A Rapid Method for the Determination of Sodium Chloride in Soap.

Abstract

The usual method f o r the determination of sodium chloride in soap calls f o r the decomposition of the soap by sulfuric acid, and the i-olumetric estimation of the sodiuiii chloride by titration with 0.11-silrer nitrate, using liotassiuni chromate as a n indicator.'When one is called upon to analyze soaps containing a large amount of silicate of soda or insoluble fillers such a s silica or talc, this inethod proves to be s'omewliat difficult of manipulation. The method described below, however, is not open to this objection and is quick and accurate. For this reason also it is valuable i n routine control analyses in soap works laboratories. d complete deterinination can be easily made in 7 min.The method depends upon the precipitation of the soap and admixed soluble fillers in a water solution by the addition of magnesium nitrate. The sodiuiii chloride may then be titrated immediately with silver nitrate solution without filtering off the precipitated magnesium soap and other insoluble compounds. The indicator used is potassium chromate.Magnesium nitrate was selected as the best precipitating agent to use became of the fact that it does not forin a n insoluble product with the potassium chromate, which is later used as a n indicator in the titration. Solubility tablesd show that magnesium chromate is very soluble in either hot or cold water, \\Thereas calcium chromate is only moderately soluble in cold water, and much less so in hot water. Barium and strontium chromates are insoluble.The method, in detail, consists of dissolving 5.65 g. of soap in 150 cc. of hot water in a beaker, boiling if necessary to effect solution of all solulile components. The soap and soluble fillers a i e precipitated out of solution by the addition of 25 cc. of a 20 per cent solution of magnesium nitrate (crystals). Without cooling, the unfiltered mixture is then titrated against 0.U' silver nitrate solution, using a small amount of potassium chromate a s a n inclicator. The appearance of a reddish brown color is the end-point, and the percentage of sodium chloride is read directly from the buret by dihiding the number of cubic centimeters by ten(1 cc. = 0.1 per cent).Results by this method have been checked in this laboratory time after tiine against the usual standard method aiid hare always agreed to within 0.01 o r 0.02 per cent. Cooling the mixture before titrating increases the accuracy of the method. Soaps to which k n o v n amounts of sOdiUllJ chloride u-ere added have also been made in the laboratory, and the analysis by this method gave coiicordant results.The author is indebted to Mr. E. L. Sorthrup, who performed the laboratory work necessary to demonstrate the feasibility of the method.

Standard Methods for the Sampling and Analysis of Commercial Soaps and Soap Products, Revised

“…(Any blank from the petroleum ether must be deducted from the weight before calculating the unsaponified and unsaponifiable matter.) (4) Unsaponifiable matter. Weigh 5 grams ( ±0.2 gram) of the prepared sample into a 200-cc.…”

“…dish or casserole containing the solution should be covered with a watch glass while adding acid so as to avoid loss by spray. (2) When the material contains mineral matter insoluble in water, or a determination of highest accuracy is not necessary, take a portion of the solution after titrating the matter insoluble in alcohol C-II (4) containing not more than 0.2 gram of silica and add 5 to 10 cc. of strong hydrochloric acid.…”

Standard Methods for Sampling and Analysis of Commercial Soaps and Soap Products, Revised

“…(A trier sample is obtained by inserting a trier into the material. A trier is a half-round steel cylinder 0.5 to 0.75 inch in diameter, 6 to 36 inches in length, pointed on one end, and having a grip handle on the other end. After insertion, the trier is turned 2 or 3 times, and upon removal a core of the material being sampled is obtained.)…”