Table IV. Effect of Time and Temperature upon the Determination of Active Hydrogen in Urethane-Terminated Polymersa in Toluene Active hydrogen Active hydrogen Polymer Reaction conditions Time, min Found, Polymer Reaction conditions Time, min Found, Ib Shaken at room temperature 5 0.01 Ib Refluxed 20 0.13 Shaken at room temperature 5
20.0
0
W
- 100
I-
15.0
10.0
IO-^
IO-^
Conc.
0.0
IO-^
H2C204
Conc.
(molar)
Figure 3. Potential of the kinetic peak as a function of H2C204 concentration
Table 11. Position of the Kinetic Peak of Peroxytungstate Solutions in NaC104 and Varying H2C204 Concentrations 2.0 X 10-'M WOd-2, 4.0 X 10-2MHz0z E in presence Concn H2C204, of 0.5M NaCIOa, E in absence of NaC104,volts us sec moles/liter volts cs sec +O. 296 +o. 255 1 . 0 x 10-2 2 . 0 x 10-1 +O ,296 +0.289
H
2
o2
x lo2
(molar)
Figure 4. Kinetic current as a function of H 2 0 2concentration 2.5 X lO-SM
186
10.0
WOa2-; 1.0 X 10-'M HzCzOa
W04
2-
15.0 x lo5
23.0 (molar)
Figure 5. Kinetic current as a function of WOd2- concentration 8.0 X 10PM HzOz; 1.0 X 10-'M HzCzOr
2.0 X 10-6M W O P ; 4.0 X 10-'M HZOZ
Conc.
5.0
IO-'
kinetic maximum with and without the addition of 0 . 5 M NaC104. All subsequent kinetic currents are reported in 1.0 X 10- 2MH2C204 and are measured at the maximum of the polarographic wave even though the potential of the maximum exhibits a small variation depending upon the total ionic concentration of the particular solution under study. Figure 4 shows the effect of hydrogen peroxide concentration upon the kinetic current. As the hydrogen peroxide concentration increases, the kinetic current also increases and slowly approaches a limiting value. A concentration 8.0 X 10- 2M in HzOzis selected for future measurements because it produces currents of moderate magnitude and because it represents a convenient aliquot of hydrogen peroxide stock solution. The position of the kinetic maximum undergoes a potential shift to more cathodic values as the hydrogen peroxide concentration increases. Current measurements are always made at the maximum of the kinetic peak. Figure 5 shows the kinetic currents observed for solutions over the tungstate concentration range 5 . 0 X 10- 6 M to 2.5 X lOP4M. All solutions contain 8.0 X 10- 2M H202and 1.0 X 10-2M H2C2O4. The first linear portion extends from tungstate concentrations less than 5.0 X 10-6M to 2.0 X 10-6M. The second linear portion extends from 4.0 X 10-5M to 2.5 X 10-4M W042-. For tungstate concentrations approaching 1.5 X lo-", the potential of the kinetic peak is relatively constant at f 0 . 2 3 3 =t0.016 volt os. SCE. At higher concentrations, the peak shifts slightly to more cathodic values. At 2.5 X 10- 4MW042-,the peak potential is $0.197 volt us. SCE. The presence of two distinct portions to the tungstate calibration curve is not unexpected. A curve of similar shape has previously been reported for tungstate in the presence of hydrogen peroxide and varying concentrations of sulfuric acid (3). The existence of more than one complex species is believed to account for the observed break in the calibration curve. Both oxalatomono and oxalatodiperoxy species are reported from evidence based upon infrared and Raman
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
The effects of Cr(VI), Fe(III), Mo(VI), Ti(IV), and V(V) upon the kinetic current observed for tungsten are listed in Table 111. Concentrations one-tenth, equal, and ten times that of the tungstate concentration are employed. No interference is assumed if the observed current is within one standard deviation of the expected value. This corresponds to a relative error of =k2.3%. All five metal ions are found to interfere significantly. The relative errors range from 1.0 to 80%. Only chromium and molybdenum could be tolerated at the smallest concentrations employed. Separation procedures would have to be employed prior to tungstate determination by this procedure.
