Table I. Polarographic Values for Sodium Bromite in 0.5M Sodium Hydroxide NaBrO2, mg -Ep?,o V 0.18 0.88 0.18 0.86 0.35 0.92 0.45 0.87 0.53 0.84 0.71 0.89 0.88 0.86 0.90 0.86 1 .OG 0.90 1.41 0.89 1.41 0.89 1.80 0.87 2.12 0.91 a Mean value was -0.88 V. b Least squares value was 5.84 pLA/mg.
i,* pa
0.84 0.86 1.86 1.83 2.44 3.3 4.8 4.8 6.2 7.5 8.0 9.6 12.4
RESULTS AND DISCUSSJON
Bromite ion is stable in solution at very high p H levels ( I , 3). For that reason, 0.5M NaOH was chosen as a suitable supporting electrolyte for analytical purposes; the sample would suffer minimum decomposition during analysis. Polarograms of sodium bromite solutions disclosed a reduction wave for BrOa- at about -0.88 V, which was well separated from the hypobromite (0.0 V) and bromate (-1.7 V) waves and could be easily analyzed. The diffusion current varied linearly with bromite concentration to about 2 mg/lO ml. A reaction between sodium bromite and mercury was noted in that a pool of mercury allowed t o remain in the bottom of the cell for an extended period of time became coated with a solid.
To optimize precision and accuracy, therefore, the dropping mercury electrode should be inserted into the cell only immediately before the measurements are to be made and the polarograms should be run as rapidly as possible. As a means of evaluating the method, samples of sodium bromite solutions containing various sodium bromite concentrations were analyzed polarographically and titrimetrically ( 9 ) over a period of about a year; these samples contained varying amounts of the other bromine species in addition t o bromite. Table I lists the data. Treatment of these accumulated data by a least squares method (12) gave a regression coefficient of 5.84 pajrng with a standard error of 0.20. This value corresponds to a diffusion current constant, I, of 3.65. pH effects. The effect of p H o n the reduction was studied briefly also. In essence, a fixed amount of pure sodium bron i t e was polarographed in inorganic buffers at several pH values above 7. Attempts to extend the study to lower pH levels were unsuccessful. Visible decomposition, including bubble formation and a color change, was noted after adding the bromite to buffers below pH 7 . A marked cathodic shift in half wave potential was observed over the range of p H 7 to 10.4. The slope of a plot of Eli2 c‘s. p H (Figure 1) corresponded to 232 mV/pH unit. which is in good agreement with the theoretical value of 236 mV/pK unit for a reduction involving four protons. Therefore, the electrode reaction, over the pH range 7-10, appears to be: Br0,-
+ 4H+ + 4e-
+
Br-
+ 2H20
RECEIVED for review June 13, 1968. Accepted July 31, 1968. (12) 0. L. Davies, Ed., “Statistical Methods in Research and Production.” Oliver and Boyd, London, 1961.
Cationic Glass Electrode Response to Alkali Metal Ions in Nonaqueous Solvents James E. McClure and Thomas B. Reddy Research Seraice Department and Chemical Department, American Cyanamid Co., I937 West Main Street, Stangord, Conn.
THEPROPERTIES of cationic glass electrodes in aqueous solvents are well known (1-3). Recent work has shown that the electrodes also respond to cations in partially aquated solvents ( 2 , 4 ) and in nonaqueous systems such as methanol ( 4 ) . Very little is known, however, about the performance of such electrodes in aprotic organic solvents. Solutions of alkali metal ion salts in aprotic solvents areof current interest because of their utility as electrolyte systems in high energy density battery systems. Boden (5) has reported the response of the cationic glass electrode to Li+ over a limited concentration range in the presence of Mg*+, K+, NH4+ and (C2H&N+ in propylene carbonate. This paper presents the results of some preliminary experiments which were carried out to determine (1) G. A. Rechnitz, Chem. Eng. News, 45 (2% 146 (1967). (2) G. A. Rechnitz, Rec. Chem. Progr., 26 (4), 242 (1965). (3) G. Eisenman, “Advances in Analytical Chemistry and Instrumentation,” Vol. 4, C. N. Reilley, Ed., Interscience, New York, N. Y.,1965. (4) Ibid., p 295. ( 5 ) D. Boden, presented at the Electrochemical Society Meeting, May 7-12, 1967, Dallas, Texas.
2064
ANALYTICAL CHEMISTRY
the behavior of the cationic glass electrode in propylene carbonate, acetonitrile, and dimethylformamide. The electrodes were evaluated in terms of response, response time, selectivity, and durability. Ions investigated included Li”, Na+, and K+. Response was tested over a concentration range of lO-“o 10-2M. EXPERIMENTAL
Reagents. Stock solutions (0.5 or 1M) of NaSCN, KSCN in propylene carbonate, acetonitrile, and dimethylformamide were prepared from reagent grade salts which had been dried to less than 0.04z HsO. Li+ solutions were prepared from G . F. Smith LiClO, which had been dried at 120 “C t o 0.2% HzO. Acetonitrile and propylene carbonate were purified according to a published procedure (6). After purification, these solvents contained less than 40 ppm H 2 0 . Baker reagent grade dimethylformamide containing 200 ppm HzO was
(6) J. E. McClure and D. L. Maricle, ANAL.CHEM.,39, 236 (1967).
