c. neutral alcohol, and warming under a reflux condenser on a steam or water-bath. When boiling, the flask is disconnected, 50 c.c. of an approximately semi-normal alcoholic potash solution carefully added from a burette, together with a few drops of phenol-phthalein solution, and the boiling under a reflux condenser continued, with frequent agitation, until saponification is complete (usually from 30-60 minutes) which is indicated by the absence of fatty globules. The excess of alkali is t.i.trated with N/1 hydrochloric or sulphuric acid.

The value of the approximately N/2 alkali solution is ascertained by taking 50 c.c. together with 30 c.c. neutral alcohol in a similar flask, boiling for the same length of time as the fat, and t.i.trating with N/1 hydrochloric or sulphuric acid. The "saponification equivalent" is the amount of fat or oil in grammes saponified by 1 equivalent or 56.1 grammes of caustic potash.

_Example._--1.8976 grammes fat required 18.95 c.c. N/1 acid to neutralise the unabsorbed alkali.

Fifty c.c. approximately N/2 alcoholic potash solution required 25.6 c.c. N/ acid..

25.6 - 18.95 = 6.65 c.c. N/1 KOH required by fat.

1.8976 1000 / 6.65 = 285.3 Saponification Equivalent.

The result of this test is often expressed as the "Saponification Value," which is the number of milligrammes of KOH required for the saponification of 1 gramme of fat. This may be found by dividing 56,100 by the saponification equivalent or by multiplying the number of c.c. of N/1 alkali absorbed, by 56.1 and dividing by the quant.i.ty of fat taken.

Thus, in the above example:--

6.65 56.1 / 1.8976 = 196.6 Saponification Value.

The _ester_ or _ether value_, or number of milligrammes of KOH required for the saponification of the neutral esters or glycerides in 1 gramme of fat, is represented by the difference between the saponification and acid values. In the example given, the ester value would be 196.6 - 15.3 = 181.3.

_Unsaponifiable Matter._--The usual method adopted is to saponify about 5 grammes of the fat or oil with 50 c.c. of approximately N/2 alcoholic potash solution by boiling under a reflux condenser with frequent agitation for about 1 hour. The solution is then evaporated to dryness in a porcelain basin over a steam or water-bath, and the resultant soap dissolved in about 200 c.c. hot water. When sufficiently cool, the soap solution is transferred to a separating funnel, 50 c.c. of ether added, the whole well shaken, and allowed to rest. The ethereal layer is removed to another separator, more ether being added to the aqueous soap solution, and again separated. The two ethereal extracts are then washed with water to deprive them of any soap, separated, transferred to a flask, and the ether distilled off upon a water-bath. The residue, dried in the oven at 100 C. until constant, is the "unsaponifiable matter,"

which is calculated to per cent. on the oil.

In this method, it is very frequently most difficult to obtain a distinct separation of ether and aqueous soap solution--an intermediate layer of emulsion remaining even after prolonged standing, and various expedients have been recommended to overcome this, such as addition of alcohol (when petroleum ether is used), glycerine, more ether, water, or caustic potash solution, or by rotatory agitation.

A better plan is to proceed as in the method above described as far as dissolving the resulting soap in 200 c.c. water, and then boil for twenty or thirty minutes. Slightly cool and acidify with dilute sulphuric acid (1 to 3), boil until the fatty acids are clear, wash with hot water free from mineral acid, and dry by filtering through a hot water funnel.

Two grammes of the fatty acids are now dissolved in neutral alcohol saturated with some solvent, preferably a light fraction of benzoline, a quant.i.ty of the solvent added to take up the unsaponifiable matter, and the whole boiled under a reflux condenser. After cooling, the liquid is t.i.trated with N/2 aqueous KOH solution, using phenol-phthalein as indicator, this figure giving the amount of the total fatty acids present. The whole is then poured into a separating funnel, when separation immediately takes place. The alcoholic layer is withdrawn, the benzoline washed with warm water (about 32 C.) followed by neutral alcohol (previously saturated with the solvent), and transferred to a tared flask, which is attached to a condenser, and the benzoline distilled off. The last traces of solvent remaining in the flask are removed by gently warming in the water-oven, and the flask cooled and weighed, thus giving the amount of unsaponifiable matter.

_Const.i.tution of the Unsaponifiable Matter._--Unsaponifiable matter may consist of cholesterol, phytosterol, solid alcohols (cetyl and ceryl alcohols), or hydrocarbons (mineral oil). Cholesterol is frequently found in animal fats, and phytosterol is a very similar substance present in vegetable fats. Solid alcohols occur naturally in sperm oil, but hydrocarbons, which may be generally recognised by the fluorescence or bloom they give to the oil, are not natural const.i.tuents of animal or vegetable oils and fats.

