Co Io rimet ric Determination of ChIo ride with Mercuric Chloranilate J. E. BARNEY II and R. J. BERTOLACINI Research Department, Standard Oil Co. (Indiana), Whifing, Ind.
b Chloride in aqueous solution can b e determined colorimetrically b y reaction with mercuric chloranilate to liberate reddish purple acid chloranilate ion. Methyl Cellosolve is added to lower the solubility of mercuric chloranilate and to suppress the dissociation of mercuric chloride, and nitric acid to give maximum absorbance. Interfering cations are removed with ion exchange resins. Sulfate, acetate, oxalate, and citrate d o not interfere. Bromide, iodide, iodate, thiocyanate, fluoride, and phosphate interfere; however, interference from the latter two can probably b e corrected for. Limit of detection is 0.2 p.p.m. o f chloride; precision and accuracy are within about 1 %; analysis time i s 30 minutes. ARIOUS anions may be determined colorimetrically with selected metal salts of chloranilic acid (2. 3). The metal chloranilate, itself only slightly soluble, reacts with the desired anion to form an insoluble or slightly dissociated salt, and liberates the highly colored acid chloranilate ion. The color is proportional to the concentration of the desired anion. Traces of sulfate are determined in this way b y reaction with barium chloranilate (a). A similar method for determining chloride mould have many applications. Silver chloranilate is unsatisfactory because it produces colloidal silver chloride ( 3 ) . This disability is not encountered in a rerent method for determining chloride ( I , 4 ) , based on the reaction
Hg(SCS)?
+ 2Fe++++ 2C1-+ HgCl? + 2Fe(SCS)++
because the mercuric chloride formed is soluble, although only very slightly dissociated. The reaction of mercuric chloranilate with chloride ion, HgCsC1204
+
2C1-
+ H' + + HC,Cl,Od-
HgCl?
which gives reddish purple acid chloranilate ion, mas selected for study. DEVELOPMENT OF METHOD
T'ariablcs investigated in the application of mercuric chloranilate to the determination of chloride included the
solvent system and the acidity. The wave length of maximum absorption and color stability of the acid chloranilate ion have been established ( 2 ) . Subsequently, the extent of interferences from cations and other anions was considered. Ethyl alcohol and methyl Cellosolve ( 2 - methoxyethanol. ethylene glycol monomethyl ether) were studied in the expectation that they would increase the sensitivity of the method by lowering the solubility of mercuric chloranilate and the dissociation constant of mercuric chloride. Ethyl alcohol had been successfully used as a solvent in previous studies ( 2 ) . Methyl Cellosolve is a highly polar solvent with a low dielectric constant and is miscible with water. Data for curves were obtained b y adding a n excess of mercuric chloranilate (0.2 to 0.3 gram) to solutions of ammonium chloride in the appropriate solvent, shaking the mixture for 15 minutes, filtering to remove the e-icess mercuric chloranilate, and reading the absorbance of the resulting solutions a t 530 mp ( 2 ) in I-em. cells with a Beckman Model B spectrophotometer a t a sensitivity setting of 2. The solvent systems are compared in Figure 1. I n neutral solution. 50YG ethyl alcohol had little effect on sensitivity, but 5070 methyl Cellosolve definitely increased it. I n a solution 0.05X in nitric acid, 507, methyl Cellosolve produced a large increase. Higher acidities should increase the sensitivity because the nbsorbance of solutions of chloranilic acid increases with decreasing pH ( 5 , 6) and reaches a maximum a t about pH 2. Studies with 50% methyl Cellosolve showed t h a t the highest sensitivity was obtained in 0.05-1- nitric acid solution. A t this acidity the reagent blank had a n abqorbance of 0.08 when measured against distilled water. Addition of acid t o 507, ethyl alcohol solutions caused d i p solution of the mercuric chloranilate and resulted in a large blank. PROCEDURE
