Soap-Making Manual Part 18

[Greek: Delta] 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 t.i.trated directly after cooling with N/40 stearic acid, using [Greek: alpha] naphtholphthalein or phenolphthalein as an indicator, without the addition of barium chloride. From the difference of the two t.i.trations 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.

INSOLUBLE MATTER.

The insoluble matter in soap may consist of organic or inorganic substances. Among the organic substances which are usually present in soap are oat meal, bran, sawdust, etc., while among the common inorganic or mineral compounds are pumice, silex, clay, talc, zinc oxide, infusorial earth, sand or other material used as fillers.

To determine insoluble matter, 5 grams of soap are dissolved in 75 cubic centimeters of hot water. The solution is filtered through a weighed gooch crucible or filter paper. The residue remaining on the filter is washed with hot water until all the soap is removed, is then dried to constant weight at 105 degrees C. and weighed. From the difference in weight of the gooch or filter paper and the dried residue remaining thereon after filtering and drying, the total percentage of insoluble matter may easily be calculated. By igniting the residue and reweighing the amount of insoluble mineral matter can be readily determined.

STARCH AND GELATINE.

Should starch or gelatine be present in soap it is necessary to extract 5 grams of the soap with 100 cubic centimeters of 95 per cent.

neutralized alcohol in a Soxhlet extractor until the residue on the extraction thimble is in a powder form. If necessary the apparatus should be disconnected and any lumps crushed, as these may contain soap.

The residue remaining on the thimble consists of all substances present in soap, insoluble in alcohol. This is dried and weighed so that any percentage of impurities not actually determined can be found by difference. Starch and gelatine are separated from carbonate, sulfate and borate by dissolving the latter out through a filter with cold water. The starch and gelatine thus remaining can be determined by known methods, starch by the method of direct hydrolysis[15] and gelatine by Kjeldahling and calculating the corresponding amount of gelatine from the percentage of nitrogen (17.9%) therein.[16]

TOTAL FATTY AND RESIN ACIDS.

To the filtrate from the insoluble matter add 40 cubic centimeters of half normal sulfuric acid, all the acid being added at once. Boil, stir thoroughly for some minutes and keep warm on a water bath until the fatty acids have collected as a clear layer on the surface. Cool by placing the beaker in ice and syphon off the acid water through a filter. Should the fatty acids not readily congeal a weighed amount of dried bleached bees-wax or stearic acid may be added to the hot mixture.

This fuses with the hot ma.s.s and forms a firm cake of fatty acids upon cooling. Without removing the fatty acids from the beaker, add about 300 cubic centimeters of hot water, cool, syphon off the water through the same filter used before and wash again. Repeat washing, cooling and syphoning processes until the wash water is no longer acid. When this stage is reached, dissolve any fatty acid which may have remained on the filter with hot 95 per cent. alcohol into the beaker containing the fatty acids. Evaporate the alcohol and dry the beaker to constant weight over a water bath. The fatty acids thus obtained represent the combined fatty acids, uncombined fat and hydrocarbons.

DETERMINATION OF ROSIN.

If resin acids are present, this may be determined by the Liebermann-Storch reaction. To carry out this test shake 2 cubic centimeters of the fatty acids with 5 cubic centimeters of acetic anhydride; warm slightly; cool; draw off the anhydride and add 1:1 sulfuric acid. A violet color, which is not permanent, indicates the presence of rosin in the soap. The cholesterol in linseed or fish oil, which of course may be present in the soap, also give this reaction.

Should resin acids be present, these may be separated by the Twitch.e.l.l method, which depends upon the difference in the behavior of the fatty and resin acids when converted into their ethyl esters through the action of hydrochloric acid. This may be carried out as follows:

Three grams of the dried mixed acids are dissolved in 25 cubic centimeters of absolute alcohol in a 100 cubic centimeter stoppered flask; the flask placed in cold water and shaken. To this cooled solution 25 cubic centimeters of absolute alcohol saturated with dry hydrochloric acid is added. The flask is shaken occasionally and the action allowed to continue for twenty minutes, then 10 grams of dry granular zinc chloride are added, the flask shaken and again allowed to stand for twenty minutes. The contents of the flask are then poured into 200 cubic centimeters of water in a 500 cubic centimeter beaker and the flask rinsed out with alcohol. A small strip of zinc is placed in the beaker and the alcohol evaporated. The beaker is then cooled and transferred to a separatory funnel, washing out the beaker with 50 cubic centimeters of gasoline (boiling below 80 degrees C.) and extracting by shaking the funnel well. Draw off the acid solution after allowing to separate and wash the gasoline with water until free from hydrochloric acid. Draw off the gasoline solution and evaporate the gasoline.