The presence of cholesterol and phytosterol may be detected by dissolving a small portion of the unsaponifiable matter in acetic anhydride, and adding a drop of the solution to one drop of 50 per cent.

sulphuric acid on a spot plate, when a characteristic blood red to violet coloration is produced. It has been proposed to differentiate between cholesterol and phytosterol by their melting points, but it is more reliable to compare the crystalline forms, the former crystallising in laminae, while the latter forms groups of needle-shaped tufts. Another method is to convert the substance into acetate, and take its melting point, cholesterol acetate melting at 114.3-114.8 C., and phytosterol acetate at 125.6-137 C.

Additional tests for cholesterol have been recently proposed by Lifschutz (_Ber. Deut. Chem. Ges._, 1908, 252-255), and Golodetz (_Chem.

Zeit._, 1908, 160). In that due to the former, which depends on the oxidation of cholesterol to oxycholesterol ester and oxycholesterol, a few milligrammes of the substance are dissolved in 2-3 c.c. glacial acetic acid, a little benzoyl peroxide added, and the solution boiled, after which four drops of strong sulphuric acid are added, when a violet-blue or green colour is produced, if cholesterol is present, the violet colour being due to oxycholesterol ester, the green to oxycholesterol. Two tests are suggested by Golodetz (1) the addition of one or two drops of a reagent consisting of five parts of concentrated sulphuric acid and three parts of formaldehyde solution, which colours cholesterol a blackish-brown, and (2) the addition of one drop of 30 per cent. formaldehyde solution to a solution of the substance in trichloracetic acid, when with cholesterol an intense blue coloration is produced.

_Water._--From 5 to 20 grammes of the fat or oil are weighed into a tared porcelain or platinum dish, and stirred with a thermometer, whilst being heated over a gas flame at 100 C. until bubbling or cracking has ceased, and reweighed, the loss in weight representing the water. In cases of spurting a little added alcohol will carry the water off quietly.

To prevent loss by spurting, Davis (_J. Amer. Chem. Soc._, 23, 487) has suggested that the fat or oil should be added to a previously dried and tared coil of filter paper contained in a stoppered weighing bottle, which is then placed in the oven and dried at 100 C. until constant in weight. Of course, this method is not applicable to oils or fats liable to oxidation on heating.

_Dregs, Dirt, Adipose Tissue, Fibre, etc._--From 10 to 15 grammes of the fat are dissolved in petroleum ether with frequent stirring, and pa.s.sed through a tared filter paper. The residue retained by the filter paper is washed with petroleum ether until free from fat, dried in the water-oven at 100 C. and weighed.

If the amount of residue is large, it may be ignited, and the proportion and nature of the ash determined.

The amount of impurities may also be estimated by Tate's method, which is performed by weighing 5 grammes of fat into a separating funnel, dissolving in ether, and allowing the whole to stand to enable the water to deposit. After six hours' rest the water is withdrawn, the tube of the separator carefully dried, and the ethereal solution filtered through a dried tared filter paper into a tared flask. Well wash the filter with ether, and carefully dry at 100 C. The ether in the flask is recovered, and the flask dried until all ether is expelled, and its weight is constant. The amount of fat in the flask gives the quant.i.ty of actual fat in the sample taken; the loss represents the water and other impurities, and these latter may be obtained from the increase of weight of the filter paper.

_Starch_ may be detected by the blue coloration it gives with iodine solution, and confirmed by microscopical examination, or it may be converted into glucose by inversion, and the glucose estimated by means of Fehling's solution.

_Iodine Absorption._--This determination shows the amount of iodine absorbed by a fat or oil, and was devised by Hubl, the reagents required being as follows:--

(1) Solution of 25 grammes iodine in 500 c.c. absolute alcohol; (2) solution of 30 grammes mercuric chloride in 500 c.c. absolute alcohol, these two solutions being mixed together and allowed to stand at least twelve hours before use; (3) a freshly prepared 10 per cent. aqueous solution of pota.s.sium iodide; and (4) a N/10 solution of sodium thiosulphate, standardised just prior to use by t.i.trating a weighed quant.i.ty of resublimed iodine dissolved in pota.s.sium iodide solution.

In the actual determination, 0.2 to 0.5 gramme of fat or fatty acids is carefully weighed into a well-fitting stoppered 250 c.c. bottle, dissolved in 10 c.c. chloroform, and 25 c.c. of the Hubl reagent added, the stopper being then moistened with pota.s.sium iodide solution and placed firmly in the bottle, which is allowed to stand at rest in a dark place for four hours. A blank experiment is also performed, using the same quant.i.ties of chloroform and Hubl reagent, and allowing to stand for the same length of time.