Chapter VI — Analytical Methods (part 1 of 3)

While it is possible to attain a certain amount of efficiency in determining the worth of the raw material entering into the manufacture of soap through organoleptic methods, these are by no means accurate. It is, therefore, necessary to revert to chemical methods to correctly determine the selection of fats, oil or other substances used in soap making, as well as standardizing a particular soap manufactured and to properly regulate the glycerine recovered.

It is not our purpose to cover in detail the numerous analytical processes which may be employed in the examination of fats and oils, alkalis, soap and glycerine, as these are fully and accurately covered in various texts, but rather to give briefly the necessary tests which ought to be carried out in factories where large amounts of soap are made. Occasion often arises where it is impossible to employ a chemist, yet it is possible to have this work done by a competent person or to have someone instruct himself as just how to carry out the more simple analyses, which is not a very difficult matter. The various standard solutions necessary to carrying out the simpler titrations can readily be purchased from dealers in chemical apparatus and it does not take extraordinary intelligence for anyone to operate a burette, yet in many soap plants in this country absolutely no attention is paid to the examining of raw material, though many thousand pounds are handled annually, which, if they were more carefully examined would result in the saving of much more money than it costs to examine them or have them at least occasionally analyzed.

ANALYSIS OF FATS AND OILS.

In order to arrive at proper results in the analysis of a fat or oil, it is necessary to have a proper sample. To obtain this a sample of several of the packages of oil or fat is taken and these mixed or molten together into a composite sample which is used in making the tests. If the oil or fat is solid, a tester is used in taking the sample from the package and if they are liquid, it is a simple matter to draw off a uniform sample from each package and from these to form a composite sample.

In purchasing an oil or fat for soap making, the manufacturer is usually interested in the amount of free fatty acid contained therein, of moisture, the titer, the percentage of unsaponifiable matter and to previously determine the color of soap which will be obtained where color is an object.

DETERMINATION OF FREE FATTY ACIDS.

Since the free fatty acid content of a fat or oil represents a loss of glycerine, the greater the percentage of free fatty acid, the less glycerine is contained in the fat or oil, it is advisable to purchase a fat or oil with the lower free acid, other properties and the price being the same.

While the mean molecular weight of the mixed free fatty acids varies with the same and different oils or fats and should be determined for any particular analysis for accuracy, the free fatty acid is usually expressed as oleic acid, which has a molecular weight of 282.

To carry out the analysis 5 to 20 grams of the fat are weighed out into an Erlenmeyer flask and 50 cubic centimeters of carefully neutralized alcohol are added. In order to neutralize the alcohol add a few drops of phenolphthalein solution to same and add a weak caustic soda solution drop by drop until a very faint pink color is obtained upon shaking or stirring the alcohol thoroughly. The mixture of fat and neutralized alcohol is then heated to boiling and titrated with tenth normal alkali solution, using phenolphthalein as an indicator. As only the free fatty acids are readily soluble in the alcohol and the fat itself only slightly mixes with it, the flask should be well agitated toward the end of the titration. When a faint pink color remains after thoroughly agitating the flask the end point is reached. In order to calculate the percentage of free fatty acid as oleic acid, multiply the number of cubic centimeters of tenth normal alkali used as read on the burette by 0.0282 and divide by the number of grams of fat taken for the determination and multiply by 100.

When dark colored oils or fats are being titrated it is often difficult to obtain a good end point with phenolphthalein. In such cases about 2 cubic centimeters of a 2 per cent. alcoholic solution of Alkali Blue 6 B is recommended.

Another method of directly determining the free fatty acid content of tallow or grease upon which this determination is most often made is to weigh out into an Erlenmeyer flask exactly 5.645 grams of a sample of tallow or grease. Add about 75 cubic centimeters of neutralized alcohol. Heat until it boils, then titrate with tenth normal alkali and divide the reading by 2, which gives the percentage of free fatty acid as oleic. If a fifth normal caustic solution is used, the reading on the burette gives the percentage of free fatty acid directly. This method, while it eliminates the necessity of calculation, is troublesome in that it is difficult to obtain the exact weight of fat.

