This section is from the book "A Practical Treatise On Friction, Lubrication, Fats And Oils", by Emil F. Dieterichs. Also available from Amazon: A practical treatise on friction, lubrication, fats and oils.
Alkali tests are made to ascertain first if an oil is a pure fatty oil, or a hydrocarbon oil, or a mixture of both. A solution of caustic soda or potash of a specific gravity of 1.340 is prepared and two volumes of this solution are shaken up with four volumes of the oil to be tested. After standing, the oil separates out, leaving an aqueous layer clear or slightly clouded. If hydrocarbon oils are in large proportion in the sample, they will form a layer on the top and the aqueous layer will be emulsified. If the fatty oil is in largest proportion, the smaller proportion of hydrocarbon oil will be more difficult to detect. To ascertain this, dissolve a piece of caustic potash the size of a pea, in 5 c.c. of alcohol. Then add a few drops of the oil to be tested and boil for two to three minutes and add from 3 to 4 c.c. of distilled water. If the solution remains clear, only fatty oil is present. Mineral oil will cause the solution to be turbid and even as small a quantity as 2 per cent present will show itself this way.
The amount of mineral oil in fatty oils is also ascertained by mixing 25 grammes of the oil with 10 to 15 c.c. of the caustic potash solution and 25 c.c. of water and 5 c.c. of alcohol. This is boiled, constantly stirring, for about one hour. By that time the fatty oil is saponified. Put the whole in a separating funnel and add more warm water and 25 c.c. petroleum ether; shake for a few minutes and allow to stand. The upper stratum is composed of the petroleum ether and the mineral oil and the lower stratum of the aqueous layer of soap formed by the fatty matter. This is run off, clean water is added, stirred together, and the whole again allowed to stand and the aqueous liquor run off. This is repeated until the latter runs off perfectly clear. Now put the ethereal layer into a weighed vessel, evaporate the ether and weigh the remaining oil; the weight multiplied by four gives the percentage of mineral oil in the sample.
A color test can be made by placing some 20 drops of the oil in a porcelain cup and adding two drops of strong sulphuric acid. As the acid drops through the oil, streaks of color are shown, and a tint of characteristic color gradually spreads through the oil. Then stir the whole and again note the coloring. Vegetable oils give various colors, shades of yellow-brown or green; fish oils turn to violet or purple, animal oils to a reddishbrown, and hydrocarbon oils turn slightly to a blackish-brown. The test should first be made with samples of known pure quality and compared with the action of the sample under test.
Agitating 4 c.c. of the oil to be tested with 10 c.c. colorless nitric acid will show, after settling, when olive oil has been adulterated with cottonseed oil, by a brownish color, while pure olive oil will not become darkened.
For a test of cottonseed oil in lard oil put 1/2 ounce of nitro-sulphuric acid and 1/2 ounce of the oil to be tested in a glass vessel and stir well. Pure lard oil becomes hard in two to three hours, but when adulterated with cottonseed oil the sample may thicken but will not become hard.
For a preliminary test for neutral oil in lard oil, shake the suspected sample violently in a bottle. If it contains neutral oil it will form beads or bubbles that will pass away when the oil is pure, but if adulterated with much neutral oil it will have a tendency to foam.
To detect small quantities of fatty oils, of 1/4 to 2 per cent, in a sample of mineral oil, some of the oil is heated for about fifteen minutes, with bits of sodium or sodium hydrate, to about 230° to 250° F. Fatty acid present will solidify to a jelly of more or less consistency, according to the amount of fatty oil therein.
To detect soap dissolved in mineral oil, five to ten per cent, of the oil is dissolved in about fifteen parts of gasoline or ether, and solution of phosphoric acid added. The formation of floc-culent precipitate indicates the presence of soap.
To detect acidity or alkali in mineral oil, shake a sample of the oil with an equal quantity of warm water, pour off the oil when settled, and test the water with litmus paper. Acidity will turn blue litmus paper red, and if alkaline, will turn red litmus paper blue, and yellow turmeric paper brown.
We will not enter any further into the inexhaustible field of chemical and technological testing of oils, but would call attention to some simple and practical methods for testing oils and oil mixtures, as to their purity and efficiency as lubricants, that can be carried out by the most inexperienced investigator.
First are to be secured samples of known purity of the different kinds of animal and vegetable oils used in the manufacture of lubricants, and then proceed with the testing of the oils to be investigated as follows:
All fatty oils of animal or vegetable origin possess an odor and taste peculiar to themselves, which becomes more distinctly noticeable when the temperature of the oil is increased. If, therefore a few drops of the oil, to be investigated as to its characteristics or origin, are placed in the palm of one hand and vigorously rubbed by the other until a burning sensation is experienced, we can ascertain the individuality of the oil by the smell from the flavor known to be peculiar to it and corresponding with that of one of the samples of known origin and purity. We can thereby tell if it is lard oil, cottonseed oil, tallow oil, fish oil, palm oil or rosin oil, etc., and by this method we can in many cases also ascertain which of these oils may be intermixed with another. When the oils are fresh and pure and carefully purified and bleached, their peculiar odor is not so readily noticed as when they are older; but by slightly heating between the hands we are enabled to recognize their characteristic odor.
Tasting oils will also enable us to ascertain their individuality, when we make comparison of their taste with that of the samples of oils, the purity and character of which is known to us.