I
I
1
I
I
1
K + in DMF
250 Y
U
40t /
bl
> 200
IO VOLUME
30 ’ (100) IN PROPYLENE CARBONATE
20
Z CHsOH
Figure 2. Selectivity ratio of Na + to Li+ as a function of volume % methanol in propylene carbonate
6
5
4
- log (Mi)
3
2
Figure 1. mV us. log Mi plots for alkali metal ions in nonaqueous solvents used without further purification. Reagent grade absolute methanol contained 0.1 % water. Dried polarographic grade tetrabutylammonium perchlorate (Southwestern Analytical Chemicals) was used as electrolyte in all experiments. Apparatus. Beckman No. 39047 (21/2inch) general cationic electrodes were used in all experiments. The reference electrode was a saturated calomel electrode which was separated from the test solution by a nonaqueous salt bridge and two very fine frits. Potentials were measured with a Weston Model 1420 integrating digital voltmeter or a Model 310 Fisher Accumet Expanded Scale p H meter. Procedure. All measurements were carried out in a beaker type cell fitted with a rubber stopper. All cell parts were dried a t 120 “C. The cell was filled in a dry bag. Solution volume in the cell was -25 mls. The electrolyte was 0.1M Bu,NCIOI in all experiments. Electrodes were soaked in 0.01M solutions of the salt t o be tested for a t least 24 hours before use and stored in the same solution when not in use. Each electrode was in contact with only one solvent throughout the course of the work. After background potential of the 0.1M BuNClO4 solvent system was measured, known amounts of stock solutions of the cations were added t o the test cell by syringe for response measurements. Argon was allowed to flow over the surface of the test solution in order to minimize leakage of moisture into the cell. Stirring was effected by either argon flow or magnetic stirring. All measurements were carried out at 27 =t0.3 “C. RESULTS AND DISCUSSION
Typical electrode response curves are shown in Figure 1. Linear response is observed in all solvents over a concentration range of 10-5 to 10-ZM. Although response t o higher metal ion concentrations is also observed, concentrations above -10-2M were not used because of the importance of junction potential and activity corrections at higher concentrations. Slopes of the mV DS. Log (M+) plots are given in
Table I. The observed slopes are less than the Nerstian value (59.5 mV at 27 “C). This difference could be due to dehydration of the glass in the nonaqueous solvent ( I , 7). Linear response curves were not observed for Li+ and K + in D M F . It was necessary to add millimolar levels of these ions to the reagent grade D M F before any electrode response was observed. This effect is thought to be due t o titration of impurities in the solvent with Li+ and Na+. Chromatographic analysis indicated that low molecular weight amines were present in the D M F . When ethylamine was bubbled into a solution of Li+, an apparent decrease in Li+ concentration was observed. Selectivity ratios ( I ) for Li+, K+, and Na+ in nonaqueous solvents are given in Table 11. The values were calculated at (M+) = 10-3M. It was assumed that all salts are equally dissociated. Very little cation selectivity was observed in the three solvents tested. This is in contrast to selectivity reported for cations in aqueous systems and in methanol (4). Apparently the selectivity of the cationic glass electrode does not manifest itself in certain solvents. It would be interesting to determine whether poor glass electrode selectivity is characteristic in aprotic organic solvents. The effect of solvent on glass electrode selectivity was tested by adding methanol t o a cell (No. 1) containing a solution of 10-*M LiC104 in propylene carbonate and to a second cell (No. 2) containing 10-2M NaSCN in propylene carbonate. Both salt solutions were 0.1M in Bu4NC104. The E M F of each cell (glass us. reference) was measured as a function of volume methanol in propylene carbonate. The selectivity ( I ) of the glass electrode was calculated for each solvent composition from the difference in E M F of the two cells (cell No. 2 - cell No. 1). Junction potential effects (due t o solvent changes) and changes (7) R. G. Bates, “Determination of pH,” Wiley, New York, N. Y . , 1964, p 298.
Table I. Slopes of mV us. Log (M+) Plots Solvent Li+ Na+ Acetonitrile 56 56 Propylene carbonate 53 52 Dimethylformamide a Li+ and Na+ apparently react with an impurity in this (see text). 0
VOL. 40, NO. 13, NOVEMBER 1968
K+ 58 53 59
solvent
2065
Table 11. Selectivity Ratios (1) Na+/LiK+/Li’ Acetonitrile 1.9 1.6 Propylene carbonate 1.5 1.1
Na+/K+ 1.2 1.4
in potential due t o dilution of the salt were conveniently cancelled because equal amounts of methanol were added to both cells. Only one glass electrode was used and was transferred between the two cells after each increment of CHPOH was added. The effect of methanol on the selectivity ratio ( K s ~ + / Li+) of the electrode is shown in Figure 2. The selectivity apparently develops as the percentage of methanol in propylene carbonate is increased. The increase in the selectivity ratio is presumably related to changes in the nature of the swollen layer on the glass surface. Variation in apparent selectivity (although not of the above magnitude) with change in solvent has been previously reported (8). The response time of the electrode was measured after injection of a solution of the alkali metal ion into a stirred (8) G. Eisenman, “Advances in Analytical Chemistry and Instrumentation,’’ Vol. 4, C. N. Reilley, Ed., Interscience, New York, N. Y., 1965, p 298.
