CHAPTER XXVIII

FOREIGN INGREDIENTS OF COMMERCIAL SOAPS

WE have seen that in addition to the soaps proper―i.e. the salts of fatty acids - commercial soaps always contain water and may contain glycerol, unsaponified fat, and free alkali. But in practice other substances are frequently added, either to lower the cost of production or to render the soap more attractive. The substances added to lower the cost of production are of two classes: (1) other detergents; (2) fillers.

Detergents

Among the detergents added to soaps are:

Sodium and Potassium Carbonates. - These are cheap and harsh alkalies and are to be regarded as adulterants, except in soaps to be used for rough cleaning. Even for such purposes the soda or potash can be more economically purchased separately, as soda ash or washing soda, and as pearl ash, respectively. Sodium carbonate cannot be added to soap in greater quantity than 5 per cent without causing a white incrustation on the surface of the soap. Potassium carbonate can be added in larger quantity, and has the property of making the soap look finer in texture and therefore more attractive.

Experiment 88. Test for Carbonates

Materials:

Soda ash (sodium carbonate).

Pearl ash (potassium carbonate).

Commercial soaps and washing powders.

Add a little dilute sulphuric acid to (1) soda ash, (2) pearl ash. Note and account for the effervescence. Write equations for the reactions.

Test the commercial soaps and washing powders for carbonates in the same way.

Sodium silicate is known as water glass." This substance gives firmness to soap, and enables it to hold more water and still remain hard. In small quantities it is a legitimate addition to soaps for some purposes. Soaps containing more than a very little silicate, when used in the laundry, leave a deposit of silica (an insoluble substance of the same composition as sand) in the clothes.

Experiment 89. Test for Silicate.

Treat the finely shaved soap with hot alcohol until nothing further dissolves; filter and wash with hot alcohol. Now wash the residue with hot water, collecting the solution obtained. Acidify this solution with hydrochloric acid, evaporate to dryness, and gently heat the residue for some time. If it chars, heat more strongly until it is completely burned. Allow to cool, add water and a little hydrochloric acid, and warm. Silica will be left as an insoluble, gritty residue.

Sodium Resinate.—Rosin (also termed colophony) consists of acids which react with alkalies to form salts called resinates, which, like soaps, have detergent properties. These resinates cannot be used separately for cleansing purposes. In dilute hot solutions they hydrolyze to so great an extent as to precipitate the rosin acids. These are deposited on the goods, causing a yellow stain having the odor of rosin. Resinates are often contained in laundry soaps, particularly yellow soaps, and are objectionable constituents, unless present in only small quantities. Yellow soaps have been analyzed which contain up to 40 per cent of resinates. Soaps containing resinates are sometimes called "rosin soaps" and are spoken of as containing rosin.

Experiment 90. Test for Rosin.

Materials:

Soaps with and without rosin.
Acetic anhydride.

FOREIGN INGREDIENTS OF COMMERCIAL SOAPS 153

Compare the odors of the soaps containing rosin with the odors of the non-rosin soaps. Dissolve the soaps in water. Acidify with sulphuric acid. Filter. Dissolve the precipitate in acetic anhydride. What is this precipitate (1) if the soap is pure? (2) if the soap contains rosin?

To 5 cc. water add 5 cc. concentrated sulphuric acid. Cool the mixture. Place about 2 cc. in a test tube and add a few drops of the acetic anhydride solution of the fatty acids. A violet coloration shows that rosin is present.

Petroleum Products. - Petroleum products, such as paraffin wax, kerosene, and naphtha (a volatile product resembling benzine) are sometimes added to soap. These, being fat solvents, have value as detergents. Kerosene itself is sometimes used in the clothes boiler, both in the household and in commercial laundries. The naphtha soap, however, cannot be used with hot water.

Borax is a sodium borate, whose detergent property is well known. It is an excellent ingredient of soaps.

Fillers

Among the "fillers," i.e. cheap, weight-making substances of little or no detergent value, used as ingredients of commercial soaps, are the sulphates of sodium, potassium, calcium, and barium, infusorial earth (a fine form of silica, SiO2, left from the decay of minute marine organisms called infusoria), fine clay, chalk, or whiting (calcium carbonate), French chalk (a soft, powdery magnesium silicate), starch, and impure vaseline. In the detection of these substances advantage is taken of the circumstances that none of them is soluble in alcohol and only the sodium sulphate and potassium sulphate are soluble in water.

Dissolve the finely shaved soap in alcohol. Filter and wash the insoluble residue with alcohol, rejecting the alcoholic solution. Boil the residue insoluble in alcohol with water. Treat the residue insoluble in water and the water solution as follows:

CHAPTER XXIX

SPECIAL SOAPS AND SCOURING POWDERS

Perfumed and Colored Soaps. The perfumes and coloring matters ordinarily added to toilet soaps are harmless, but sometimes excessive quantities of perfume are used to conceal disagreeable odors due to the use of decomposing fats. Strongly perfumed soaps are, therefore, to be regarded with suspicion.

Transparent Soaps. The best transparent soaps are made by dissolving the soap in alcohol, filtering off the undissolved residue, then removing the alcohol by evaporation. Glycerin is often added to give a pleasant emollient feel. Cheaper transparent soaps are made by the cold process from tallow, castor oil, palm oil, or coconut oil. These usually contain free alkali. Some contain sugar, an undesirable adulterant, because, being so soluble, it causes rapid wasting of the soap.

Dissolve the soap in water, acidify with dilute sulphuric acid, filter off the precipitated fatty acids. Boil the filtrate for about half an hour, neutralize with sodium hydroxide, add a little of the neutralized solution to Fehling-Benedict solution, and boil for a minute. A red or yellow precipitate (cuprous oxide, Cu2O) shows sugar.

Floating Soaps. — Floating soaps are made by beating the molten soap to incorporate air bubbles.

Marine Soap. Marine soap is a soap made from palmnut or coconut oil, and takes its name from the fact that it will form a lather with sea water. Marine soap has been known to contain as much as 70 per cent of water.