2716
Anal. Chem. 1987, 59, 2716-2718
ACKNOWLEDGMENT The authors thank Donald L. Beduhn of Mattson Instruments, Inc., for providing the library of vapor-phase spectra and for helpful discussions concerning software development. LITERATURE CITED Wilkins, C. L.; Giss, G. N.; Brissey, 0. M.; Seiner, S. Anal. Chem. 1081, 53. 113. Crawford, R. W.; Hirshfeid, T.; Sanborn, R. H.; Wong. C. M. Anal. Chem. 1982, 5 4 , 817. WHkins, C. L.; ass, 0. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1082, 5 4 , 2260. Wilklns, C. L. Science 1083, 222, 291. Laude, D. A., Jr.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163. Laude, D. A., Jr.; Johiman, C.; Wilkins, C. L. Opt. Eng. 1985, 2 4 , 1011.
Cooper, J. R.; Bowater, 1. C.; Wilkins, C. L. Anal. Chem. 1088, 58, 279 1. Williams, S.S.; Lam, R. B.; Sparks, D. T.; Isenhour, T. L.; Hass, J. R. Anal. Chlm. Acta 1982, 138, 1. Williams, S. S.; Lam, R. B.; Isenhour, T. L. Anal. Chem. 1983, 55, 1117.
(10) Whke, R. L. J. Autom. Chem. 1087, 9 ,66. (11) White, R. L. Appl. Spectrosc. Rev., in press. (12) Coffey, P.; Mattson, D. R.; Wright, J. C. Am. Lab. (FairfieM, Conn.) 1078, 10, 126. (13) DeHaseth, J. A.; Isenhour, T. L. Anal. Chem. 1977, 49, 1977. (14) Hanna, D. A.; Hangac, G.; Hohne, B. A,; Small, G. W.; Wieboidt, R. C.; Isenhour, T. L. J . C h r m t o g r . Sc;. 1070, 17, 423. (15) Hohne, B. A.; Hangac, G.; Small, G. W.; Isenhour, T. L. J. Chromatogr. Sci. 1081. 19, 283. (16) Gurka, D. F.; Betowski, L. D. Anal. Chem. 1982, 5 4 , 1819. (17) Hanna, A.; Marshall, J. C.; Isenhour, T. L. J. Chromatogr. Sci. 1970, 17, 434. (18) Fields, R. E., 111; White, R. L. Appl. Spectrosc. 1987, 41, 705. (19) Lowry, S. R.; Huppler, D. A. Anal. Chem. 1981, 53, 889. (20) Morris, W. W. J. Assoc. Off. Anal. Chem. 1073, 56, 1037. (21) Cooper. J. R.; Taylor, L. T. Anal. Chem. 1984, 56, 1989. (22) Lee, M. L.; Vassibros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1079, 51, 768. (23) Sprouse, J. F.: Varano, A. Am. Lab. (Fairfield, Conn.) 1984, 16,54.
RECEIVED for review March 23, 1987. Accepted July 7, 1987. This work was supported by funds provided by the oMahoma University Associates Foundation.
Spectrophotometric Determination of Poly(viny1 alcohol) in Cadmium Hydroxide Pastes Charles E. Baumgartner
GE Company, Corporate Research and Development, Schenectady, New York 12301
A colorknetrlc method has been developed to spectrophote metrically determtne the poly(vlnyl alcohol) (PVA) content of aqueous Cd(OH), pastes. The method, which Is based upon the formailon of a blue PVA-loQne-borlc acid complex, can quantitatively determine PVA in aqueous soiutlon concentrations of 2-40 mg/L corresponding to PVA levels within the pasle as low as 0.02 % Solution absorbance d the complex, however, depend8 upon the concentration of Cd2+in solution necessitatingcognizance of this level for routine quantitative analysis.
.
