platinum to the ratio of the cross sectional areas of the metal and platinum atoms. For gold and platinum with atomic radii of 1.44 A and 1.38 A, respectively, this ratio is 1.09. The latter value compares well with the experimental result, 1.05. Comparisonwith Bulk Gold. We repeated our experiments under oxidation conditions corresponding to those of Brummer and Makrides (2), i.e., oxidizing the electrode at +1.20 V for 5 minutes and then scanning between +1.20 V > E > -0.26 V. On plotting these data as in Figure 2, we obtained an average oxidation state of the gold film of 1.92 us. Brummer and Makrides’ value of 2.0. Hence, it would appear that
thin gold deposits, 8 < 0.35, oxidize in a manner similar to bulk gold. The agreement between these two methods, which differ quite widely in approach, leaves little doubt that the average oxidation state of gold is Au(1I) at f1.20 V, and that only a true surface film has formed. These results strongly imply that the oxygen atoms are adsorbed rather than that a phase oxide layer is formed.
RECEIVED for review May 5, 1972. Accepted July 10, 1972. The support of the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 70-1832 is gratefully acknowledged.
Electrode Indicator Technique for Measuring Low Levels of Cyanide M. S . Frant, J. W. Ross, Jr., and J. H. Riseman Orion Research Incorporated, I 1 Blackstone Street, Cambridge, Mass. 02139 A RECENT PUBLICATION (1) has studied the response of the silver specific ion electrode (2) in solutions containing silver and cyanide ions. The authors suggest that a trace cyanide determination might be based on electrode measurements in samples to which a silver indicator has been added, and refer to unpublished data from our laboratory, where we have developed a cyanide monitoring system which uses a silver indicator in a manner similar to that suggested by the authors. The method we have developed, although intended for use in a monitoring system, applies equally well to a single sample laboratory analysis, and has many advantages over the standard trace cyanide methods presently being used (3, 4 ) . In view of the widespread interest in trace cyanide determinations in waste water from metal plating, steel coking, and other processing installations, we wish to report our method as modified for individual sample analysis, using a silver ion electrode, and a simple masking procedure for removing metals, such as nickel, which form stable cyanide complexes. THEORY
The cyanide ion electrode ( 5 ) can be used for cyanide determinations down to about 10-5M (about 0.3 ppm), and will respond to cyanide complexed to some metals, such as zinc or cadmium (6). Further, an ion-exchange procedure has been suggested for removing copper from cyanide solutions before making electrode measurements (6). However, most regulatory agencies require minimum cyanide levels of 0.1 ppm or lower. This is below the operating level of the (1) B. Fleet and H . Von Storp, Anal. Leu., 4,425-35 (1971). (2) J. W. Ross, Jr., and M. S. Frant, Abstracts, Eighth Eastern Analytical Symposium, New York, N.Y., Nov. 1966, p 30. (3) Method D-2036-68, American Society for Testing and Materials, “1970 Annual Book of ASTM Standards,” part 23, ASTM, Philadelphia, Pa. (4) R . Weiner and C . Leks, Galraizotechnik, 62, 366-75 (1971). ( 5 ) J. W. Ross, Jr., in “Ion Selective Electrodes,” R. Durst, Ed., NBS Special Publication No. 314, U S . Government Printing Office, Washington, D.C., 1969. ( 6 ) M. S. Frant, Plating. 58, 686 (1971).
cyanide electrode, which also suffers from the drawback that it is itself slowly attacked by cyanide solutions. To reach lower operating levels and to have extended electrode life, we looked for methods which would use the silver sulfide membrane electrode. A variety of analytical approaches were tried using silver indicator solutions. Direct determinations of trace cyanide as suggested in ( I ) were of poor precision because of the difficulty of preparing stable standard cyanide solutions in the sub part-per-million range and the difficulty in matching sample and standard solution ionic strength. Conventional potentiometric titrations also gave poor results because of the very small potential change at the end point in the low ranges of cyanide which were of interest. The method finally chosen was a known addition method (7) using a Gran’s plot (8) for calibration of the cyanide concentration. The method requires taking a known volume, V , of the sample, adjusting the pH if necessary, and adding a small volume of an Ag(CN)*- indicator solution. The electrode potential is noted in this solution and again each time after three to five successive additions of a standard cyanide solution. The volume of each addition is constant and small compared with V. The measurement potentials are plotted cs. the increase in sample cyanide concentration from the known cyanide additions, using commercially available Gran’s plot paper (9). The original sample cyanide concentration is obtained by extrapolation. The success of the method depends on the fact that the silver ion concentration in a solution containing a concentration Z of Ag(CN)?- is related to the free cyanide concentrations (CN-) by
where p2 is the overall formation constant for Ag(CN)2-. Strictly, activities rather than concentrations should be used, (7) Orion Research, Inc., Newslerrer/Specific Ion Technology, 1(2), 9 (1969) and 2(2), 5 (1970). ( 8 ) G. Gran, Analyst, 77, 661 (1952). (9) Cheni. Eng. News, Feb. 15, 1971, p 89.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2227
470
460
450
* m 440
6
-430
=
?
