Ag, AgCl/[Cl-] = O.OlM, [NO,-] = 0.09MITHAN03 0.05Min benzene [NO3-] [Cl-] = O.lM, [NO3-] = variablejAgC1, Ag with P2.l = 40 and V(mV) = -32.8 59 log ([Cl-] 40 WO3-1)
+
+
+
The data of Figure 2 show that in the case of the determination [NOB-] = O.lM, the of nitrate ions in a medium [Cl-I concentration of the membrane to be used is dictated by the order of magnitude of the nitrate ion concentration in the solution. In fact for a nitrate ion concentration of the order of 10-2M, system (a) gives a AV of 13 mV for AC = 1.10-22.10-2M, while system (b) gives a AV of 15 mV. In this case, since both values 13 mV and 15 mV can be read with the same precision, it can be worthwhile to use system (a). This system, which makes use of a T H A N 0 3 0.3M benzene solution represents a low impedance electrode with respect to system (6). When the nitrate concentration is of the order of 1 X 10-4M, the two systems give AV = 0.5 mV
+
(system a) and AV = 1 mV (system b) for a AC of 1.10-42.10-4M. It is clear in this case that system (b) is more convenient than system (a) despite its greater impedance due to the use of a THAN03 0.05M benzene solution. Therefore the choice of the liquid electrode concentration depends on the required value of the selectivity factor P2.1.The examples discussed show that a reasonable compromise between impedance and precision requested can be worked out when the characteristics of the electrode are well known. ACKNOWLEDGMENT
The authors thank Mr. F. Salvemini for his contribution in the membrane potential determinations and Mr. M. De Carolis for performing the dielectric constant measurements. RECEIVED for review April 9, 1971. Accepted July 23, 1971.
Polarographic Determination of Chloride, Cyanide, Fluoride, Sulfate, and Sulfite with Metal Chloranilates Ray E. Humphrey and Clyde E. Laird Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340 RELATIVELY INSOLUBLE METAL chloranilates have been used for the spectrophotometric determination of a number of anions. The less soluble or undissociated compound of the metal with the respective anion is formed releasing the absorbing chloranilate species into solution. For example, the determination of chloride involves the use of mercuric chloranilate as shown in Equation 1. HgCh
+ 2C1-
+ HgC12
+ Ch2-
(1)
The chloranilate ion released, represented by Ch2-, absorbs at 525 nm and 330 nm. Chloride, (1, 2) cyanide (3, 4) and sulfite (4) have been determined spectrophotometrically using mercuric chloranilate, fluoride has been measured using thorium chloranilate (5), and for sulfate ion barium chloranilate has been employed (6). Chloranilic acid and chloranilate ion undergo a reversible two-electron reduction at the dropping mercury electrode (7). In this work, the current from the reduction of the released chloranilate ion was measured and related to the concentration of the anion sought. No report was found in the literature for the determination of chloride by means of a polarographic reduction current. Cyanide has been determined polarographically by measuring the reduction current for mercuric cyanide (8). Sulfite is converted, by (1) J. E. Barney I1 and R. J. Bertolacini, ANAL.CHEM., 29, 1187 ( 1957). ( 2 ) R. J. Bertolacini and J. E. Barney 11, ibid., 30, 202 (1958). (3) E. Hoffmann, Z . Ami. Chem., 185, 372 (1962). 43, 1100 (1971). (4) R. E. Humphrey and W. Hinze, ANAL.CHEM., (5) A. L. Hensley and J. E. Barney 11, ibid., 32, 828 (1960). (6) R. J. Bertolacini and J. E. Barney 11, ibid., 29, 281 (1957). (7) J. Weissbart and D. Van Rysselberghe, J . Phys. Chem., 61, 765 (1957). (8) J. S . Hetman, Lab. Practice, 10, 155 (1961).