Poly(viny1 alcohol) (PVA) has been added recently as a binder to many oxide systems as a means of improving the integrity of the formed product. One such area has been its increasing use as a binder in pasted Cd(OH)2battery electrodes (1-4), where it is used to improve the handling characteristics of formed electrodes. This use in sealed battery systems, which require careful monitoring of all electrode constituents, has required the establishment of a sensitive analytical method for control of the electrode's PVA level. Previous analytical techniques, based upon the formation of a blue PVA-iodineboric acid complex, are well documented in the literature (5-9), however, these methods were typically developed for nonoxide systems such as paper and textile coatings. It has been found that these methods required modification to provide routine quantitative PVA analysis for this system due to sensitivity of the absorptivity to dissolved Cd2+concentrations. This modified technique allows quantitative detection of PVA in Cd2+-containingsolution from concentrations of 2 to 40 mg/L (ppm). EXPERIMENTAL SECTION Reagents. Boric acid solution is prepared at 0.65 M by the dissolutionof 40.0 g of, H3B03into 1L of HzO. An Iz/KI solution is prepared by dissolving 25.0 g of KI into 100 mL of HzO, followed
by the addition of 12.7 g of sublimed 12. Following dissolution, this is diluted to a 1-L volume to yield concentrations of 0.05 M Iz and 0.15 M KI. HC1 solution for dissolution of the Cd(OH), paste is obtained via 1 O : l dilution of concentrated HCl yielding 1.2 M HC1. Procedure. Optimum analytical results are obtained for a sample possessing between 0.5 and 2.5 mg of PVA/100 mL of solution. For a Cd(OH)zpaste containing between 0.1 arid 0.5% PVA by weight, this corresponds to a sample size of approximately 0.5 g. The samples of interest here are always predominately (>90%) Cd(OH)> For samples of lower or variable Cd(OH)z content, the Cd(OH)z level should be known to within a few percent. The electrode or Cd(OH)2paste should be dried briefly at 105 "C prior to accurately weighing out a sample between 0.25 and 0.75 g. Samples prepared by using high molecular weight PVA of low residual acetate content can readily be dried at this temperature overnight, while low molecular weight PVA pastes should be heated only to dryness to avoid PVA loss due to evaporation. Following drying, the sample is placed into a 100-mLErlenmeyer flask along with 50 mL of 1.2 M HC1 and is stirred until all Cd(OH)zhas dissolved (e.g., 30-60 min at room temperature). The solution is filtered to remove undissolved constituents within the electrode, i.e., insoluble oxides or hydroxides,metallic Cd, organic fibers, etc., added to the electrode to alter either physical or electrochemicalproperties, and is then quantitatively transferred to a 100-mLvolumetric flask and placed into a 25 "C constant temperature HzO bath. To this solution is added 25 mL of 0.65 M H3B03. The solution is well mixed and allowed to equilibrate thermally in the water bath. A 3.0-mL aliquot of the Iz/KI solution is pipetted into the sample flask and the volume brought to 100 mL by using 25 "C HzO. (Final solution concentrations M Iz, 4.5 X are 0.16 M H3B03,1.5 X M KI, and 0.6 M HCl.) After 15 min, the absorbance of the yellow-green solution is measured at 660 nm, the PVA complex's absorption maximum, vs a PVA-free reference solution prepared by using 50 mL of HCl solution,25 mL of boric acid solution,and 3.0 mL of Iz/KI solution diluted to a 100-mL total volume. The concentration of PVA in the unknown solution is obtained from a previously determined calibration plot relating the linear
0003-2700/87/0359-2716$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987 poiyiodide Chain
=
0.8 -
2+
0.7 -
0
a
0.6 0.5 0.4 -
0
m
2 0.1 0 v)
s
2 mg PVA per 100 ml
B
/t f
2 0.3=o_ 0.2 -
v)
-
2717
4
1 mg PVA per 100 rnl
I
I
I
1
I
I
I
I
I
I
0
12 16 20 24 20 32 36 40 44 m l 0 . 6 5 M HgBOg ADDED PER 100 rnl VOLUME
Figure 2. Solution absorbance at 660 nm as a function of H3B03 concentration.
g
\>”+
1.1
1.0
a 0.9
5
2 $
$
9 / F w e 1. Helix structure of poly(viny1alcoholkiodine-boric acM complex (adapted from ref 6).
change in solution absorbance to the milligrams of PVA in solution (see below).
