er Ion in Parts per Selective Ion Electrode Cller, Philip W. West, and Ralph H. Muller Coates Chemical Laboratories, Louisianu State University, Baton Rouge, La. 70803 Silver ion in the 50 parts per billion (4.63 X ~.O-%I) range has been determined by direct potentiometry using a silver sulfide membrane electrode with an accuracy of 0.2% over the range of 22-28 ‘C. Reproducibility of EMF values over a three-month period is 2%. ~ e r n s ~ ~slopes an at seven temperatures etlcal mean deviation of &0.03/0.1 O C the theoretical value of &0.02/0.1 O 6 . s for the effect of chloride ion are given and a simple empirical equation permits calculation ent loss of Ag+ due to the presence of
IN ENVIRONMENTAL studies under way in these laboratories, the need arose for the determination of silver in the range of 50 parts per billion (4.63 X lO-’M). This has been done with precision by direct potenliometry using an Orion silver sulfide membrane electrode. The advent of a considerable number of solid-state membrane electrodes, selective for various cations and anions, has opened new horizons for electroanalytical methods. Much of this is due to Dr. James W. ROSS, Jr. and his associates at Orion Research, Inc. of Cambridge, Mass. Rechnitz ( I ) has discussed selective ion electrodes and has made numerous contributions to the subject. In a recent paper (2), he has exslmined the sulfide ion membrane electrode in detail, including its response to silver ion over the range 10-1 to 10-4Mand shown that it exhibits Nernstian behavior, accurately. The present work confirms this behavior down to three orders of magnitude lower in concentration ( l O W M ) . It has been reported that a glass electrode is responsive to Ag+ at the 10+M level (3). EXPERIMENTAL
All chemicals were reagent grade, used without further purification. It was found necessary to use deionized water which had been doubly distilled, a continuous supply of which was maintained. Stock solutions of 0.1M in silver(1) were prepared daily by direct weighing of AgN03. Anhydrous sodium nitrate was added to adjust all sample solutions to 0.1M in nitrate to maintain an ionic strength of p = 0.1. A p ~ a r a ~All ~ ~data . herein reported were obtained with an Orion “Ionalyzer,” Model 801, a digital voltmeter with automatic decimal point location and sign indication and least count of 10.1 mV. All EMF values were recorded to the nearest 0.1 mV after attainment of equilibrium. The indicating electrode was an Orion Sulfide Ion Activity Electrode, Model 94-16. A saturated calomel electrode was used as the reference with an intermediate nitrate salt bridge. Measurements were made over a limited range of temperatures as discussed below. All glassware used in the preparation of the solutions, as containers for sample solutions and the glass reaction vessel, (1) G . A. Rechnitz, Chem. Eng. News, 4.5 (23, 146-58 (1967). (2) Tong-Ming Hseu and G. A. Rechnitz, ANAL.Cmw, 40,1054-60
(1968). (3) L. L. Gerchman and 6.A. Rechnitz, Z . Anal. Chem., 230,
265 (1967). 2038
M
10’
10
20
2
4
6
I
I
1
40
8 I
lo6
15
I
60 80 100
150 200
ppb A Q
Figure 1. Semi-logarithmic plot of EMF us. ppb Ag+ for data in Table I
were washed in Dreft, allowed to stand overnight in cleaning solution, rinsed six times in tap water and seven times with double distilled and deionized water and then dried at 100 “C. Procedure. With interest in Ag+ centered in the general range of 50 ppb., measurements were made over the range of approximately 10-130 ppb. (1.205 X 10-7 to 1.205 X 10-8M). The experimental conditions were directed toward practical analytical conditions and, in the interest of ultimate automatic monitoring. An indicating electrode and a nitrate salt bridge, connected to a saturated caIomel electrode, were immersed in the glass measuring vessel along with a Teflon-covered stirring bar. A thermometer, immersed in the solution, permitted temperature readings to 0.1 “C. Starting with 200 ml. of 0.1M NaNO3, uniform increments of freshly prepared silver nitrate standard were added. Potentials were not measured at a stated time interval, but only when no increase in potential exceeded 1 0 . 1 mV. The small increase in volume for each addition was corrected by the usual V/o V factor. Conditions in the air-conditioned laboratory were such that no sudden temperature gradient arose, although temperatures were not always the same from day to day. No particular heat insulation lagging of the system was required and the small addition of heat from the magnetic stirrer motor could be controlled quite easily by interposing one or more disks of filter paper. The adequacy of this technique is illustrated by the reproducibility of the data, particularly as a function of temperature. This approach is considered to have the following advantages. The first is the speed with which a precise calibration can be made. It is limited in this respect to the time required for potentials to come within 1 0 . 1 rnV of the equilibrium value after each concentration increment. The second is that it eliminates the rinsing and blotting of the indicator electrode before each new concentration increment.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