Table 111. Effect of Diverse Ions on'the Polarographic Determination of Tungsten 3.0 X 10-6M WOaZ-, 8.0 X 10-'M H202, 1.0 X 102-M HzCzOi Concentration, Re1 InterIon Added as moles/liter error, ference VOaNHaVOs 3.0 X -6.8 Yes Cr6+
K2Cr207
Fe3+
Fe(C104)J
MOO^^-
Na2Mo04
Ti4+
3.0 x 3.0 X 3.0 X 3.0 x 3.0 X 3.0 X 3.0 X 3.0 X 3.0 X 3.0 x
?.o x
, . .
10-5 10-5 loe4 10W 10-5 10-6 10-4
3.0 x 3.0 x 10-5 3.0 X
-16.7 -19.5 -1.8 -4.1 -16.7 -4.1 -16.7 -46.2 +0.9 +11.7 +79.6 -24.4 -24.4 No peak
Yes
Yes No Yes Yes
Yes Yes Yes No Yes Yes Yes
CONCLUSIONS
Yes
Yes
studies (5). The nonlinear shape of the curve in Figure 5 points out the necessity of constructing a calibration curve for tungsten determined by this method. To test the reproducibility of the polarographic measurements, twenty solutions of 3.0 X 10-3M W042-, 8.0 X 10-2M H 2 0 2 ,and 1.0 X 10-2M H G 0 4 were run and the kinetic currents and potentials recorded. The mean current observed is 22.0 pA with a standard deviation of *0.5 pA. The mean potential observed for the kinetic peak is +0.231 volt us. SCE with a standard deviation of 10.009 volt us. SCE. ( 5 ) W. P. Griffith and T. D. Wickins, J. Chem. SOC.,A , 1967, 590.
A procedure is presented for the quantitative determination of tungsten by polarographic measurement of the kinetic wave produced in hydrogen peroxide and oxalic acid medium. The lowest reported tungstate concentration of 5.0 X 10-FM is comparable to that reported for tungstate kinetic waves in hydrogen peroxide-sulfuric acid medium (2). It is less than that of a conventional dc polarographic method utilizing hydrochloric acid-phosphoric acid medium (6) and approaches the limit of 5 X 10-7Mreported for tungstate in perchloric acid-tartaric acid medium (7). As with other polarographic procedures, prior separation is necessary to avoid the interfering effects of other species.
RECEIVED for review June 1,1971. Accepted August 2,1971, (6) R. M. h a , B. A. Abd-el-Nabey, and A. M. Hindawey, 2. Aiial. Chem., 240, 9 (1968). (7) R. Bock and B. Bockholt, ibid.,216, 21 (1966).
An Instrument for the Direct Measurement of the Resistance of Glass pH Electrodes W. Michael Krebs Imtrumentation Laboratory Inc., 113 Hartwell Aoenue, Lexington, Mass. 02173 THE GLASS pH electrode has its origin in a paper by Lord Kelvin ( I ) , which appeared in 1875, in which it was suggested that glass is an electrolyte-i.e., can possess ionic conductivity. Experimental verification of this hypothesis was obtained some thirty years later by Cremer (2). The relationship between the observed potential and the difference in the acidity of solutions contacting both sides of the glass membrane was made quantitative by Haber and Klemensiewicz (3) a short time thereafter. This relationship, which is Nernstian, can be expressed as :
(1) W. Thomson, Proc. Roy. SOC.,A23, 463 (1875). (2) M. Cremer, Z. Biol., 47, 562 (1906). (3) F. Haber and Z. Klemensiewicz, Z. Pliysik. Chem. (Leipzig), 67, 385 (1909).
where subscripts 1 and 2 refer to the solutions on either side of the membrane, R and F a r e constants, Tis the absolute temperature, and aH+is the hydrogen ion activity. The glass electrode is not an electrode in the same sense as the calomel, hydrogen, or silver-silver chloride electrodes. The potential established at the glass surface does not result from electron transfer between oxidized and reduced forms of a material, but rather involves an ion exchange process very similar to that operative at conventional ion exchange membranes. Thus, the glass electrode is more precisely described as a cation exchange membrane. The number of different glass formulations which can be utilized for pH glass membranes is quite limited. Not only must an acceptable glass exhibit close to theoretical response over a wide range of pH, it also must be mechanically strong, workable in a flame, have low solubility in aqueous media, and have an electrical resistance which is under cu. 1OI2 ohm-cm within the temperature range of interest.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
187