-420 410 400
- 390
05
04
02
03 ppm
01
0
1
OF CYANIDE IN UNKNOWN
2
3
4
5
MI OF CYANIDE SOLUTION ADDED
Figure 1. Known additions of 0.01 ppm cyanide to “unknown” solutions containing various initial levels of cyanide (from 0 to 0.4 ppm) Results are plotted on Gran’s Plot Paper without volume correction. Initial solution volume, 100 ml; pH 11.0; silver indicator concentration, 10-5M
but since the ionic strength of the sample solution does not change appreciably during the course of a determination, it is permissible to write equilibrium expressions in terms of concentrations. If C is the original sample cyanide concentration and S the concentration of cyanide in the standard cyanide solution, then the silver ion concentration after n additions is
L
v
J
where V, is the volume of each added increment of the standard solution. Substitution of Equation 2 in the Nernst equation for a silver ion sensing electrode gives the electrode potential after n known additions.
E, = constant
I + 2.3RT -log - F
P2
( 2 ) (2.3RT) log F
[ c + n?]
(3)
The Gran’s plot is made by plotting the potentials E, us. nSV,/V on a piece of graph paper whose vertical axis is scaled in units of antilog [EF/(2)(2.3RT)J. Rearranging Equation 3 we have :
where k 1 is constant. The Gran’s plot should give a straight line whose intercept in the zero horizontal axis occurs when nS V, --c V
By approximate choice of V, and S , the units in the horizontal axis can be made ppm or any other convenient quantity. The concentration, S, of a standard solution must be chosen 2228
with due regard for the anticipated levels of sample cyanide, C . Convenient and precise extrapolations require that
where C,,, is the maximum level of cyanide anticipated in the sample. For example, if Vis taken as 100 ml, V, as 1 ml, and Cm,, is 1 ppm, then an approximate choice of S would be 100 ppm. Standard cyanide solutions this concentrated are easily prepared, are stable, and can be stored for long periods. SAMPLE CONDITIONS
The indicator concentration is not crucial within rather broad limits, since the only requirement imposed by the method is that Z be constant during the known additions. From a practical standpoint, however, the indicator concentration should not be more than about ten times the minimum concentration of cyanide to be detected. Indicator solutions require the presence of a small excess (about 1 %) of cyanide to prevent precipitation of AgAg(CN)*. This excess cyanide will be measured as sample cyanide in the Gran’s plot. At the same time, too low an indicator concentration results in slow electrode response time. A reasonable compromise is an indicator level of about 10-5M which permits detection of cyanide to about 0.03 ppm. An upper limit of sample cyanide concentration is imposed by the formation of higher silver cyano complexes. The method assumes that the only silver complex present is Ag(CN)z-. At sample cyanide levels above 10-2M(260 pprn), significant amounts of Ag(CN)3- are present. Such samples should be diluted prior to analysis. The permissible sample pH range is limited on the acid side by the formation of HCN. On the basic side, the indicator may react to form AgOHCN. As a result, it is necessary to adjust sample pH to the range 11-12 prior to analysis.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
+laol
Table I.
Cyanide Analyses without Metals Present
[Indicator level 1 X 10-5M KAg(CN)J
+loo{
Total cyanide, ppm r20-
Sample
Theory
Found
Error,
10-3
26.0 2.6 0.4 0.26
26.0 2.6 0.4 0.26
0.1 0.05
0.04
0 0 0 0 2.0 20.0
10-4
1.5 x 10-4 10-5 3.8 x 1.9 x 10-6
-60-
-140
i
-zzol
Table 11. Cyanide Analyses in the Presence of Complexing Metals
!!
