The Handbook of Soap Manufacture Part 24

_Glycerine._--Five grammes of soap are dissolved in water, decomposed with dilute sulphuric acid, and the clear fatty acids filtered and washed. The filtrate is neutralised with barium carbonate, evaporated to 50 c.c., and the glycerol estimated by the bichromate method detailed under Crude Glycerine.

_Starch_ or _gum_ may be detected by dissolving the soap in alcohol, filtering, and examining the residue on the filter paper. Starch is readily recognised by the blue coloration it gives with a solution of iodine in pota.s.sium iodide.

_Sugars_ are tested for by means of Fehlings' solution, in the liquor separated from the fatty acids, after first boiling with dilute acid to invert any cane sugar.

_Mercury_ will be revealed by a black precipitate produced when sulphuretted hydrogen is added to the liquor separated from the fatty acids, and may be estimated by filtering off this precipitate on a tared Gooch's crucible, which is then dried and weighed.

_Borax or borates_ are tested for in the residue insoluble in alcohol.

This is dissolved in water, rendered faintly acid with dilute hydrochloric acid, and a strip of turmeric paper immersed for a few minutes in the liquid. This is then dried in the water-oven, when if any boric acid compound is present, a bright reddish-pink stain is produced on the paper, which is turned blue on moistening with dilute alkali.

The amount of the boric acid radicle may be determined by incinerating 5-10 grammes of soap, extracting with hot dilute acid, filtering, neutralising this solution to methyl orange, and boiling to expel carbon dioxide. After cooling, sufficient pure neutralised glycerine is added to form one-third of the total volume, and the liquid t.i.trated with N/2 caustic soda solution, using phenol-phthalein as indicator. Each c.c. of N/2 NaOH solution corresponds to 0.031 gramme crystallised boric acid, H_{3}BO_{3} or 0.0477 gramme crystallised borax, Na_{2}B_{4}O_{7}10H_{2}O.

LYES.

The amounts of caustic alkali (if any), carbonated alkali, and salt present are determined in the manner already described under Alkali and Alkali Salts. The glycerol content is ascertained by taking 2.5 grammes, adding lead subacetate solution, and filtering without increasing the bulk more than is absolutely necessary; the solution is concentrated to about 25 c.c., and the oxidation with bichromate and sulphuric acid conducted as described in the examination of Crude Glycerine. The solution, after oxidation, is made up to 250 c.c., and t.i.trated against standard ferrous ammonium sulphate solution, the formula for the calculation being:--

{0.25 - 2.5} Per cent. of glycerol = { ---} 40 { n }

where n equals the number of c.c. of oxidised lyes required to oxidise the ferrous ammonium sulphate solution.

The estimation of actual glycerol in this is necessarily a matter of considerable importance, and a very large number of processes, which are constantly being added to, have been suggested for the purpose.

Hitherto, however, only two methods have been generally adopted, _viz._ the acetin and the bichromate processes. Unfortunately the results obtained by these do not invariably agree, the latter, which includes all oxidisable matter as glycerol, giving sometimes considerably higher results, and it has been suggested that a determination should be made by both methods, and the average of the two results considered the true value. This involves a considerable amount of time and trouble, and it will generally be found sufficient in a works laboratory to determine the glycerol by one method only in the ordinary course, reserving the other process for use as a check in case of dispute or doubt.

_Acetin Method._--This consists in converting the glycerol into its ester with acetic acid, the acetic triglyceride, or triacetin being formed. This is then saponified with a known volume of standard alkali, the excess of which is t.i.trated with acid, and the percentage of glycerol calculated from the amount of alkali absorbed.