1\Iercuric chloranilate, which is not available commercially, was prepared by adding dropwise a 570 solution of reagent grade mercuric nitrate in 270 nitric acid to a stirred 0.1% solution of
Eastman practical grade chloranilic acid a t 50" C., until no additional precipitate of mercuric chloranilate formed. After the supernatant liquid was decanted, the precipitate was washed three times by decantation with ethyl alcohol and once with diethyl ether, and dried at 60" C . in a vacuum oven. The final product was a greenish black crystalline solid with a pronounced metallic luster. For the determination of chloride, a n aqueous solution containing the ion is passed through a column 1.5 em. in diameter and 15 em. long containing Dowex resin 50 X 8, 20 to 50 mesh, hydrogen form. If the pH of the effluent is belom- 2 or above 12, it is adjusted t o 7 rrith dilute nitric acid or ammonium hydroxide and p H paper. To an aliquot containing not more than 1 mg. of chloride in less than 45 ml. of water in a 100-nil. volumetrii. flask are added 5 ml. of I S nitric acid, prepared from reagent grade nitric a d , and 50 ml. of methyl Cellosolve (Union Carbide Chemicals Co.). The mixture is diluted to volume n i t h distilled water, 0.2 gram of mercuric chloranilate is added, the flask is shaken intermittently for 15 minutes, and the excess mercuric chloranilate is removed by filtration. The absorbance of the filtrate is measured with a colorimeter or spectrophotometer a t 530 nip us. a blank prepared in the same manner. The chloride concentration is then obtained from a calibration curve prepared from standard ammonium chloride solution. RESULTS
Precision and accuracy of the method were established by analysis of standard solutions of ammonium chloride. The method was applied to the determination of chloride in water and in naphthas. -411 measurements were made with a Beckman Model B spectrophotometer a t a sensitivity setting of 2 in 1-cm. cells. The calibration curve was a straight line. Table I s h o w replicate analyses for chloride in six standard solutions containing 1 to 100 p.p.m. of chloride as ammonium chloride. The results possess a standard deviation and a relative error of 0.1 p.p.m. below 10 p.p.m. and a standard deviation and re1a t 'ive error of 1% above 10 p.p.m. Results of the analysis of five niunicipal n-aters, two well waters, and three lake waters for chloride b y the chlorVOL. 29, NO. 8, AUGUST 1957
1187
anilate colorimetric method are shown in Table 11. They were conipared with those obtained by the ferric thiocyanate colorimetric method ( 1 4 ); agreement is excellent. Table I.
Taken,
P.P.11. 1 0
3 0 10 0 25 0 61 100 122
Replicate Analyses of Known Chloride Solutions
z
0.6
a
c
METHYL
/%
0.5
0
m
Found, P.P.11, 096,11,11 30,31,32 9 9, 10 0 , 10 0 , 10 0 , 9 9 24 9 , 25 0 , 24 5 , 24 6 61, 60, 63, 62, 61 100, 101, 101, 101 120, 122, 123, 122, 122
m
a
0.4
0.3 0.2 0.1 n
0
Table II.
/
10
20
30
Chloride Content of Waters
Chloride Found, P - .P - .\T .- . Ferric Chlor- thioSource anilate cyanate Chesterton, Ind. 5 6 60 Chicago Heights, Ill. 4 8 4 2 Gary, Ind. 7 6 8 0 Hammond, Ind. 7 2 74 T'alparaiso, Ind. 7 8 76 2 7 2 4 Dyer, Ind., well South Holland, Ill., well 17 0 17 6 Lake Michinan 6 4 6 4 Lake George, Ind. 6 6 6 5 Kolf Lake, Ind. 14 9 15 0
40
50
60
70
80
90
100
C H L O R I D E , ?!?hi.
Figure 1.