MOISTURE.

To calculate the amount of moisture contained in a fat or oil 5 to 10 grams are weighed into a flat bottom dish, together with a known amount of clean, dry sand, if it is so desired. The dish is then heated over a water bath, or at a temperature of 100-110 degs. C., until it no longer loses weight upon drying and reweighing the dish. One hour should elapse between the time the dish is put on the water bath and the time it is taken off to reweigh. The difference between the weight of the dish is put on the water bath and the time it is taken off when it reaches a constant weight is moisture. This difference divided by the original weight of the fat or oil × 100 gives the percentage of moisture.

When highly unsaturated fats or oils are being analyzed for moisture, an error may be introduced either by the absorption of oxygen, which is accelerated at higher temperature, or by the formation of volatile fatty acids. The former causes an increase in weight, the latter causes a decrease. To obviate this, the above operation of drying should be carried out in the presence of some inert gas like hydrogen, carbon dioxide, or nitrogen.

TITER.

The titer of a fat or oil is really an indication of the amount of stearic acid contained therein. The titer, expressed in degrees Centigrade, is the solidification point of the fatty acids of an oil or fat. In order to carry out the operation a Centigrade thermometer graduated in one or two-tenths of a degree is necessary. A thermometer graduated between 10 degs. centigrade to 60 degs. centigrade is best adapted and the graduations should be clear cut and distinct.

To make the determination about 30 grams of fat are roughly weighed in a metal dish and 30-40 cubic centimeters of a 30 per cent. (36 degs. Baumé) solution of sodium hydroxide, together with 30-40 cubic centimeters of alcohol, denatured alcohol will do, are added and the mass heated until saponified. Heat over a low flame or over an asbestos plate until the soap thus formed is dry, constantly stirring the contents of the dish to prevent burning. The dried soap is then dissolved in about 1000 cubic centimeters of water, being certain that all the alcohol has been expelled by boiling the soap solution for about half an hour. When the soap is in solution add sufficient sulphuric acid to decompose the soap, approximately 100 cubic centimeters of 25 degs. Baumé sulphuric acid, and boil until the fatty acids form a clear layer on top of the liquid. A few pieces of pumice stone put into the mixture will prevent the bumping caused by boiling. Siphon off the water from the bottom of the dish and wash the fatty acids with boiling water until free from sulphuric acid. Collect the fatty acids in a small casserole or beaker and dry them over a steam bath or drying oven at 110 degs. Centigrade. When the fatty acids are dry, cool them to about 10 degs. above the titer expected and transfer them to a titer tube or short test tube which is firmly supported by a cork in the opening of a salt mouth bottle. Hang the thermometer by a cord from above the supported tube so it reaches close to the bottom when in the titer tube containing the fatty acids and so that it may be used as a stirrer. Stir the mass rather slowly, closely noting the temperature. The temperature will gradually fall during the stirring operation and finally remain stationary for half a minute or so then rise from 0.1 to 0.5 degs. The highest point to which the mercury rises after having been stationary is taken as the reading of the titer.

DETERMINATION OF UNSAPONIFIABLE MATTER.

In order to determine the unsaponifiable matter in fats and oils they are first saponified, then the unsaponifiable, which consists mainly of hydrocarbons and the higher alcohols cholesterol or phytosterol, is extracted with ether or petroleum ether, the ether evaporated and the residue weighed as unsaponifiable.