system. The electrode attained a potential within 1 mV of the equilibrium value in 5 to 10 seconds. This apparent response time may be in part related to slow mixing. Some indication of the durability of the electrodes in nonaqueous solvents was obtained by measuring their response (to alkali metal ions) after storage in the solvents containing 10-2M alkali metal ion. An electrode which was stored in propylene carbonate (10-2M Li+) for six months showed no signs of deterioration. A linear response of mV cs. log &if) was observed for the electrode after this time. Response times were comparable to those observed for a new electrode. The glass electrode is less durable in CHsCN. Thus, after three months, the electrode became pitted and ceased to respond. The above observations indicate that the properties of the cationic glass electrode in aprotic nonaqueous solvents are such that it should be suitable for use in these media. The response rate is rapid enough to allow use of the electrode as an indicator electrode in nonaqueous titrations. Other applications include use as an indicator electrode for studies of ion pairing and as a reference electrode in nonaqueous systems (responding to, for example, Li+) (5).
RECEIVED for review August 30, 1967. Accepted August 1, 1968.
Determination of Low Levels of Polyacrylic or Polymethacrylic Acids in Electrolyte Solutions by Precipitation of Cupric Salts Andrew G. Tsuk and Thomas E. Ferington W. R . Grace & Co., Research Dicision, Clarksciiie, Md. 21029
POLYACRYLIC AND
POLYMETHACRYLIC acid salts are being used increasingly in boiler and process waters as scale preventing or dispersing agents. A common example is the use of sodium polyacrylate t o prevent scale formation on the heating surfaces in phosphate treated low and medium pressure boilers. To get the most effective use from these water treatment agents, it is important t o be able to monitor their concentrations at the very low levels at which they are ordinarily used. In the case of boiler water and sea water, the few methods of analysis for polyacrylates listed in the literature (I, 2) either failed completely or gave only semi-quantitative results, partly because of interference from the high concentrations of salts present and partly because of the low concentration range of polyelectrolyte employed (0.5 to 20 ppm). Polyacrylate or polymethacrylate anion forms precipitates with many divalent metal cations. The precipitate with cupric ion has been studied in some detail by several investigators (3-6), who described precipitations from solutions
(1) J. Lomonte, W. R. Grace Co., Clarksville, Md., personal communication, 1966. (2) F. Sweett and P. F. Rolfe, ANAL.CHEM., 38, 1958 (1966). (3) H. P. Gregor, L. B. Luttinger, and E. M. Loebl, J. Pliys. Chem., 59, 34 (1955). (4) M. Mandel and J. C. Leyte, J. Polymer Sci., Pt. A , , 2, 3771 (1964). (5) H. Morawetz and A. Y . Kandanian, J . Phys. Cliern., 70, 2995 (1966). (6) L. Costantino, V. Crescenzi, F. Quadrifoglio, and V. Vitagliano, J . Polymer Sci., P I . A-2, 5, 771 (1967). 2066
ANALYTICAL CHEMISTRY
much more concentrated than our range of interest. Our aim was to find ways for essentially quantitative precipitation from very dilute solutions, and with reproducible copper content, so that polyacrylate content could be calculated from a determination of precipitated copper. EXPERIMENTAL
Reagents used were as follows: Polyacrylic acid, or its Na salt, were samples of Goodrite K702 (B. F. Goodrich Chem. Co.) and a sample prepared from the monomer. Stated molecular weights are slightly under lo5 for both. Their analytical behavior appeared to be identical. Polymethacrylic acid, Na salt, of about 9000 molecular weight, was a sample prepared from the monomer. Simulated boiler water was prepared with electrolyte contents as follows (values in ppm): Na2Si03 25, NaCl 60, N a N 0 3 21, Na2S04221, N a 2 C 0 31030, Na3P0434.5, Na2S03 20, and NaOH 280. Any hardness is assumed to be precipitated as boiler sludge-i.e., phosphates and hydroxidesand filtered off before the assay. Sodium lignosulfonate, Maracell-E (American Can Co.) was employed to study possible interference. India Ink, Higgins Engrossing Ink 892, was used to make the floc more easily visible. Procedure. The sample was filtered if not free of suspended matter, The p H of 100 ml of the sample was adjusted to an exact value, usually 3.90 + 0.03, with 1N HCI, then heated to 70-90 “C, and 1.0 ml of a filtered solution of Cu(NOJ2.3 H 2 0 (150 grams/liter) was introduced with stirring. The solution was removed from the heat, and allowed to stand, with occasional stirring, for 1-2 hours. The precipitated pale