Dissolve the residue in neutral alcohol and t.i.trate with standard alkali using phenolphthalein as an indicator. One cubic centimeter of normal alkali equals 0.346 grams of rosin. The rosin may be gravimetrically determined by washing the gasoline extract with water, it not being necessary to wash absolutely free from acid, then adding 0.5 gram of pota.s.sium hydroxide and 5 cubic centimeters of alcohol in 50 cubic centimeters of water. Upon shaking the resin acids are rapidly saponified and extracted by the dilute alkaline solution as rosin soaps, while the ethyl esters remain in solution in the gasoline. Draw off the soap solution, wash the gasoline solution again with dilute alkali and unite the alkaline solutions. Decompose the alkaline soap solution with an excess of hydrochloric acid and weigh the resin acids liberated as in the determination of total fatty acids.

According to Lewkowitsch, the results obtained by the volumetric method which a.s.sumes a combining weight of 346 for resin acids, are very likely to be high. On the other hand those obtained by the gravimetric method are too low.

Leiste and Stiepel[17] have devised a simpler method for the determination of rosin. They make use of the fact that the resin acids as sodium soaps are soluble in acetone and particularly acetone containing two per cent. water, while the fatty acid soaps are soluble in this solvent to the extent of only about 2 per cent. First of all it is necessary to show that the sample to be a.n.a.lyzed contains a mixture of resin and fatty acids. This may be done by the Liebermann-Storch reaction already described. Glycerine interferes with the method. Two grams of fatty acids or 3 grams of soap are weighed in a nickel crucible and dissolved in 15-20 cubic centimeters of alcohol. The solution is then neutralized with alcoholic sodium hydroxide, using phenolphthalein as an indicator. The ma.s.s is concentrated by heat over an asbestos plate until a slight film forms over it. Then about 10 grams of sharp, granular, ignited sand are stirred in by means of a spatula, the alcohol further evaporated, the mixture being constantly stirred and then thoroughly dried in a drying oven. The solvent for the cooled ma.s.s is acetone containing 2 per cent. water. It is obtained from acetone dried by ignited sodium sulfate and adding 2 per cent. water by volume. One hundred cubic centimeters of this solvent are sufficient for extracting the above. The extraction of the rosin soap is conducted by adding 10 cubic centimeters of acetone eight times, rubbing the ma.s.s thoroughly with a spatula and decanting. The decanted portions are combined in a beaker and the suspended fatty soaps allowed to separate. The mixture is then filtered into a previously weighed flask and washed several times with the acetone remaining. The solution of rosin soap should show no separation of solid matter after having evaporated to half the volume and allowing to cool. If a separation should occur another filtration and the slightest possible washing is necessary. To complete the a.n.a.lysis, the acetone is completely evaporated and the ma.s.s dried to constant weight in a drying oven. The weight found gives the weight of the rosin soap. In conducting the determination, it is important to dry the mixture of soap and sand thoroughly. In dealing with potash soaps it is necessary to separate the fatty acids from these and use them as acetone dissolves too great a quant.i.ty of a potash soap.

TOTAL ALKALI.

In the filtrate remaining after having washed the fatty acids in the determination of total fatty and resin acids all the alkali present as soap, as carbonate and as hydroxide remains in solution as sulfate. Upon t.i.trating this solution with half normal alkali the difference between the half normal acid used in decomposing the soap and alkali used in t.i.trating the excess of acid gives the amount of total alkali in the soap. By deducting the amount of free alkali present as carbonate or hydroxide previously found the amount of combined alkali in the soap may be calculated.

To quickly determine total alkali in soap a weighed portion of the soap may be ignited to a white ash and the ash t.i.trated for alkalinity using methyl orange as an indicator.

UNSAPONIFIED MATTER.

Dissolve 5 grams of soap in 50 cubic centimeters of 50 per cent.

alcohol. Should any free fatty acids be present neutralize them with standard alkali. Wash into a separatory funnel with 50 per cent. alcohol and extract with 100 cubic centimeters of gasoline, boiling at 50 degrees to 60 degrees C. Wash the gasoline with water, draw off the watery layer. Run the gasoline into a weighed dish, evaporate the alcohol, dry and weigh the residue as unsaponified matter. The residue contains any hydrocarbon oils or fats not converted into soap.

SILICA AND SILICATES.

The insoluble silicates, sand, etc., are present in the ignited residue in the determination of insoluble matter. Sodium silicate, extensively used as a filler, however, will only show itself in forming a pasty liquid. Where it is desired to determine sodium silicate, 10 grams of soap are ashed by ignition, hydrochloric acid added to the ash in excess and evaporated to dryness. More hydrochloric acid is then added and the ma.s.s is again evaporated until dry; then cooled; moistened with hydrochloric acid; dissolved in water; filtered; washed; the filtrate evaporated to dryness and again taken up with hydrochloric acid and water; filtered and washed. The precipitates are then combined and ignited. Silicon dioxide (SiO_{2}) is thus formed, which can be calculated to sodium silicate (Na_{2}Si_{4}O_{9}). Should other metals than alkali metals be suspected present the filtrate from the silica determinations should be examined.

GLYCERINE IN SOAP.

