Soap-Making Manual Part 17

It may be further stated that methyl orange at the neutral point is orange in color.

To calculate the percentage of effective alkali from the above t.i.trations, it must be first pointed out that in the case of caustic potash or soda aliquot portions are taken. This is done to reduce the error necessarily involved by weighing, as the absorption of water is decided. Thus we had, say, exactly 5 grams which weighed 5.05 grams by the time it was balanced. This was dissolved in 500 cubic centimeters of water and 50 cubic centimeters or one tenth of the amount of the solution was taken, or in each 50 cubic centimeters there were 0.505 grams of the sample. We thus reduced the error of weighing by one tenth provided other conditions introduce no error. In the case of the carbonates the weight is taken directly.

One cubic centimeter of a normal acid solution is the equivalent of:

Grams.

Sodium Carbonate, Na_{2}CO_{3} 0.05305 Sodium Hydroxide, NaOH 0.04006 Sodium Oxide, Na_{2}O 0.02905 Carbonate K_{2}CO_{3} 0.06908 Pota.s.sium Hydroxide, KOH 0.05616 Pota.s.sium Oxide, K_{2}O 0.04715

Hence to arrive at the alkalinity we multiply the number of cubic centimeters, read on the burette, by the factor opposite the terms in which we desire to express the alkalinity, divide the weight in grams thus obtained by the original weight taken, and multiply the result by 100, which gives the percentage of alkali in the proper terms. For example, say, we took the 0.505 grams of caustic potash as explained above and required 8.7 cubic centimeter normal acid to neutralize the solution, then

8.7 .05616 = .4886 grams KOH in sample

.4886 ----- 100 = 96.73% KOH in sample.

.505

Caustic potash often contains some caustic soda, and while it is possible to express the results in terms of KOH, regardless of any trouble that may be caused by this mixture in soap making, an error is introduced in the results, not all the alkali being caustic potash. In such cases it is advisable to consult a book on a.n.a.lysis as the a.n.a.lysis is far more complicated than those given we will not consider it. The presence of carbonates, as already stated, also causes an error. To overcome this the alkali is t.i.trated in absolute alcohol, filtering off the insoluble carbonate. The soluble portion is caustic hydrate and may be t.i.trated as such. The carbonate remaining on the filter paper is dissolved in water and t.i.trated as carbonate.

SOAP a.n.a.lYSIS.

To obtain a sample of a cake of soap for a.n.a.lysis is a rather difficult matter as the moisture content of the outer and inner layer varies considerably. To overcome this difficulty a borer or sampler may be run right through the cake of soap, or slices may be cut from various parts of the cake, or the cake may be cut and run through a meat chopper several times and mixed. A sufficient amount of a h.o.m.ogeneous sample obtained by any of these methods is preserved for the entire a.n.a.lysis by keeping the soap in a securely stoppered bottle.

The more important determinations of soap are moisture, free alkali, or fatty acid, combined alkali and total fatty matter. Besides these it is often necessary to determine insoluble matter, glycerine, unsaponifiable matter, rosin and sugar.

MOISTURE.

The a.n.a.lysis of soap for moisture, at its best, is most unsatisfactory, for by heating it is impossible to drive off all the water, and on the other hand volatile oils driven off by heat are a part of the loss represented as moisture.

The usual method of determining moisture is to weigh 2 to 3 grams of finely shaved soap on a watch gla.s.s and heat in an oven at 105 degrees C. for 2 to 3 hours. The loss in weight is represented as water, although it is really impossible to drive off all the water in this way.

To overcome the difficulties just mentioned either the Smith or Fahrion method may be used. Allen recommends Smith's method which is said to be truthful to within 0.25 per cent. Fahrion's method, according to the author, gives reliable results to within 0.5 per cent. Both are more rapid than the above manipulation. To carry out the method of Smith, 5 to 10 grams of finely ground soap are heated over a sand bath with a small Bunsen flame beneath it, in a large porcelain crucible. The heating takes 20 to 30 minutes, or until no further evidence is present of water being driven off. This may be tested by the fogging of a cold piece of gla.s.s held over the crucible immediately upon removing the burner. When no fog appears the soap is considered dry. Any lumps of soap may be broken up by a small gla.s.s rod, weighed with the crucible, and with a roughened end to more easily separate the lumps. Should the soap burn, this can readily be detected by the odor, which, of course, renders the a.n.a.lysis useless. The loss in weight is moisture.

By Fahrion's method[13], 2 to 4 grams of soap are weighed in a platinum crucible and about three times its weight of oleic acid, which has been heated at 120 degrees C. until all the water is driven off and preserved from moisture, is added and reweighed. The dish is then cautiously heated with a small flame until all the water is driven off and all the soap is dissolved. Care must be exercised not to heat too highly or the oleic acid will decompose. The moment the water is all driven off a clear solution is formed, provided no fillers are present in the soap.