DISCUSSION Basis for Colorimetric Method. The existence of a blue PVA-boric acid-iodine complex has provided the basis for quantitative analytical methods for both PVA and boric acid in solution and paper coatings. Zwick postulated the complex to consist of a helical PVA structure with an associated intramolecular polyiodide chain based upon a comparison with the well-characterized amylose-iodine helical complex (6,101. The presence of triiodide ions in solution induce the PVA molecule to helix formation at isolated stereoregular locations such that 12 vinyl alcohol groups encircle each iodine atom; the stability of this PVA-iodine complex is further enhanced by boric acid linkage of two OH groups belonging to successive turns of the helix, Figure 1. A slight increase in color intensity within the solution, followed by a decrease in complex intensity, results from individual helix association followed by continued solution aging with helix recrystallization and complex precipitation. These features, i.e., association followed by recrystallization and precipitation, have been observed here; however, precipitation was found to occur much more rapidly in the present HCl plus Cd2+solution than in pure HzO, thereby imposing some time limitations on the analytical determination. Influence of Solution Concentrations. The wavelength of maximum absorption for the PVA-iodine-boric acid complex shifts from 690 nm in pure HzO to 660 nm in the presence of 1.0 g/L Cd(OH), in HC1. In addition, the solution absorbance, measured 15 min after complex formation, increased from 0.48 to 0.64, with the higher absorbance in the acidified solution likely arising from enhanced helix association in the acid medium. The level of reagents used was optimized empirically to provide a high level of reproducibility. For example, Figure 2 shows the 660-nm solution absorbance recorded by using a Bausch and Lomb Spectronic 20 for samples containing 0.50 g of Cd(OH)z, 0.60 M HC1, and either 1.00 or 2.00 mg of
Z
0.8 0.7
0.6 0.5 0.4 0.3
SOLUTIONS
0= NO Cd(0H)
a=
C? I
C
3 (1 1 0 VI
PASTE
@ = 0.2Cd(OH), PASTE @ = 0.5Cd(OH), PASTE 1.0Cd(0H); PASTE I 05
10
I 15
I
I
I
2 0 25 3 0 rng PVA PER 100 ml VOLUME
I
35
Flgure 3. Solution absorbance at 660 nm as a function of PVA and Cd(OH), concentration.
PVA/ 100 mL volume as a function of the solution’s H3B03 concentration. All solutions were prepared by dissolving 0.50 g of Cd(OH)z into 50 mL of 1.2 M HC1, the PVA was added from a previously prepared standard solution, and each solution contained 3.0 mL of the 12/KI solution. Absorbances were measured 15 min after complex formation vs. an identically prepared, PVA-free reference. In both cases the measured solution absorbance initially increased commensurate with an increase in volume of reagent added until reaching a level beyond which further reagent addition yielded little or no absorption change. A similar behavior was found for variations in the total 12/KI concentration, with the regions of fairly constant solution absorption used to select the finalized respective levels of and 12/KIsolution additions. A complication to the analytical method was the slight suppression in 660-nm solution absorbance found with increasing Cd(OH), sample size. This suppression, seen in Beer’s law plots of Figure 3, requires a different calibration curve relating the measured solution absorbance to the solution’s PVA content be used for each Cd(OH)2sample size. Failure to take this into account can lead to a sizable error in the sample’s determined PVA level. As an example, consider the analysis of two Cd(OH)zelectrodes possessing the disparate PVA levels of 0.2% and 1.0%. If the final solution is to contain 2 mg of PVA/100 mL volume, the sample size selected for analysis of these two electrodes would be 1000 and 200 mg,respectively. However, even though both solutions contain the same PVA content, the measured 660-nm solution absorbance following formation of the blue PVA-iodine-boric acid complex would be 0.64 and 0.69 absorbance units, respectively, due to the suppression in solution absorbance associated with the higher Cd2+presence. This would correspond to an analytical error of 8% if the proper calibration
2718
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987 Table 036
r
I. A n a l y s i s of PVA P o s t c o a t e d Cd(OH)* E l e c t r o d e s
Cd(OW2 sample size, g
0.4464 0.4790 0.5877 0.5943 0.7327 0 31
I 0.1
I
I I I I I I I I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Cd(OH)* SAMPLE SIZE PER 100 rnl VOLUME (GRAMS)
Flgure 4. Analytical deterrninatlon factor relating sdution absorbance at 660 nrn/rng of PVA to the Cd(OH)* electrode sample size.