[Indicator level 1 X 10-5MKAg(CN)J
-300
A. Total cyanide 7.6 X 10-5M ( 2 ppm) Metal EDTA CN found 2 x 10-3 Cd2+ 0.2M 2.05 2 x 10-3 Cr3+ 0.2M 1.95 1 x 10-3 cut+ 0.05M 2.0 1X NiZC 0.05M 1.95 1 x 10-3 Zn2+ 0.05M 1.95 B. Total cyanide 7.6 X 10-6M (0.2 ppm) 1 x 10-4 Cdz+ 0.02M 0.205 1 x 10-4Cr3' 0.02M 0.20 1 x 10-4 C U ~ + 0.02M 0.205
-380-
-460't
/
-5404
-620 O
I
0.98
2
3
4 MI
5
6
7
OF NoOCI
8 9 ADDED
1
0
1
1
1
2
1X
Figure 2. Titration of 100 ml of 10-3MKCN (pH 11, containing 10 -5M silver indicator) by 10-*M NaOCl
In wastewaters and other samples containing unknown components, interference from metal ions which form some stable cyanide complexes can be expected, and require the addition of a masking agent prior to the sample solution. We have found disodium EDTA to be effective for this purpose. In normal waste waters 10-3-10-4M EDTA levels are adequate. If high levels of metals are expected, in large excess compared to the total cyanide, 10-2MEDTA or more may be required. Anions will not interfere unless they can react with the very stable Ag(CN),-- indicator complex. The only anion likely to be present which would interfere is sulfide, which can be removed by the addition of a slight excess of Pb?+. Up to a hundredfold excess of lead over cyanide did not appear to cause any problems. Ammonia, which forms weaker complexes with silver, will not interfere, even when present at a 103-fold excess. Only the metals which form extremely stable complexes such as cobalt, iron, gold, and silver, will not be dissociated and measured by this procedure. EXPERIMENTAL
All potentiometric measurements were made using an Orion Model 94-16 silver-sensing electrode and an Orion double junction reference electrode with 10 potassium nitrate and 0.001M KOH in the outer compartment. An Orion Model 801 pH/mV meter was used. The silver indicator stock solution was prepared by titrating a small aliquot of an approximately IOp3MKCN solution with 10-zM AgN03, using the silver sensing electrode. From the plotted potentiometric titration curve, the millivolt reading corresponding to 98-99z of the way to the first end point [formation of Ag(CN)?-] is noted. The remainder of the KCN solution is then titrated with the AgN03 solution to the same millivolt reading. The solution is stable for at least several months if kept in a stoppered bottle.
1
x
Ni2+ Zn2+
0.02M 0.02M
0.20 0.21
Error, 2.5
2.5 0 2.5 2.5
2.5 0 2.5 0 5.0
Other solutions were prepared from reagent grade chemicals and standardized by conventional procedures. Gran's Plot Paper, uncorrected for volume changes, Orion catalog number 90-00-92, was used for Figure 1. RESULTS AND DISCUSSION
A number of synthetic samples were prepared and analyzed by the following procedure: To 100 ml of the sample solution, 1 ml of the silver indicator solution was added. For samples containing Zn2+,Cd2+,NiZ+,Cu+, and Cr 3+, typically 1 ml of 10-lM disodium EDTA was added as a masking agent. In the case of Cr3+,Cu+, and Ni*+, it was necessary to acidify the sample to pH 4 with acetic acid and heat to about 50 "C for 5 minutes in order to destroy these cyanide complexes. No loss of HCN was observed during this operation, even though carried out in open beakers. All samples were then adjusted to pH 11 with KOH. Electrodes were inserted and potential readings taken initially and after SUCcessive incremental additions of a 0.01 ppm standard KCN solution (1-ml increments of 3.8 x 10-4M KCN into 100 ml of solution). Figure 1 shows the results plotted on Gran's Plot Paper of a series of known addition experiments at different initial cyanide levels. It would appear that the minimum detection level is about 0.025 ppm. Lower levels may be possible with further dilution of the indicator solution, but such solutions are not stable for long periods of time, and electrode response time becomes quite long. The values obtained by extrapolating Gran's plot known addition in similar experiments are shown in Table I for pure KCN solutions, and in the presence of various metals in Table 11. It is apparent that all of the cyanide originally present in the sample can be detected.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2229
In actual practice, cyanide is often destroyed by chlorination under alkaline conditions. To demonstrate that intermediate products, such as CNO-, do not interfere, the silver indicator procedure was used to follow the titration of a KCN solution by hypochlorite (Figure 2). When the end point is passed, all of the cyanide has been destroyed, including the indicator itself. The fact that a single, smooth curve is obtained, with an extremely large end-point break, suggests that none of the reaction products interfere with the electrode response. The method should perform well in the analysis of both chlorinated waste waters and untreated samples. The successful application of this method to the laboratory
analysis of actual cyanide waste samples has been reported (10).