To determine the amount of glycerine contained in soap dissolve 25 grams in hot water, add a slight excess of sulfuric acid and keep hot until the fatty acids form as a clear layer on top. Cool the ma.s.s and remove the fatty acids. Filter the acid solution into a 25 cubic centimeter graduated flask; bring to the mark with water and determine the glycerine by the bichromate method as described under glycerine a.n.a.lysis.

When sugar is present the bichromate would be reduced by the sugar, hence this method is not applicable. In this case remove the fatty acids as before, neutralize an aliquot portion with milk of lime, evaporate to 10 cubic centimeters, add 2 grams of sand and milk of lime containing about 2 grams of calcium hydroxide and evaporate almost to dryness.

Treat the moist residue with 5 cubic centimeters of 96 per cent.

alcohol, rub the whole ma.s.s into a paste, then constantly stirring, heat on a water bath and decant into a 250 cubic centimeter graduated flask.

Repeat the washing with 5 cubic centimeters of alcohol five or six times, each time pouring the washings into the flask; cool the flask to room temperature and fill to the mark with 96 per cent. alcohol, agitate the flask until well mixed and filter through a dry filter paper. Take 200 cubic centimeters of the nitrate and evaporate to a syrupy consistency over a safety water bath. Wash the liquor into a stoppered flask with 20 cubic centimeters of absolute alcohol, add 30 cubic centimeters of absolute ether 10 cubic centimeters at a time, shaking well after each addition and let stand until clear. Pour off the solution through a filter into a weighed dish and wash out the flask with a mixture of three parts absolute ether and two parts absolute alcohol. Evaporate to a syrup, dry for one hour at the temperature of boiling water, weigh, ignite and weigh again. The loss is glycerine.

This multiplied by 5/4 gives the total loss for the aliquot portion taken. The glycerine may also be determined by the acetin or bichromate methods after driving off the alcohol and ether if so desired.

SUGAR IN SOAP.

To determine sugar in soap, usually present in transparent soaps, decompose a soap solution of 5 grams of soap dissolved in 100 cubic centimeters of hot water with an excess of hydrochloric acid and separate the fatty acids as usual. Filter the acid solution into a graduated flask and make up to the mark. Take an aliquot containing approximately 1 per cent. of reducing sugar and determine the amount of sugar by the Soxhlet method.[18]

GLYCERINE a.n.a.lYSIS.

The methods of a.n.a.lyzing glycerine varied so greatly due to the fact that glycerine contained impurities which acted so much like glycerine as to introduce serious errors in the determinations of crude glycerine.

This led to the appointment of committees in the United States and Europe to investigate the methods of glycerine a.n.a.lysis. An international committee met after their investigations and decided the acetin method should control the buying and selling of glycerine, but the more convenient bichromate method in a standardized form might be used in factory control and other technical purposes. The following are the methods of a.n.a.lysis and sampling as suggested by the international committee:

SAMPLING.

The most satisfactory method available for sampling crude glycerine liable to contain suspended matter, or which is liable to deposit salt on settling, is to have the glycerine sampled by a mutually approved sampler as soon as possible after it is filled into drums, but in any case before any separation of salt has taken place. In such cases he shall sample with a sectional sampler (see appendix) then seal the drums, brand them with a number for identification, and keep a record of the brand number. The presence of any visible salt or other suspended matter is to be noted by the sampler, and a report of the same made in his certificate, together with the temperature of the glycerine. Each drum must be sampled. Glycerine which has deposited salt or other solid matter cannot be accurately sampled from the drums, but an approximate sample can be obtained by means of the sectional sampler, which will allow a complete vertical section of the glycerine to be taken including any deposit.

a.n.a.lYSIS.

1. _Determination of Free Caustic Alkali._--Put 20 grams of the sample into a 100 cc. flask, dilute with approximately 50 cc. of freshly boiled distilled water, add an excess of neutral barium chloride solution, 1 cc. of phenolphthalein solution, make up to the mark and mix. Allow the precipitate to settle, draw off 50 cc. of the clear liquid and t.i.trate with normal acid (_N_/1). Calculate the percentage of Na_{2}O existing as caustic alkali.

2. _Determination of Ash and Total Alkalinity._--Weigh 2 to 5 grams of the sample in a platinum dish, burn off the glycerine over a luminous Argand burner or other source of heat,[19] giving a low temperature, to avoid volatilization and the formation of sulphides. When the ma.s.s is charred to the point that water will not be colored by soluble organic matter, lixiviate with hot distilled water, filter, wash and ignite the residue in the platinum dish. Return the filtrate and washings to the dish, evaporate the water, and carefully ignite without fusion. Weigh the ash.

Dissolve the ash in distilled water and t.i.trate total alkalinity, using as indicator methyl orange cold or litmus boiling.

3. _Determination of Alkali Present as Carbonate._--Take 10 grams of the sample, dilute with 50 cc. distilled water, add sufficient _N_/1 acid to neutralize the total alkali found at (2), boil under a reflux condenser for 15 to 20 minutes, wash down the condenser tube with distilled water, free from carbon dioxide, and then t.i.trate back with _N_/1 NaOH, using phenolphthalein as indicator. Calculate the percentage of Na_{2}O.