curve corresponding to the initial Cd(OH)2sample size was not referred to for calculating the sample's PVA level. This feature is corrected here by graphically presenting the slope of the absorbance vs. PVA content curves from Figure 3, that is the measured solution absorbance corresponding to a PVA concentration of 1 mg, as a function of the initial Cd(OH)2 sample size. This plot, Figure 4,is used to obtain the analytical determination factor, ADF, used in eq l to calculate measd 660-nm absorbance X 100% = %PVA (1) ADF X sample size (mg) the sample's PVA content from the measured solution absorbance. For example, the value of the solution's absorbance measured at 660 nm is divided by the initial sample size (e.g., 500 mg) and the ADF value correspondingto that sample size (e.g., 0.335) to yield the sample's PVA content. Method Evaluation. PVA-containing Cd(OH)2samples were prepared by treating a known quantity of a Cd(OH)2, Cd, and Ni(OH)2paste with an accurate volume of an aqueous PVA solution. After the samples were dried, the PVA content was determined from the increase in sample weight. These samples were subsequently dried for 2 h at 105 "C in a forced air oven and were then analyzed for the PVA content by using the above method. Absorbances, recorded 15 min following complex formation, are given in Table I. Sample sizes varied between 0.45 and 0.73 g of Cd(OH)2paste and corresponding ADF values of between 0.336 and 0.327 were obtained from Figure 4. As is seen in the table, the analyzed PVA content agrees to within a few percent with the theoretical PVA level predicted from the increase in sample weight coincident with PVA addition. Two method limitations which are inherent to the PVAiodine-boric acid complex are worthy of note. The literature (6) reports that recrystallization of the complex in aqueous solutions is slow and, in fact, solution absorbances had been
ADF'
0.336 0.335 0.332 0.331 0.327
measured 660-nm solution absorbance
analyzed
theoretical
0.230 0.190 0.285 0.235 0.270
0.153 0.118 0.146 0.119 0.113
0.161
% PVA
0.114 0.143 0.121 0.110
Analytical determination factor i s t h e solution's 660-nm absorbance/mg of PVA in a 100 mL volume.
reported to change only a few percent following several days at constant temperature. It has been found here, however, that the complex posesses a rather limited stability in this solution environmentand, in fact, initial complex precipitation has been seen in times as short as 1h at the highest solution cadmium concentrations examined here. This limits the analysis time considerably as the solution absorbance must be measured within an hour following complex formation. Since only a few minutes are required for the solution to attain its maximum absorbance, it is therefore recommended that the absorption be measured between 15 and 25 min following addition of the 12/KI solution. A second method limitation is related to the strong temperature coefficient on the measured complex absorbance. The solution absorbance was 10% higher when the complex was formed a t 20 "C than a t 25 "C and 14% lower at 30 "C. An absorbance decrease of a few percent due to solution heating while handling the sample tube was also observed, thereby necessitating caution during analysis. All data reported here were obtained at 25.0 f 0.2 "C by maintaining solutions within a water bath as prescribed in the analytical procedure. Registry No. PVA, 9002-89-5; H3B03,10043-35-3;KI,768111-@ Cd(OH)z, 21041-95-2.
LITERATURE CITED (1) Kunlckl, J.; Kasprzak, G. Chem. Inz. Chem. 1878, 13, 171-178. (2) Kurihara, K.; Yanagklate, T. Jpn. Kokal Tokkyo Koho 79, 30, 433, 06 Mar 1979. (3) Suzuki, S.;Kurlhara, K. Jpn. Kokal Tokkyo Koho 79, 106, 829, Aug. 22, 1979. (4) Furukawa Electric Co. Jpn. Kokai Tokkyo Koho 80, 53, 874, April 19, 1980. (5) Water-soluble Synthetic Polymers: Propertles and Behavior; CRC Press: Boca Raton, FL, 1984; Voi. I, p 145. (8) Zwick, M. M. J . Appl. Polym. Sci. 1865, 9 , 2393. (7) Joshi. D. P.; Lan-Chun-Fung, Y. L.; Prltchard, J. G. Anal. Chim. Acta 1878, 104, 153. (8) Finley, J. H. Anal. Chem. 1861, 33, 1925. (9) Po&vinylalcohol;Finch, C. A.. Ed.; Wiley: London, 561-572. (10) Zwlck, M. M. J. Powm. Sci., Polym. Chem. Ed. 1866, 4 , 1642. (11) West, C. D. J . Chem. Phys. 1849, 17, 219.
RECEIVED for review March 26,1987. Accepted June 19,1987.
