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 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 (1cc. = 0.1per 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 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 D R U G 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 fact 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 take 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 by 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
From the above it is obrious that the Talue of one division of the inimersion refractometer scale expressed in per 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 per 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 ~ hat 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.
T H E J O U R N A L OF IATDUSTRIAL -4ND E N G I N E E R I N G C H E B I I S T R Y
814
TABLEI.-THE EFFCCT OB TEXPERATKRE CHAKGEU P O K N-KO Formula P e r cent. n-no Expressed i n Immersion Reof Salt. of Salt. fractameter Scale Divisions 18°C. 2ooc. 23°C. 25°C. SaCl 0.68 3.00 3.04 3.02 3.0i 6.66 29.75 * 29.68 29.65 29.62
........ IZ2Cr,07 ...... 2.01
9.38 46.70
9.35 46.70
0.38 46.70
9.39 46.67
1.79 11.54
9.37 59.98
9.43 60.82
9.37 60.35
9.31 60.45
10.00
Sa2S203
......
,I.IlCl*
.......
1.61 6.71
8.47 37.51
8.39 37.50
8.35 37.45
5.51 37.52
Kclo,
.......
1.05 5.13
2.20 10.69
2.15 10.72
2.19 10.70
2.18 10.68
14.05 25.26
14.01 25.21
14.03 25.19
14.06 28.17
12.89 40.80
12.90 40.81
12.93 40.79
12.89
AgS@y
CaCl,
....... 5.19 10.46 ........ 2.23 7.06
40.82
Getmanl find 110 effect due to hydration. The analytical data which follo~vlater in tlie present paper tend to c o m ~ i t i the above work. The foregoing establishes the fact that c / w - no is, for ai: practical purposes, a constant independent of teniperature and unaffected by dissociation, hydration, and the formation of complexes. This constant represents the per cent by volume of salt equivalent to one division of tlie immersion refractometer scale. Several of these constants hare been worked out and are shown in the following table : TABLEII.-REFRACTIVE IBDEX FACTORS
. . . . . . . . . . . . . . . . .. .0.224 l i 2 C r 0 4 . . . . . .. . . . . . . . . . .0.153 K I . . . . . . . . . .. . . . . . . . .. .0.298 Na2S203 . . . . . . . . . . . . . .. .0.191 JillClz . . . . . . .. . . . . . . . .. .0.179 IiZCl'ZOi . . . . . . . . . . . . . .. .0.214 S H 4 N a H P 0 4 . . . . . . . . . .. .0.206 K C 1 0 . . . . . . . . . . . . . . . . .0.479 .\lgso,R . . . . . . . . . . . . . . .. .0.186 SaCl
.
Zn(C2H3O2)z . .0.242 TABLEIII.-DCTERJIINATION O F
. . . . . . . . . . . . . . . ... . . . . . . . . . . ...
AgS@3 . . . . . . . . . . . . . . . .0.370 IiSO3 .0.422 BaCI, . . . . . . . . . . . . . . . . .0.264 CaCI2 . . . . . . . . . . . . . . . ..0.173 CU(CzH30g)z .0.264 HgClz . . . . . . . . . . . . . . . . . .0.448 I L F e (CN ) . . . . . . . . . . . .0.191 KiSOe . . . . . . . . . . . . . . . . .0.321 ("4) zCz04 . .0.201
...........
A SINGLE SALT Ih A Q U E O U S T I O S BY THE I M M E R S I O X REPRACTOIIETER
..
SOLE-
-Refratti1 .e Index7 reP--- , cent. of Salt.-Formula of S a l t Water Solution Present Found Difference Sac1 . . . . . . . . . ..14.70 17.70 0.68 0.00 0.65 I~zCr04 .13.98 27.73 2.50 2.52 +0.02 Ia .14.20 18.60 1.33 1.31 -0.02 Ka&O3 .14.50 6.84 50.21 6.82 -0.02 MnC1, .13.98 51.00 6.71 6.63 -0.08 IZ2Cr20.14.10 37.57 5.05 5.02 -0.03 h'H4SahPO4 . , , .14.34 46.43 6.69 6.67 -0.02 r w m 8 . . . . . . . ..13.81 18.11 2.06 2.06 0.00 Zn(CzHjOz)2 ,. .14.20 4.59 33.30 4.62 +0.03 .4gN0~ .14.15 42.41 10.45 10.4G +O.Ol IiNOs .14.05 30.60 7.06 6.97 -0.09 BaClz .14.32 22.61 2.17 2.19 +o.oz CaL12. . . . . . . . . . . .. .14.80 44.25 5.21 5.18 -0.03 CUiC2HoOz)z . . . . .14.21 25.71 3.01 8.04 .14.13 HgCla 25.39 5.00 5.04 I i 4 F e ( C N ) , .,. , .14.33 64.48 9.61 9.55 -0.03 KZSo4 .14.33 23.95 3.07 3.09 +0.02 .14.46 i"4)2C204 30.94 3.34 3.31 -0.03 LkPPLICATTON OF hfETHOD SIXGLE S A L T - T ~ ~ concentration of all sohtioiis used in
........ ............ ........ ......... . . . . . ... ................... ..........