+
The third is that the procedure is useful for interpolation with standards and unknowns with little need to be concerned about the absolute value of the potentials. The fourth is the most important consequence of increased speed which is to minimize errors due to adsorption of the silver ion. The definitive studies of West, West, and Iddings (4, 5 ) in this laboratory, using the radioactive isotope ll0Ag as a tracer, showed that all containers, including plastics and various coatings, adsorb silver ion in varying degrees. Their methods gave sensitive measures of adsorption by counting disintegrations and also the “geographical” distribution of the deposits by autoradiography. They concluded that, in all cases, complexation of the silver, especially by thiosulfate ion, is the best way to minimize absorption errors. In the results reported here, no detectable losses by adsorption occurred in the relatively short periods of observation. Conversely, the potentiometric method would seem to be well suited for more precise measurements of adsorption rates although not as sensitive as the radiometric method. RESULTS AND DISCUSSION
Figure 1 illustrates the semilogarithmic plot of EMF us. ppb of Ag+ for the data shown in Table I. The corresponding concentrations, expressed in molarity, are also indicated. All measurements reported in this paper were treated in the same manner, of which Figure 1 is a single example. During measurements, the known values of concentration (ppb) of Ag+ were plotted on 2-cycle semilogarithmic paper as soon as the equilibrium potential was attained. A quick estimate of the slope is readily obtained graphically from this plot and this is independent of the concentration units employed, (ppb or molarity). However, the intercept in the equation:
E = K (constant)
+ 2.303 RT - log aAgt F
is not and must be expressed in terms of molarity. All data and evaluations in this report are based on the solution of an equation of the form:
E
Table I. Data for Figure 1 Temperature = 24 “C Agi Ag+ taken, ppb 13.5 27.0 40.0 53.0 67.0
EMF, mV 136.5 154.2 164.3 171.5 177.5 182.1 80.0 189.3 106.0 194.9 132.0 Calculated from equation for these data: E = 69.89 58.94 log (Ag+). Mean error: =tO.l%; max. error: =t0.2%. 5
+
Table 11. Effect of Temperature on Nernstian Slope. Slope, obsd Slope, theor Temp “C 22 58.60 58.56 23 58.76 58.76 24 58.94 58.96 25 59.11 59.16 26 59 44 S9,36 27 59.60 59.56 28 59.77 59.76 a Mean deviation obsd: 10.03 mV. Theoretical deviation/ 0.1 “C = 0.02 mV. I
Table 111. Comparison of Silver Values Taken and Found Ag+ taken,a PPb 13.5 27.0 40.0 53.0 67.0
Ag+
calcd,
Av error, % 0.10
PPb
13.50 27.00 40.00 53 .OO 66 98
0.08
0.05 0.06 0.08
I
80.00 Uncertainty ca. 0.20%. 80.0
a
+ K (constant) + m log (Ag)+
calcd.5, ppb 13.50 26.95 39.98 52.97 66.96 80.14 106.20 132.10
0.06
Max error, % 0.15 0.19
Nosb
detns 18 18
0.10
18 18 18 18
0.11 0.15 0.20
Probable error of single measurement: 0.15-0.2%,
in which the slope and intercept were calculated by the arithmetic mean. Table I1 illustrates the effect of temperature on the slope, RT expressed by 2.303 - and amounts to 0.02 mV/O.1 O C . F Table I11 illustrates the comparison of taken us. found for a large number of measurement values. It should be pointed out that the high accuracy is accounted for by the fact that the calculated values are based upon the best equation representing the data for at least eight different concentrations. What is more significant analytically is the long term reproducibility of the electrode measurements. Table IV shows the range of EMF reproducibility based upon measurements efttending over a three-month period. Effect of Chloride Ion. When successive increments of silver ion are added to the system containing known amounts of chloride ion, good straight lines are obtained in the semilogarithmic plot but both slope and intercept are markedly changed. By comparison with the straight line obtained in
(4) F. K. West, P. W. West, and F. A. Iddings, ANAL.CHEW,38, 1566 (1966). (5) F. K. West, P. W. West, and F. A. Iddings, Anal. Chirn. Acta, 37, 112 (1967).