reaction with hydrogen ion, to sulfur dioxide which is polarographically reducible (9, 10). Fluoride has been determined by the decrease in the reduction current for Fe3+(11) or the A13+-Solochrome Violet RS complex (12) on adding this ion to a solution containing one of these species. Apparently, very little has been done in the way of a metathesis reaction to exchange chloride, cyanide, fluoride, or sulfite for a polarographically reducible ion. Sulfate has been determined polarographically by reaction with barium chromate to yield the reducible chromate ion (13). EXPERIMENTAL
Polarography. Polarograms were recorded with a SargentWelch Model XVI Polarograph in conjunction with a Sargent Constant Head Dropping Mercury Electrode assembly. A conventional H-cell, with a saturated calomel electrode (SCE) on one side, was employed. The drop time of the capillary electrode was in the range of 4-5 seconds. Solutions were purged with nitrogen prior to measurements. Polarograms were recorded for all of the reaction solutions. The maximum current value was used in establishing calibration plots and obtaining recovery data. Maximum pen travel was determined at the appropriate potential. For the lowest concentration, a current sensitivity setting of 0.01 pA/mm was used. Reagents and Solvents. Barium chloranilate and methyl cellosolve were Matheson, Coleman and Bell products. (9) D. B. Aulenbach and J. C. Balmat, ANAL. CHEM., 27, 562 (1955). (10) L. L. Ciaccio and T. Cotsis, ibid., 39, 260 (1967). ( 1 1 ) C. E. Shoemaker, ibid., 27, 552 (1955). (12) B. J. MacNulty, G. F. Reynolds, and E. A. Terry, Airalyst, 79, 190 (1954). (13) J. Mayer, E. Hluchan, and E. Abel, ANAL.CHEM., 39, 1460 (1967).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1895
M
x
104 0.5 3.1 10 20
Chloride' (PPm) (1.8) (11) (36) (71)
PAb 0.24 1.29 4.12 8.10
Table I. Current-Concentration Data Cvanidea M x 104 PA= 0.5 3.0 8.1 30
104
1.0 3.0 5.0 10
(PPm)
CtAf
(1.9) (5.7) (9.5) (19)
0.22 0.65 1.04 2.06
M
x
x
104
1 .o 5.0 10 20
0.19 1.15 3.10 13.3
Fluoride8
x
M
Sulfite" M
1.o 3.0 8.0 10
(8.0) (40) (80) ( 160)
Ct Ad 0.24 2.10 4.48 9.94
Chloranilate ion
-
104
(PPm)
PAh
M X 104
(PPd
pAi
0.29 0.72 2.51 3.11
0.5 1.0 3.0 10
(10.5) (20.9) (62.7) (209)
0.36 0.63 1.85 5.85
Mercuric chloranilate used. E = 0 volts us. SCE. C E = -1.5 volts US. SCE. d E = -0.7 volts US. SCE.
Thorium chloranilate used, volt US. SCE. Barium chloranilate used. E = -0.5 volt US SCE. E = -0.7 volt US. SCE.
' E = -0.6
0
Chloranilic acid and mercuric chloranilate were from Eastman Organic Chemicals. Thorium chloranilate was obtained from Fisher Scientific Co. Ammonium chloride, potassium cyanide, sodium fluoride, sodium sulfate, and sodium sulfite were used to provide the appropriate anion. These chemicals, and all others used to prepare buffers or for supporting electrolytes, were the best available reagent material. Buffers and Electrolytes. A phosphate buffer, pH 6.86, was prepared using KH2P04 and Na2HP04 in the proper amounts. The buffer with pH 4.75 was made using acetic acid and scdium acetate. Sodium acetate alone was used in the cyanide analysis while N a N 0 3 and H N 0 3 were employed in the chloride work. General Procedure. A small amount of the appropriate metal chloranilate was added to the solutions of the anion being determined. The solution and solid were stirred for 15-20 minutes and the solid was then removed by centrifuging and, in some cases, filtering. A buRer or electrolyte was then added to those solutions which did not contain these substances. The solutions were placed in the H-cell, purged with nitrogen, and the maximum diffusion current measured at the required applied potential. A blank was run for each procedure to correct for the current due to the solubility of each metal chloranilate. Determination of Chloride. The solvent used for this anion was 1 :1 (by volume) methyl cellosolve-water which was 0.1N in HNOl and 0.5M in NaN03. Both electrolytes were present when the mercuric chloranilate was added. Current was measured at -0.5 volt us. SCE. No wave is observed for chloranilate ion as the reduction occurs at a positive potential. The chloride concentration ranged from 1 X lO-5M to 5 X 10-3M. The current for the blank for analyses with mercuric chloranilate could be minimized by washing with aqueous ethanol (14). Determination of Cyanide. The standard solutions of KCN were prepared in 50% aqueous ethanol. The solid mercuric chloranilate was added and stirring continued for 15 minutes. After removal of the mercuric chloranilate, solid sodium acetate was added so that the solution was 0.1M in this salt. After purging, the maximum current was measured at -1.5 volts US. SCE. The cyanide Concentration range was from 1 X l W 5 Mto 5 X 10-3M. (14) C. F. Hammer and J. H. Craig, ANAL.CHEM., 42 1588 (1970).