Comparison of solvents
~~
The method was tested for use in combination n-ith lamp combustion for determining chlorine in naphthas in the parts-per-million range ( 1 ) . Synthetic standards mere prepared by adding chlorocyclohexane to n-heptane. They were burned in a lamp-combustion apparatus, and aliquots analyzed for chloride by the chloranilate and the ferric thiocyanate methods. Results. expressed in parts per million of ehloride, were: Ferric Chloranilate thiocyanate 5.7, 5.9 5.6, 5.7 1 1 . 9 , 11.9 11.6, 11.8 23.4, 23.3 2 3 . 6 , 23.1 Agreement is within the reproducibility of the two met.hods. INTERFERENCES
Cations interfere in the colorimetric method by forming insoluble chloranilates. The per cent error produced by 250 p.p.m. of 12 cations on the determinatipn of 25 p.p.m. of chloride was : 1 32 X a A Al+++ 3 C u + + 0 10 Fe++++ 100 NH4+ Ba+2 P b + + 100 Cat+ 6 K 6 Zn++ 37 Cd-+ 28 M g + + However, interfering cations are easily removed with the ion exchange resin; tests with solutions containing ferric and plumbous ions showed t.he error could be reduced to less than 1%.
1 188
ANALYTICAL CHEMISTRY
Because of the complete lack of interference by ammonium ion, calibration curves should be prepared with standard ammonium chloride solutions rather than sodium or potassium chloride solutions. Anions interfere more seriously in the colorimetric method for chloride than in the method for sulfate (S), because many react with mercuric ion to form insoluble or slightly dissociated compounds, or decompose the mercuric chloranilate. Ten anions a t a concentration of 250 p.p.m. were studied for interference in the method in the determination of 25 p.p.m. of chloride. Those which interfered seriously were studied a t one-tenth concentrations of anion and chloride. The per cent errors produced are listed in Table 111. Sulfate, acetate, oxalate, and citrate ions do not interfere. Bromide, iodide, iodate, thiocyanate, fluoride, and phosphate produce serious interferences, but the latter two can probably be corrected for, if their concentrations are known to only an order of magnitude. CONCLUSION
Determination of chloride in aqueous solutions b y this method can be carried out in 30 minutes. Limit of detection (based on the assumption that a difference of 0.005 absorbance unit can be detected with certainty) for 5-em. cells is 0.2 p.p.m. of chloride in the original sample. Other direct applications of the method include the determination of chloride in blood serum, wine, and waste waters. It could be adapted to the determination of bromide, iodide, and thiocyanate because they all form slightly ionized mercuric compounds. The chloranilate and ferric thiocyanate colorimetric methods are essentially equivalent in accuracy, precision, and ease of operation. Interferences from cations and anions are nearly the
Table 111.
Per Cent Error Caused by Anions
;inion, p.p.m. Chloride, p.p,m. FBrIBr03p04---
sc3.-~ -
so4--
25 25 -8 +11 $4
250 250 25 2.5 -11 -16 a a
-16 -44 +17
-65 -52
...
-1
0
a
-100 -52 n
I~
...
C2H302-(acetate) . . , 0 ... C104-- (oxalate) . . . -1 .. C6H507--- (cib rate) ... -1 a Solutions so highly colored that absorbance could not be measured.
same for both methods. The ferric thiocyanate method is slightly more sensitive; however, a correction must be made for chloride in the ferric alum used (4), whereas in the chloranilate method no correction is required. The particular application will determine the choice between the two methods. ACKNOWLEDGMENT
R. W. Fish assisted with the studies on interferences of cations and anions. LITERATURE CITED
(1) Bergmann, J. G., Sanik, J., ANAL. CHEM.29, 241 (1957). (2) Bertolacini, R. J., Barney, J. E., Zbid., 29, 281 (1957). (3) Coutinho, A. B., Almeida, M. D., Anais assoc. qufm. B r a d 10, 82 (1951). (4) Iwasaki, I., Utsumi, S.,Ozawa, T., Bull. Chem. SOC. Japan 25, 226 (1952). (5) Schwarzenbach, G., Suter, H., Helv. Chim. Acta 24, 617 (1941). (6) Tyner, E. H., -4NAL. CHEIU.20, 76 (1948). RECEIVED for review December 17, 1956. Accepted April 17, 1957. Eighth Conference on -4nalytical Chemistry and Applied Spectroscopy, Pittsburgh, March 1957.