:;0$
......... . . . . .. . . . .
....
TABLE IT'.-DETERMINATIOh'
the following work is expressed as per cent by volume (g. solute per 1 0 0 cc. solution). All solutions were made up at 20°C. KO temperatures were read while taking the refractive indices ; instead, controls of distilled water were kept in the bath at the same temperatures as the solutions. Unless otherwise stated, refractive index refers to the immersion refractometer reading. I n all calculations the anhydrous form of the salt mas used. It is assumed that the salts are linown qualitatively in every case. Tables mag be conveniently used in the methods described in this paper. If the table is prepared for temperature t and it is desired to work a t temperature t', the readings may be correeted by subtracting the refractive index of water a t t' from the refractire index of the solution a t t' and adding the refractive index of water a t t, thus obtaining the reading of the solution at the temperature t . XISTURES O F TWO SALTS, OXE OP KNOWN APTD OXE O F UX-KXOWN C@XCENTR+4TIOX--The percentage of the salt of
known concentration divided by its refractive index factor gives in terms of scale divisions the effect on the refractive iiidex due to that salt. When this value is subtracted from the refractive index of the solution containing the two salts, the remainder is equivalent to the refractive index of the salt of unknown concentration in aqueous solution, and the percentage of salt present is determined in the manner explained before. Table I V gives illustrations of this application MIXTURE O F TWO SALTS, BOTH O F UNKROWN CONCENTRA-
wox-The procedure in this case is based upon that used by Leach and Lythgoe' in determining methanol in the presence of ethyl alcohol. This involves the use of the specific gravity or density of the solutions, and again we find a great lack of reliable data. However, it was found that a factor similar to the refra'etive index factor could be worked out. The specific gravities at 20°C./40C. of various solutions were taken, and the value f o r water (0.99823) was subtracted. The values thus obtained were then divided by the respectiPe percentages (by volume) of salt in the solutions. This gave the effect of the addition of 1 per cent of salt upon the density of the solution a t 20°C. Table V gives several of these constants. TABLEV.-DENSITY
.............. ............. ..............
. . . . . . . . ..0.00820
. . . . . . . . ..0.00602
. . . . . . . . ..0.00866 . . . . . . . . . .0.00753
. . . . . . . . ..0.00585 . . . . . . . . . . .0.00817 . . . . . . . . ..0.00652 . . . . . . . . ..0.00778
. . . . . . . . . ..0.00470
DifP: 47.22
of Known ConcentrationAmount Calculated Present Effect on Formula P e r cent' Ref. Index NaCl ,4.67 20.85 NaCl .2.33 10.40 NaCl .1.40 6.25 NaCl .1.17 5.22 NaCl .3.74 16.70
13.91 13.91 13.91
32.84 29.27 39.18
18.93 15.36 25.27
13.88 14.00
36.79 29.30
22.91 14.70
14.06 14.05
31.18 42.74
17.13 28.69
15.15 15.15
25.43 24.75
10.28 9.60
AgNO, AgPTOa
14.40 14.40
23.37 25.00
8.97 10.60
("4)2C204
cit.
20°C
OF ONE SALT I N A MIXTURE OF T W O SAmX BY MEANS OF THE IMMBRSION RERRACTOXETER, THE S E C O N D SALT UEING DETERMINED BY OTHER MEANS
............. ........... ........................ ............ NazS20s . . . . . . . . . .1.95 NazSeOs . . . . . . . . . .0.99 Na2S203 . . . . . . . . . .3.96 MnCI, . . . . . . . . . . .3.35 MnCl, . . . . . . . . . . . l . l 6 Na2SpOe . . . . . . . . . .2.04 NazSZO3 . . . . . . . . . .1.02
' Lce.
