Seat.. 1921
T H E J O U R N A L OF Ih’DUSTRIAL A N D ENGINEERING C H E M I S T R Y
813
A Rapid Method for the Determination of Sodium Chloride in Soap’ By H. C. Bennett Los ANCELESSOAPC o x P A v Y , Los AXCELES,CALIFORNIA
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 o r 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
o r 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. T h e 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 b y ten (1cc. = 0.1p e r cent). Results by this method have been checked in this laboratory time after tiine against the usual standard method aiid h a r e always agreed to within 0.01 o r 0.02 p e r 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. S o r t h r u p , who performed the laboratory work necessary t o demonstrate the feasibility of the method.
The Application of the Immersion Refractometer to the Analysis of Aqueus Salt Solutions’ By C. A. Clemens TERJIILION, SOUTHDAKOTA SOT-TI%DAKOTASTATE FOOD AXD DRUG LABORATORIES,
I n the past the immersion refractometer has been little used in the quantitative analysis 0; inorganic salt solutions, and then largely with solutions containing but a single salt. This f a c t may be cxplained by the lack and unreliabilitg of data o n refractive indices. Practically all data upon aqueous salt solutions prepared f o r quantitative work have been worked out and compiled by Wagner.5 They t a k e the form of tables worked out f o r definite temperatures, necessitating the accurate control of temperature, which is a serious drawback in ordinary analytical work. I n this paper it is proposed to demonstrate that the immersion refractometer furnishes a very rapid and, at the same time, fairly accurate method f o r the determination of soluble salts in aqueous solution, both singly and in simple mixtures; and, furthermore, that the use of tables can be avoided b y the deterinination of a constant f o r each salt, which constant is applicable a t ordinary rooin temperatures. THE R E r R I C T 1 k . C
IXDEX FACTOR
Jones and Getman6 measured t h e refractive indices of several solutions, and in every case they found the refractire index to be a linear function of the concentration. Hallcvachs7 and Bender8 obtained practically a constant value for IReceived J u n e 8. 1921. Z T JOURNAL, ~ ~ f,l (1919) ~ 786. 3 Van Yostrand’s ChemicLl Annual.” 4 t h Ed. Ileceived April 2 1921. u b e r q u a n t i t a t i d Bestimmungen wiisseriger Lcisunpen mit Clem Zeiss”schen Eintauchrefraktometer,” Sonderhausen, 1907. BAm. Chew?. J., 31 ( 1 9 4 ) , 303. 7 Wied. A ~ I L4 . 7, ( 1 8 9 2 ) , 380. 8Zbid., 39 (1690), 90.
-7Z0 -
C
nhere n =refractive index of s c h t i o n n 3=refractive indcx of imter at t h e same temperature c=grarr,s solute per 100 cc. solution
F r o m the above i t is obrious that the Talue of one division of the inimersion refractometer scale expressed in p e r cent by volume (g. in 100 cc. of solution) is equivalent to the the >slues of 9% and no being exreciprocal of n--n,/e, pressed i n terms of the immersion refractometer scale, and that this value is a constant. It should be noted that when p (g. solute p e r 100 g. solution) is substituted f o r e, the values obtained increase with increased concentration. Robertson’ has morked out such a constant f o r caseinates, while Zwick and Lalin3 established factors f o r tannins and starch, respectively. Robertson finds that “ the difference between refractive index of the solution and that of water a t the same temperature remains appreciablx constant.” Salt solutions were tested a t different temperatures, and it was found that the above statement held true f o r thein as well as f o r caseinates. Cheveneau4 has shown t h a t ionizatioii has n o sensible influence on refractive index; Rimbach and W i i ~ t g e n ,t ~h a t complexes have no measurable effect, vhile Jones aiid 1 J .
2
Plzbs. Chew?., 13 (1909), 4G9.
C h e n & . - Z t g ,32 ( 1 9 0 9 ) , 406. ges. Braznw, 32 (19101, 231. Z. plaustk. Chem., 74 (19101,233.
3 2 . 4
2
Compt. r e n d , 150 (1910),866.