From 1 to 1.5 grammes of the glycerine is weighed into a conical flask of about 150 c.c. capacity, 7 or 8 c.c. of acetic anhydride added, together with about 3 grammes of anhydrous sodium acetate, and the whole boiled on a sand-bath under a reflux condenser for one to one and a half hours, after which it is allowed to cool, 50 c.c. water added, and the ester dissolved by shaking, and gently warming, the reflux condenser still being attached as the acetin is very volatile. The solution is then filtered from a white flocculent precipitate, which contains most of the impurities, into a larger conical flask, of some 500-600 c.c.

capacity, and after cooling, rendered just neutral to phenol-phthalein by means of N/2 caustic soda solution, the exact point being reached when the solution acquires a reddish-yellow tint; 25 c.c. of a strong caustic soda solution is then added, and the liquid boiled for about fifteen minutes, the excess of alkali being t.i.trated after cooling, with N/1 or N/2 hydrochloric acid. A blank experiment is carried out simultaneously, with another 25 c.c. of the soda solution, and the difference in the amounts of acid required by the two, furnishes a measure of the alkali required to saponify the acetin formed, and hence the amount of glycerol in the crude glycerine may be calculated.

_Example._--1.4367 grammes crude glycerine, after treatment with acetic anhydride, and neutralising, was saponified with 25 c.c. of a 10 per cent. caustic soda solution.

The blank experiment required 111.05 c.c. N/1 hydrochloric acid.

Flask containing acetin " 75.3 c.c. " "

----- 35.75 c.c. " "

Hence, the acetin formed from the glycerol present in 1.4367 grammes of the crude glycerine required 35.75 c.c. N/1 caustic alkali for its saponification, so that the percentage of glycerol may be calculated from the following formula:--

35.75 0.03067 100 Per cent. glycerol = --------------------- = 76.3.

1.4367

_Bichromate Method._--This process was originally devised by Hehner (_Journ. Soc. Chem. Ind._, 1889, 4-9), but the modification suggested by Richardson and Jaffe (_ibid._, 1898, 330) is preferred by the authors, and has been practised by them for several years with perfectly satisfactory results.

Twenty-five grammes of the crude glycerine are weighed out in a beaker, washed into a 250 c.c. stoppered flask, and made up to the graduation mark with water. Twenty-five c.c. of this solution are then measured from a burette into a small beaker, a slight excess of basic lead acetate solution added to precipitate organic matter, the precipitate allowed to settle, and the supernatant liquid poured through a filter paper into another 250 c.c. flask. The precipitate is washed by decantation until the flask is nearly full, then transferred to the filter, and allowed to drain, a few drops of dilute sulphuric acid being added to precipitate the slight excess of basic lead acetate solution, and the contents of the flask made up with water to 250 c.c. This solution is filtered, 20 c.c. measured from a burette into a conical flask of about 150 c.c. capacity, 25 c.c. of a standard pota.s.sium bichromate solution containing 74.86 grammes bichromate per litre added, together with 50 c.c. of 50 per cent. sulphuric acid, and the whole placed in a boiling water-bath for one hour, after which it is allowed to cool, diluted with water to 250 c.c., and this solution run in to 20 c.c. of a 3 per cent. ferrous ammonium sulphate solution until the latter is completely oxidised, as shown by no blue coloration being produced when one drop is brought into contact with one drop of a freshly prepared solution of pota.s.sium ferricyanide on a spot-plate. The ferrous ammonium sulphate solution is previously standardised by t.i.tration with a pota.s.sium bichromate solution of one-tenth the above strength, made by diluting 10 c.c. of the strong solution to 100 c.c.

with water.

The reaction taking place in the oxidation may be represented by the equation:--

3C_{3}H_{5}(OH)_{3} + 7K_{2}Cr_{2}O_{7} + 28H_{2}SO_{4} = 9CO_{2} + 40H_{2}O + 7K_{2}SO_{4} + 7Cr_{2}(SO_{4})_{3}.