FACTORS FOR
K I . . . . . . . . . . . . . . . . . 0.00118 . NazSz08 .0.00791 .0.00821
-Salt
-Refractive Index Water. Mixture. 13.48 60.70 13.98 38.00 14.23 28.83 14.39 33.42 14.39 39.29
Vol. 13, No. 9
24.02 14.60 19.03 24.90
. . . . . . . . . . ..2.61
. . . . . . . . . . . .1.3a (NH4)sCeOi . . . . . . . .0.73
........ 1 . 6 7
10.21
5.18
20.73
18.72 6.48 10.68 5.34 7.06 3.63 3.64
8.31
of Unknown Concentration.--Effect on -4mount Amount Ref. Index Present, Found, Formula Due t o P e r cent P e r cent. 5.00 4.83 Ii2CrOL ........ .26.37 .13.62 2.50 2.49 K2CrO4 IC2CrOs 8.35 1.53 KzCrOa. . . . . . . . .13.81 2.53 2.50 1.60 1.50 K2Cr01 ........ 8.20
IOKST THIOSULFATE IS
OR P O T A S S I U X IODIDE.AKD A Q u ~ o v sJ I I X T U R E
SODILhI
as
O~SERVED DATA: Specific g r a v i t y 20'/4". . . . . . . . . . . . . . . . immersion refractometer reading of m i s t Iiiimersioii refractometer reading of n a t Tile obserred specific g r a v i t y minus t h e specific graTitj- of w a t e r , 1,02729 - 0.99823 = 0.02974. divided b y t h e respective specific g r a T i t s f a c t o r s of t h e s a l t s (Table Pi,gives t h e percentage concentration of single salt solutions h a r i n g t h e s a m e specific g r a r i t y as t h e mixture. 0.02074/0.00i18 = 4.14 per cent. potassium iodide 0.02074/0.00791 = 3.74 per cent sodium thiosulfate The per cent. of salt calculated froin t h e specific gral-ity divided b y t h e a p p r o p r i a t e r e f r a c t i r e index factor (Table II), 4.14/0.208 = 13.90 3.74/0.191 = 19.56 ulmn t h e addition of tlie observcd reading f o r w a t e r , g i r e s t h e ref r a c t i v e index reaclings of single s a l t solutions h a v i n g t h e same specific graT-ity a s t h e mixture. 13.90 13.91 = 27.81, refractometer reading f o r IC1 19.66 13.91 = 33.47, refractometer reading f o r Xa25203 The difference in refractometer readings of t h e respective salts, 33.47 - 27.81 = 5.66 ,"... .,lvided i n t o t h e difference between t h e obserx-ed refractometer readi1.g a n d t h e r e a d i n g for sodium thiosulfate alone, 33.47 - 29.27 = 4.20 y i r e s t h e proportion of potassium iodide i n t h e m i x t u r e t a k e n as UnltJ-. 4.20/5.66 = 0.742 potassium iodide 1.000 - 0.742 = 0.258 sodium thiosulfate Referring back to t h e possible content of each salt calculated from t h e specific gravitj-, a n d multiplying each by t h e i r respective iirbportional p a r t s j u s t found, w e ' h a v e : 4.14 X 0.742 = 3.07 per cent potassium iodide 3.54 X 0.268 = 0.96 per c e n t sodium thiosulfate
+ -+
__
4 . 0 3 per cent t o t a l s a l t s .
I f the proclncts of the refractive index and specific grayity factors lie close together, a n error in the determination of refractive inclex or specific gravity causes a larger error in the final result than if they lie farther apart. The above method offers a rapicl means of analyzing some mixtures which cause considerable difficulty when determined by ordinary analytical methods. Table- VI gives a suniinary of a large number of determinations made by the aboye method. The range of concentration of the components is s l i o ~ ~as i , well as tlie maximum and minimum variation of the results froin the truth. TABLE VI
Combinations of S a l t s Used
Difference Between Concentration Range of Conceiitratioii Found and t h a t of Components K n o w n t o be P r e s e n t Max. hfin. &lax. Min. P e r cent P e r cent P e r cent. P e r cent
k2Cr04+NaC1 . . . . . . . . . 5 0 0 1.17 K I f K a 2 S 2 0 3 . . . . . . . . . . . .3.96 0.99 NaClfKI ,285 0.94 KaCr04 Va2SpOs . . . . 5 . 0 0 0.99 >InC12+3