1896
*
Determination of Fluoride. A small amount of thorium chloranilate was added to the sodium fluoride solutions in a mixed solvent of 25% methyl cellosolve-75% water, 0.1M in acetic acid and sodium acetate. After removing the excess thorium chloranilate and precipitated thorium fluoride, the solution was placed in the H-cell, purged, and the current measured at -0.6 volt us. SCE. The fluoride concentration was from 1 X lO+M to 5 X 10-3M. Determination of Sulfate. The solvent used for this analysis was 50% aqueous methyl cellosolve which was 0.01M in acetic acid and sodium acetate. A small amount of barium chloranilate was added and the mixture stirred for 30 minutes. The maximum diffusion current was measured at -0.5 volt us. SCE. Sulfate was determined over the range of 5 X 1 0 - 5 ~ t o1 x i o - 3 ~ . Determination of Sulfite. Sodium sulfite solutions were prepared in distilled water which contained 5 % glycerol to retard oxidation of the anion. A small amount of mercuric chloranilate was added and stirring continued for 15 minutes. After removing the excess mercuric chloranilate, a mixture of solid Na2HP04.12H20 and KH2P04 was added to produce a pH of 6.86. The maximum current was measured at -0.7 volt cs. SCE. The useful concentration range is from 5 x 1 0 - 5 to ~ 5 x 10-3~. Chloranilic Acid. Polarograms were obtained for chloranilic acid in the pH 6.86 phosphate buffer and in the acetic acid-sodium acetate buffer with pH 4.75. The current concentration plots were linear over a concentration range o f 5 x 1 0 - 5 ~ t o5 x 1 0 - 3 ~ . RESULTS AND DISCUSSION
Apparently the most common application of chloranilate salts in the analysis for anions involves measuring visible or ultraviolet absorption. A polarographic procedure could provide a useful alternate method where samples contain other absorbing species or where higher concentrations are present. The chloranilate method allows the use of readily obtainable reduction current for these ions. Also, the chloranilate ion seems to be stable for several hours. Reproducible results were obtained at rather low concentrations where very small currents were measured. The highest sensitivity setting of the Polarograph, 0.01 pA/mm was used and the maximum current measured. Current-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
Table 11. Recovery of Anions from Synthetic Samples Cyanide Fluoride Chloride Present Found Present Found Present" Found 1.78 3.55 17.8 35.5
1.30 2.60 13.0 26.0
1.77 3.55 17.8 36.2
1.31 2.60 12.5 26.1
Sulfate
0
0.95 1.90 9.50 19.0
Sulfite
4.0
0.96 1.90 10.1 19.7
,
3 .O
Present
Found
Present
Found
4.81 9.61 48.1 96.1
5.28 9.60 47.3 95.5
4.01 8.01 40.1 80.1
4.12 7.81 41.8 82.4
w
2.0
Parts per million.
Table 111. Current Sensitivity Data Anion i d p 4.3 c1CN4.2 F2.1 S032so42-
1.c
4.3 3.1 6.0
C
Ch2wA/mmole/l. The concentration value used is that of the given anion. a
20
I
I
I O
6 0
I
I
80
ANION, ~ p m
Figure 1. Calibration plots for anions Applied potential values are given in Table I
concentration plots were reasonably linear as shown by Figure 1 and the data in Table I. Reproducibility was reasonably satisfactory, as indicated by the data on recovery from synthetic samples presented in Table 11. The current obtained for these anions depends primarily on the extent of the reaction with the relatively insoluble metal chloranilate. However, a second reducible species, the mercury compound of the anion, is obtained with chloride, cyanide, and sulfite which increases the current measured for these substances. Relative current sensitivities are shown in Table I11 as diffusion current constants and compared to the value for chloranilic acid. These values give some indication of the extent of reaction and applicable concentration levels which could be measured with some accuracy. Since two chloride or cyanide ions are required to release one chloranilate ion, it is apparent from the sensitivity values in Table 111 that the slightly dissociated mercuric chloride and mercuric cyanide are making an appreciable contribution to the current measured in determining these anions. Although separation of the excess chloranilate salt by centrifugation or filtration was always done in this work, it probably would not be necessary in many instances. For determining CI-, F-, and Sod2-, where the electrolyte or
buffer was present during the reaction of the anion with the metal chloranilate, probably no separation is needed. In other instances, a high blank resulted if a buffer system was present during the reaction due to the somewhat high solubility of the metal chloranilate. For this reason the buffer components were added after centrifuging or filtering. It is entirely possible that a suitable buffer or electrolyte could be found for each reaction so that no centrifuging or filtering would be necessary. Eliminating this step might make the polarographic measurement advantageous in some instances compared to spectrophotometric methods. In the work with chloride ion the current plateau was achieved at a positive potential cs. the SCE since the hydrogen ion concentration was rather high. Maximum current was flowing a t zero volts us. SCE. It seems probable that the solutions resulting from the reactions of some of the other anions with the metal chloranilate could be made sufficiently acid so that the current could be measured with no applied potential. Possibly other complicating factors might arise, especially with cyanide and sulfite. However, for those reactions where this is feasible, the equipment need is minimal. RECEIVED for review May 19, 1971. Accepted July 23, 1971.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1897