To carry out the process first saponify about 5 grams of fat or oil with an excess of alcoholic potassium hydrate, 20-30 cubic centimeters of a 1 to 10 solution of potassium hydroxide in alcohol until the alcohol is evaporated over a steam bath. Wash the soap thus formed into a separatory funnel of 200 cubic centimeters capacity with 80-100 cubic centimeters water. Then add about 60 cubic centimeters of ether, petroleum ether or 86 degs. gasoline and thoroughly shake the funnel to extract the unsaponifiable. Should the two layers not separate readily, add a few cubic centimeters of alcohol, which will readily cause them to separate. Draw off the watery solution from beneath and wash the ether with water containing a few drops of sodium hydrate and run to another dish. Pour the watery solution into the funnel again and repeat the extraction once or twice more or until the ether shows no discoloration. Combine the ether extractions into the funnel and wash with water until no alkaline reaction is obtained from the wash water. Run the ether extract to a weighed dish, evaporate and dry rapidly in a drying oven. As some of the hydrocarbons are readily volatile at 100 degs. Centigrade, the drying should not be carried on any longer than necessary. The residue is then weighed and the original weight of fat taken divided into the weight of the residue × 100 gives the percentage unsaponifiable.

TEST FOR COLOR OF SOAP.

It is often desirable to determine the color of the finished soap by a rapid determination before it is made into soap. It often happens, especially with the tallows, that a dark colored sample produces a light colored soap, whereas a bleached light colored tallow produces a soap off shade.

To rapidly determine whether the color easily washes out of the tallow with lye, 100 cubic centimeters of tallow are saponified in an enameled or iron dish with 100 cubic centimeters of 21 degs. Baumé soda lye and 100 cubic centimeters of denatured alcohol. Continue heating over a wire gauze until all the alcohol is expelled and then add 50 cubic centimeters of the 21 degs. Baumé lye to grain the soap. Allow the lyes to settle and with an inverted pipette draw off the lyes into a test tube or bottle. Close the soap with 100 cubic centimeters of hot water and when closed again grain with 50 cubic centimeters of the lye by just bringing to a boil over an open flame. Again allow the lyes to settle and put aside a sample of the lye for comparison. Repeat the process of closing, graining and settling and take a sample of lye. If the lye is still discolored repeat the above operations again or until the lye is colorless. Ordinarily all the color will come out with the third lye. The soap thus obtained contains considerable water which makes it appear white. The soap is, therefore, dried to about 15 per cent. moisture and examined for color. The color thus obtained is a very good criterion as to what may be expected in the soap kettle.

By making the above analyses of fats or oils the main properties as to their adaptability for being made into soap are determined. In some cases, especially where adulteration or mixtures of oils are suspected, it is necessary to further analyze same. The methods of carrying out these analyses are fully covered by various texts on fats and oils and we will not go into details regarding the method of procedure in carrying these out.

TESTING OF ALKALIS USED IN SOAP MAKING.

The alkalis entering into the manufacture of soap such as caustic soda or sodium hydroxide, caustic potash or potassium hydrate, carbonate of soda or sodium carbonate, carbonate of potash or potassium carbonate usually contain impurities which do not enter into combination with the fats or fatty acids to form soap. It is out of the question to use chemically pure alkalis in soap making, hence it is often necessary to determine the alkalinity of an alkali. It may again be pointed out that in saponifying a neutral fat or oil only caustic soda or potash are efficient and the carbonate contained in these only combines to a more or less extent with any free fatty acids contained in the oils or fats. Caustic soda or potash or lyes made from these alkalis upon exposure to the air are gradually converted into sodium or potassium carbonate by the action of the carbon dioxide contained in the air. While the amount of carbonate thus formed is not very great and is greatest upon the surface, all lyes as well as caustic alkalis contain some carbonate. This carbonate introduces an error in the analysis of caustic alkalis when accuracy is required and thus in the analysis of caustic soda or potash it is necessary to remove the carbonate when the true alkalinity as sodium hydroxide or potassium hydroxide is desired. This may be done by titration in alcohol which has been neutralized.

