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 Twitchell 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 titrate 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 potassium 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 assumes 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 analyzed 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 mass 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 mass 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 mass 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 analysis, the acetone is completely evaporated and the mass 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 quantity 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 titrating this solution with half normal alkali the difference between the half normal acid used in decomposing the soap and alkali used in titrating 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 titrated for alkalinity using methyl orange as an indicator.