The time required per analysis is about 5-10 min for those samples which do not contain inert transition metal cyanide complexes, such as Ni and Cr. Inert complexes increase the time of analysis by approximately 5-10 min. per analysis. No prior distillation should be necessary for samples of practical interest. RECEIVED for review May 8, 1972. Accepted July 10, 1972. ~_____ ~~~~
(10) L. E. Lancy, Lancy Laboratories,Zelienople, Pa., personal com-
munication, 1972.
Some Characteristics of Several Commercially Available Cation-Responsive Glass Electrodes Sonny Phangl and B. J. Steel Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, S . Australia ALTHOUGH THE LITERATURE abounds with reports of various uses and properties of cation-responsive glass electrodes, it appears that there is a need for more information regarding the properties of commercially available cation-responsive glass electrodes ( I , 2). At present most of the reports concern electrodes which have been made from glasses which were either supplied by the manufacturers or which have been produced by the workers themselves in their laboratories. This report contains the characterization and the conditions necessary for such electrodes to be useful in the determination of activity coefficients. EXPERIMENTAL Apparatus. The low impedance side of the circuit consisted of a micro-step potentiometer (Type 44248, Cambridge Instrument Co. Ltd., U.K.), one terminal of which was connected to the Ag, AgCl electrode while the other was grounded. The high impedance side of the circuit consisted of a Cary Model 31CV vibrating reed electrometer. The latter was used as a null-detector. A grounded water-bath was used throughout this investigation and an opaque brass lid was used to cover the bath. It acted as both a light and an electrical shield. Table I shows the type of glass electrodes used in this work. The GEA33, GEA 33/C, and the Beckman 39278 electrodes are sodium-responsive and are designed for use in routine measurement of sodium ion concentrations. The Beckman 39137 Cationic Electrode is meant generally for monovalent cations. Reagents. Recrystallized AR sodium chloride was used. Nitric acid and sodium hydroxide were of AR grade. The hydrochloric acid was obtained from a constant boiling mixture. All the solutions were prepared by using doubly distilled water. 1 Present address, School of Chemical Sciences, Science University of Malaysia, Penang, Malaysia.
(1) “Glass Electrodes for Hydrogen and Other Cations,” G. Eisenman, Ed., Marcel Dekker, Inc., New York, N . Y . , 1967.
(2) G. Mattock, Chimia, 21,209 (1967). 2230
Table I. Glass Electrodes Used in This Work Manufacturer Label on electrode Type EIL GEA 33 1 Beckman 39278 3 Beckman 39278 4 Beckman 39137 5 Beckman 39278 6 Beckman 39137 7 EIL GEA 33 8 EIL GEA 33/C 10
Procedure. The electrodes were tested at 25 “C. Earlier tests had shown no electrical leakage by using a water-bath and thus the inconvenience of an oil-bath was avoided. Each cell was magnetically stirred and only aged thermal electrolytic type Ag, AgCl electrodes were used. RESULTS AND DISCUSSION Selectivity Constant.
By considering a cell of the type,
Ag, AgCl~HClor NaCl soln~GlassElectrode and the glass electrode potential written as
E
=
constant
+ RT - ln(aH + K H N ~ C J N ~ ) F
it can be shown ( 3 , # )that El - E2 = RT -In F
(1)
KHN~
El is the emf of the cell in O.lm NaCl solution in the limit as H+ ions + 0. E? is the emf of the same cell in O.lm HC1 solution. The selectivity constant, K H N ~shows , how effective an electrode “sees” the Na+ ions relative to the H+ions for Na-re(3) G. Eisenman, D. 0. Rudin. and J. U. Casby, Science, 126, 831 (1957). (4) G. Eisenman et a/., “The Glass Electrode,” Interscience Publishers, John Wiley and Sons, New York, N.Y., 1962, p 232.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
