The determination of the reichert-meissl number is the most important of the chemical processes applied to butter fat analysis.
502. Saponification Value and Reichert Number.—It may often be convenient to make the same sample of butter fat serve both for the determination of the saponification value and of the reichert number. For this purpose it is convenient to use exactly five grams of the dry filtered fat. The saponification may be accomplished either under pressure or by attaching a reflux condenser to the flask as suggested by Bremer.[503] When the saponification, which is accomplished with alcoholic potash lye containing about 1.25 grams in each ten cubic centimeters of seventy per cent alcohol, is finished, and the contents of the flask are cooled, the residual alkali is titrated with a set sulfuric acid solution, using phenolphthalein as indicator. When the color has almost disappeared, an additional quantity of the indicator is added and the titration continued until the liquid is of an amber tint. A sample of the alkali, treated as above, is titrated at the same time and from the two sets of data obtained, the saponification number is calculated as indicated in paragraph ([345]).
A few drops of the alcoholic lye are added to the contents of the flask and the alcohol removed by evaporation. The residual soap and potassium sulfate are dissolved in 100 cubic centimeters of recently boiled water, some pieces of pumice added, and the volatile acids removed by distillation in the usual way after adding an excess of sulfuric acid. It is important to conduct blank distillations in the same form of apparatus to determine the magnitude of any corrections to be made. The size of the distilling flask and the form of apparatus to prevent mechanical projection of sulfuric acid into the distillate should be the same in all cases.
503. Modification of the Reichert-Meissl Method.—Kreis has proposed the use of strong sulfuric acid for saponifying the fats, the saponification and distillation being accomplished in one operation. A source of error of some inconvenience in this method is due to the development of sulfurous acid by the reducing action of the organic matter on the oil of vitriol. Pinette proposes to avoid this difficulty by adding, before the distillation is begun, sufficient potassium permanganate to produce a permanent red coloration. By this means the sulfurous acid is completely oxidized and its transfer to the standard alkali during distillation entirely prevented. The same result is accomplished by Micko by the use of potassium bichromate. The details of the manipulation are as follows:[504]
About five grams of the fused fat (butter or oleomargarin) are placed in a flask of approximately 300 cubic centimeters capacity. After cooling, there are added ten cubic centimeters of sulfuric acid containing three grams of water to each ninety-seven grams of the strongest acid.
The fat and acid are well mixed by a gentle rotatory motion of the flask and placed in a water bath at a temperature of 35° (circa) for fifteen minutes. At the end of this time the flask is removed from the bath and 125 cubic centimeters of water added, little by little, keeping the contents cool. Next are added four cubic centimeters of a four per cent solution of potassium bichromate. The contents of the flask are vigorously shaken and, after five minutes, a solution of ferrous sulfate is added gradually from a burette until the reaction with a drop of potassium ferrocyanid shows a slight excess of the iron salt. The volume of the liquor in the flask is then increased to 150 cubic centimeters by the addition of water and 110 cubic centimeters distilled. After mixing and filtering through a dry filter, the acid in 100 cubic centimeters is determined by standard tenth normal barium hydroxid solution and the number thus obtained increased by one-tenth representing the total acid obtained.
504. Elimination of Sulfurous Acid.—Prager and Stern[505] propose to eliminate the sulfurous acid by a stream of air, succeeded by one of carbon dioxid, and proceed as follows: Five grams of the butter fat are brought into a liter flask, ten cubic centimeters of strong sulfuric acid are added and the flask is kept for ten minutes at 30-32° with constant agitation. When the liquid is cold, air is bubbled through it until the odor of sulfurous acid has disappeared. One hundred cubic centimeters of water are added, with precautions against rise of temperature, and carbon dioxid is bubbled through for ten minutes. This is then displaced by a stream of air for another ten minutes, the delivery tube is washed into the flask with fifty cubic centimeters of water and the distillation is effected. The following results are quoted:
Cubic centimeters of tenth normal alkali required by five grams of butter fat:
| Reichert-Meissl. | Prager-Stern. | ||
| Sample | a | 29.86 | 29.60 |
| ” | b | 30.23 | 29.65 |
| ” | c | 28.34 | 27.76 |
| ” | d | 28.20 | 28.10 |
The authors do not comment on the possibility of loss of acids other than sulfurous in the stream of air, but they admit that further investigation is requisite to render the suggestion of Kreis serviceable.