Table IV. Long Term Reproducibility of EMF Values. t = 22-28 “C
Concn Ag+, ppb 13.5 27.0
a
EMF, mV Av, mV 136.7 0.23 154.5 0.27 40.0 164.6 0.30 53.0 171.8 0.33 67.0 177.8 0.34 80.0 182.4 0.39 106.0 189.6 0.41 132.0 195.2 0.51 56 observations over a three-month period.
% Deviation 0.17 0.18 0.18
0.19 0.19 0.21 0.21 0.26
the absence of chloride ion, one can quickly calculate the “apparent” loss in silver ion as measured by the specific ion electrode. This loss can be calculated without reference to the complicated ionic equilibria which may be involved, such as the various chloro complexes of silver. If one designates the apparent loss in silver ion L as the difference between silver ion added and silver ion found potentiometrically, then :
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
*
2039
by rinsing with distilled water and subsequent immersion in
Table V. Loss in Ag+ in Presence of C1-
0.05M S2- solution and renewed washing in distilled water restored the surface to its original luster. It then responded to Ag+ yielding the proper EMF values. It is not to be
At 30 ppb Ag +
Loss
Ag+
C1- ppm
Ag+ found found ppb 100 30 90 27 77.7 23.3
0 2 10
2 10
90 80.0
At 40 ppb Ag + 40 36 32.3
0 2 10
100 90.6 83.0
At SO ppb Ag + 50 45.3 41.5
0
100
Loss Ag+
Ag+ calcd
0.0
3.0 6.7
0.0 3.001 6.706
0.0 4.0 7.7
0.0 4.003 7.702
0.0 4.7 8.5
0.0 4.675 8.493
L=- C a bC
+
where C = concentration of chloride ion. That this expression is valid is easily checked by plotting C/L us. C yielding a straight line with slope = b and intercept = a. A few examples of observed us. calculated losses are shown in Table V. For the case of 50 ppb of Ag+ the expression: C holds. Appropriate values of a b L = 0.24 0.09386C were used at other concentrations. At concentration ranges of Ag+ and CI- when the solubility product of silver chloride can be exceeded, two straight lines are obtained and the product of the respective ion concentrations at the point of intersection yields an acceptable value of Kep. At 23 “ C in the 0.1M NaN08 medium (where p = 0.1) a value of 1.12 X 10-lo was obtained. This is not proposed as a new value for K,, but indicates that, in such a system, the selective ion electrode behaves in expected fashion. On two occasions, with C1- concentrations greater than 100 ppm (2.82 x 10-3M), malfunction of the electrode occurred and it failed to respond to changes in Ag+ concentrations. The electrode appeared to be tarnished. Brief immersion (a few moments) in about 0.005M NHkOH followed
+
2040
+
inferred that this constitutes an inherent defect of the Orion specific ion electrode, but it does seem understandable that excess chloride ion could affect the equilibria at the membrane interface. Although these anomalies and sources of error can be calculated, either empirically or from fundamental concepts if the chloride ion concentration is known, the latter is not known in a practical sample and it seems more sensible to get rid of it. In several other instances, removal by ion exchange has been recommended (6) and Orion Research, Inc. sells “buffers” and gives directions for preparing them as well, for the elimination of interferences and the maintenance of constant ionic strength. The authors are engaged in a study of suitable ion exchange systems to eliminate halide ion interference. There is little question that this can be done, but at the parts per billion level, the question arises as to what extent secondary adsorption or entrainment factors will affect the precision. CONCLUSIONS
These measurements have convinced the authors that analyses with the specific ion electrode system permit precision ordinarily associated with macro quantities of sample and that these results require no unusual precautions or elegance of execution. The precision seems to be limited to the degree to which the standards can be prepared, in our case to h 0 . 2 X . The results would seem to demolish the idea held by many trace analysts that appreciable error or “order of magnitude” values are the best that can be expected. RECEIVED for review December 6, 1968. Accepted August 27, 1969. The financial support of U. S. Public Health Service Research Grant AP-UI-00724, National Center for Air Pollution Control, Bureau of Disease Prevention and Environmental Control, is gratefully acknowledged. (6) Orion Research, Inc., 11 Blackstone St., Cambridge, Mass., Application Bulletin, No. 7.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