The dish is then cooled in a dessicator and reweighed. The loss in weight of acid plus soap is moisture and is calculated on the weight of soap taken. This determination takes about fifteen minutes.

FREE ALKALI OR ACID.

(_a_) _Alcoholic Method._

Test a freshly cut surface of the soap with a few drops of an alcoholic phenolphthalein solution. If it does not turn red it may be a.s.sumed free fat is present; should a red color appear, free alkali is present. In any case dissolve 2 to 5 grams of soap in 100 cubic centimeters of neutralized alcohol and heat to boiling until in solution. Filter off the undissolved portion containing carbonate, etc., and wash with alcohol. Add phenolphthalein to the filtrate and t.i.trate with N/10 acid and calculate the per cent. of free alkali as sodium or pota.s.sium hydroxide. Should the filtrate be acid instead of alkaline, t.i.trate with N/10 alkali and calculate the percentage of free fatty acid as oleic acid.

The insoluble portion remaining on the filter paper is washed with water until all the carbonate is dissolved. The was.h.i.+ngs are then t.i.trated with N/10 sulfuric acid and expressed as sodium or pota.s.sium carbonate.

Should borates or silicates be present it is possible to express in terms of these. If borax is present the carbon dioxide is boiled off after neutralizing exactly to methyl orange; cool, add mannite and phenolphthalein and t.i.trate the boric acid with standard alkali.

(_b_) _Bosshard and Huggenberg Method._[14]

In using the alcoholic method for the determination of the free alkali or fat in soap there is a possibility of both free fat and free alkali being present. Upon boiling in an alcoholic solution the fat will be saponified, thus introducing an error in the a.n.a.lysis. The method of Bosshard and Huggenberg overcomes this objection. Their method is briefly as follows:

_Reagents._

1. N/10 hydrochloric acid to standardize N/10 alcoholic sodium hydroxide.

2. Approximately N/10 alcoholic sodium hydroxide to fix and control the N/40 stearic acid.

3. N/40 stearic acid. Preparation: About 7.1 grams of stearic acid are dissolved in one liter of absolute alcohol, the solution filtered, the strength determined by t.i.tration against N/10 NaOH and then protected in a well stoppered bottle, or better still connected directly to the burette.

4. A 10 per cent. solution of barium chloride. Preparation: 100 grams of barium chloride are dissolved in one liter of distilled water and filtered. The neutrality of the solution should be proven as it must be neutral.

5. [Greek: alpha] naptholphthalein indicator according to Sorenson.

Preparation: 0.1 gram of [Greek: alpha] naphtholphthalein is dissolved in 150 cubic centimeters of alcohol and 100 cubic centimeters of water.

For every 10 cubic centimeters of liquid use at least 12 drops of indicator.

6. Phenolphthalein solution 1 gram to 100 cubic centimeter 96 per cent.

alcohol.

7. Solvent, 50 per cent. alcohol neutralized.

MANIPULATION.

First--Determine the strength of the N/10 alcoholic sodium hydroxide in terms of N/10 hydrochloric acid and calculate the factor, e. g.:

10 c.c. N/10 alcoholic NaOH = 9.95 N/10 HCl} 10 c.c. N/10 alcoholic NaOH = 9.96 N/10 HCl} 9.96

The alcoholic N/10 NaOH has a factor of 0.996.

Second--Control the N/40 stearic acid with the above alkali to obtain its factor, e. g.:

40 c.c. N/40 alcoholic stearic acid = 10.18 c.c. N/10 NaOH } 40 c.c. N/40 alcoholic stearic acid = } 10.2 10.22 c.c. N/10 NaOH }

10.2 F N/10 NaOH (0.996) = Factor N/40 stearic acid

Therefore Factor N/40 stearic acid = 1.016.

Third--About 5 grams of soap are weighed and dissolved in 100 cubic centimeters of 50 per cent. neutralized alcohol in a 250 cubic centimeter Erlenmeyer flask over a water bath and connected with a reflux condensor. When completely dissolved, which takes but a few moments, it is cooled by allowing a stream of running water to run over the outside of the flask.

Fourth--The soap is precipitated with 15 to 20 cubic centimeters of the 10 per cent. barium chloride solution.

Fifth--After the addition of 2 to 5 cubic centimeters of [Greek: alpha]

naphtholphthalein solution the solution is t.i.trated with N/40 alcoholic stearic acid. [Greek: alpha] naphtholphthalein is red with an excess of stearic acid. To mark the color changes it is advisable to first run a few blanks until the eye has become accustomed to the change in the indicator in the same way. The change from green to red can then be carefully observed.

Let us presume 5 grams of soap were taken for the a.n.a.lysis and 20 cubic centimeters of N/40 stearic acid were required for the t.i.tration then to calculate the amount of NaOH since the stearic factor is 1.016.

20 1.016 = 20.32 N/40 stearic acid really required.

1 cubic centimeter N/40 stearic acid = 0.02 per cent. NaOH for 5 grams soap.