Now the strong pota.s.sium bichromate solution above mentioned is of such a strength that 1 c.c. will oxidise 0.01 gramme glycerine, and 20 c.c.

of the ferrous ammonium sulphate solution should require about 10 c.c.

of the one-tenth strength bichromate in the blank experiment. If it requires more or less than this, then the amount of ferrous ammonium sulphate solution which would require exactly 10 c.c. (corresponding to 0.01 gramme glycerine) is calculated, and the oxidised glycerine solution run into this until oxidation is complete.

The formula for the calculation of the percentage of glycerol then becomes:--

{0.25 -(250 0.01)} Per cent. of glycerol = { ---------- } 500, { n }

where n equals the number of c.c. of oxidised glycerine solution required to oxidise the ferrous ammonium sulphate solution.

Example:--

In the blank experiment 20 c.c. ferrous ammonium sulphate solution required 9.8 c.c. one-tenth strength bichromate solution, so that 20.4 c.c. ferrous solution would equal 10 c.c. bichromate.

20.4 c.c. ferrous solution required 27.8 c.c. of oxidised glycerine solution before it ceased to give a blue coloration with pota.s.sium ferricyanide.

{0.25 - (250 0.01)} Therefore, per cent. of glycerol = { ------------} 500 { 27.8 }

= 80.04 per cent.

Other methods have been suggested for the preliminary purification, _e.g._, silver oxide, silver carbonate and lead subacetate, and copper sulphate and caustic potash, but the lead subacetate alone with care gives satisfactory results.

Other determinations include those of specific gravity, alkalinity, proportion of salts and chloride, and tests for metals, a.r.s.enic, sulphur compounds, sugar, and fatty acids.

_Specific gravity_ is determined at 15 C., and may be taken in specific gravity bottle, or with a Westphal balance or hydrometer It usually ranges from 1.3 to 1.31.

_Alkalinity_, which is usually sodium carbonate, and may be somewhat considerable if the soap has been grained with caustic alkali, is determined after dilution with water by t.i.trating with N/2 acid, using methyl orange as indicator.

_Salts._--These may be determined by gently incinerating 5-6 grammes of the glycerine, extracting the carbonaceous ma.s.s with distilled water, filtering, and evaporating the filtrate on the water bath. The dried residue represents the salts in the weight taken.

_Chloride of sodium_ (common salt) may be estimated by dissolving the total salts in water, adding pota.s.sium chromate, and t.i.trating with N/10 silver nitrate solution.

_Copper_, _lead_, _iron_, _magnesium_, and _calcium_ may also be tested for in the salts, by ordinary reactions.

_a.r.s.enic_ is best tested for by the Gutzeit method. About 5 c.c. is placed in a test-tube, a few fragments of granulated zinc free from a.r.s.enic, and 10 c.c. dilute hydrochloric acid added, and the mouth of the tube covered with a small filter paper, moistened three successive times with an alcoholic solution of mercury bichloride and dried. After thirty minutes the filter paper is examined, when a yellow stain will be observed if a.r.s.enic is present.

_Sulphates._--These may be precipitated with barium chloride in acid solution, in the usual way, dried, ignited, and weighed.

_Sulphites_ give with barium chloride a precipitate soluble in hydrochloric acid. If the precipitate is well washed with hot water, and a few drops of iodine solution together with starch paste added, the presence of sulphites is proved by the gradual disappearance of the blue starch-iodine compound first formed.

_Thiosulphates_ are detected by precipitating any sulphite and sulphate with barium chloride, filtering, acidifying, and adding a few drops of pota.s.sium permanganate solution, when in the presence of a mere trace of thiosulphate, the solution becomes cloudy.

_Sulphides._--Lewkowitsch recommends testing for these by replacing the mercury bichloride with lead acetate paper in the Gutzeit a.r.s.enic test.

Any sulphide causes a blackening of the lead acetate paper.

_Sugars_ may be tested for both before and after inversion, by boiling with Fehlings' solution, when no reduction should take place, if pure.

_Fatty acids_ are detected by the turbidity they produce when the diluted glycerine is acidified.