In order to determine the alkalinity of any of the above mentioned alkalis, it is first necessary to obtain a representative sample of the substance to be analyzed. To do this take small samples from various portions of the package and combine them into a composite sample. Caustic potash and soda are hygroscopic and samples should be weighed at once or kept in a well stoppered bottle. Sodium or potassium carbonate can be weighed more easily as they do not rapidly absorb moisture from the air.

To weigh the caustic soda or potash place about five grams on a watch glass on a balance and weigh as rapidly as possible. Wash into a 500 cubic centimeter volumetric flask and bring to the mark with distilled water. Pipette off 50 cubic centimeters into a 200 cubic centimeter beaker, dilute slightly with distilled water, add a few drops of methyl orange indicator and titrate with normal acid. For the carbonates about 1 gram may be weighed, washed into a 400 cubic centimeter beaker, diluted with distilled water, methyl orange indicator added and titrated with normal acid. It is advisable to use methyl orange indicator in these titrations as phenolphthalein is affected by the carbon dioxide generated when an acid reacts with a carbonate and does not give the proper end point, unless the solution is boiled to expel the carbon dioxide. Litmus may also be used as the indicator, but here again it is necessary to boil as carbon dioxide also affects this substance. As an aid to the action of these common indicators the following table may be helpful:

Indicator.	Color in Acid Solution.	Color in Alkaline Solution.	Action of CO2.
Methyl orange	Red	Yellow	Very slightly acid
Phenolphthalein	Colorless	Red	Acid
Litmus	Red	Blue	Acid

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 titrations, 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, Na2CO3	0.05305
Sodium Hydroxide, NaOH	0.04006
Sodium Oxide, Na2O	0.02905
Carbonate K2CO3	0.06908
Potassium Hydroxide, KOH	0.05616
Potassium Oxide, K2O	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 analysis as the analysis 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 titrated in absolute alcohol, filtering off the insoluble carbonate. The soluble portion is caustic hydrate and may be titrated as such. The carbonate remaining on the filter paper is dissolved in water and titrated as carbonate.

SOAP ANALYSIS.

To obtain a sample of a cake of soap for analysis 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 homogeneous sample obtained by any of these methods is preserved for the entire analysis 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 analysis 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 glass 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 glass 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 glass 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 analysis 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 assumed 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 titrate with N/10 acid and calculate the per cent. of free alkali as sodium or potassium hydroxide. Should the filtrate be acid instead of alkaline, titrate 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 washings are then titrated with N/10 sulfuric acid and expressed as sodium or potassium 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 titrate 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 analysis. 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 titration 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. α naphtholphthalein indicator according to Sorenson. Preparation: 0.1 gram of α 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}	9.96
10 c.c. N/10 alcoholic NaOH	= 9.96 N/10 HCl}

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

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

∴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 α naphtholphthalein solution the solution is titrated with N/40 alcoholic stearic acid. α 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 analysis and 20 cubic centimeters of N/40 stearic acid were required for the titration 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.

Δ 20.32 cubic centimeters N/40 stearic acid = 0.02 × 20.32 per cent. NaOH for 5 grams soap.

Hence the soap contains 0.4064 per cent. NaOH.

It is necessary, however, to make a correction by this method. When the free alkali amounts to over 0.1 per cent. the correction is + 0.01, and when the free alkali exceeds 0.4 per cent. the correction is + 0.04, hence in the above case we multiply 0.004064 by 0.04, add this amount to 0.004064 and multiply by 100 to obtain the true percentage. Should the alkalinity have been near 0.1 per cent. we would have multiplied by 0.01 and added this.

If carbonate is also present in the soap, another 5 grams of soap is dissolved in 100 cubic centimeters of 50 per cent. alcohol and the solution titrated directly after cooling with N/40 stearic acid, using α naphtholphthalein or phenolphthalein as an indicator, without the addition of barium chloride. From the difference of the two titrations the alkali present as carbonate is determined.

If the decomposed soap solution is colorless with phenolphthalein, free fatty acids are present, which may be quickly determined with alcoholic N/10 sodium hydroxide.