stian. The commercial electrode and the PVC-disk electrode gave stable responses for one month. The PVC-wire electrode could be used for a period of two weeks before it lost linear response, Hydroxide ion is reported to interfere with the measurement of perchlorate (6). This interference is shown in Figures 3 and 4 by the mixed solution procedure in which the activity of the interfering ion is constant and the activity of the ion of interest is varied (7). T h e selectivity coefficients of these electrodes were calculated from
where a, and a, are the activities of the perchlorate and the hydroxide ion, respectively. K,, equals 1.2 x for the commercial perchlorate electrode and 1.3 X for the PVC-disk electrode in 0.1MS a O H solution. Interference by iodide, bromide, and nitrate ions was determined by the separate solution method and calculated from,
303RT / Z F
=
logK,,
+ log a
(2)
The selectivity coefficients determined for these ions using the PVC-disk electrode are 5.0 X l W 3 (lO-lM I - j , ( 7 ) G . J Moody a n d J. D. R. Thomas. Taianta. 1 9 , 6 2 3 ( 1 9 7 2 )
1.0 x 1 0 - 6 (10-2M B r - ) and 2.9 x 10W5 (10-2M N O s - ), respectively. The selectivity coefficients for the same ions are reported to be 1.2 X lo-* for iodide, 5.6 X for bromide, and 1.5 x for nitrate, with the commercial liquid electrode (6). The response times for measurements of the more concentrated solutions of perchlorate were of the order of 30 to 60 seconds. The response time for measurements of the 10-5M solution was approximately 120 seconds. CONCLUSION Electrodes prepared by using liquid ion-exchangers in PVC show approximately the same characteristics as the commercially available electrode. The PVC-disk perchlorate electrode has the same linear range as the commercial electrode, but the PVC-wire perchlorate electrode had a shorter linear range. The selectivity of the electrode was improved by incorporating the exchanger in a PVC matrix.
ACKNOWLEDGMENT The authors wish to thank J . D. R. Thomas for a generous supply of poly(viny1 chloride). Received for review J u n e 6, 1973. Accepted October 25, 1973. The financial assistance of the Environmental Protection Agency (Grant No. R-800359) is gratefully acknowledged.
Change in Potential of Reference Fluoride Electrode without Liquid Junction in Mixed Solvents Kathleen M . Steltingl and Stanley E. Manahan2 Department of Chemistry. University of Missouri-Columbia.
Columbia. Mo. 6520 1
A major problem in the determination of formation constants of metal-organic solvent complexes is liquid junction potentials in mixed solvent systems. This is particularly true in the determination of the formation constants of weak complexes where it is necessary to add organic solvent ligand to such a n extent that an appreciable fraction of the solvent medium no longer is water. The fluoride electrode ( I ) used in cells without liquid junction provides some unique possibilities as a reference electrode. The electrode is quite stable: it has a low impedance; it is relatively interference-free: and. in many cases. the low concentrations of fluoride ion required to poise the potential of the electrode can be tolerated in a medium without detrimental effect upon the system. This paper describes the use of the fluoride electrode as a reference in a cell without liquid junction for the determination ofsilver-acetonitrile complexes. It shows that discrepancies in this system can be explained on the basis of altered solubility of lanthanum fluoride in a medium containing an appreciable mole fraction of solvent as acetonitrile. Present address. Department of Chemisrry. California State University. Fresno. Calif. 93710. .4uthor to whom inquiries should be addressed. ( 1 ) Stanley E. Manahan.Ana/. C h e m . . 42, 128 (1970)
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 4 , A P R I L 1974
EXPERI-MENTAL Apparatus. For the attempred deterniinarion or formation constants in a cell wirhout liquid junction. a dual electrode sysrem consisting of a solid-stare silver, sulfide indicating electrode (Orion S o . 94-16)and a solid-state fluoride elecrrode (Orion S o . 94-0921) was used. The elecrrode system was conlained in a jackered glass cell regulated to 25.00 i 0.05 "C with a constant temperarure bath. The solution in the cell was srirred rnagrietically during nieasurernents. The silver $sulfide elecrrode wab curinected to the measuring side of a Corning Model 12 expanded scale p H meter and rhe fluoride electrode was connected directly to the reference input. a configuration permitted by the low impedance of the fluoride elecrrode. An electrode system with liquid junction consisted ot a silver sulfide electrode and a conventional aqueous calomel electrode with a 1 . O S S a C l filling solution bridged with an agar bridge made up with O,lOOAbf sodium perchlorate. For studies involving the glass sodium electrode (Beckman S o . 39137). the glass electrode was connected to the high impedance terminal of the merer and the fluoride or calomel electrode to the low impedance terminal. Subsequent mention of "reference electrode" in this work does not imply the terminal t o which the electrode was connected--e.p.. the glass electrode used as a reference was actually connected to the measuring terminal. Reagents. Acetonitrile. Eastman Chromatoqualit:,- Reagent Grade. was used without additional purification. Sodium perchlorate supporting electrolyte was prepared by neutralization of perchloric acid (hlallinckrodt AR Grade) with sodium hydroxide. Solutions were stored in polyethylene bottles.
Procedure. To solutions containing 1.00 X 10-3FAg(I), 1.00 X 10 - 4 FNaF, and 0.100M sodium perchlorate were added solutions consisting of the same constituents containing also 1 2 N acetonitrile. Potential readings were taken in media ranging up to 4M in acetonitrile.
2ot
RESULTS AND DISCUSSION T h e shift in potential upon addition of acetonitrile solution as a function of log[CH&N] is shown in Figure 1. T h e complex formation curve for the solution having a cell with liquid junction is of the normal shape expected for such a complexation system. Overall formation constants calculated from this curve are ( 2 ) = 2.6 and p 2 = 6.0 for the complex species AgCH3CN- and Ag(CHsCN)z-, respectively. I n calculating these formation constants. correction was made for liquid junction potential changes a t the reference electrode resulting from the addition of organic solvent. This was done by following the potential of the reference electrode us. a glass sodium electrode as acetonitrile was added. Details of the correction are given in Reference 2. Basically, the assumption is made t h a t the potential of a relatively nonhydrated glass electrode is essentially independent of the composition of a variety of water-organic solvent mixtures ( 3 - 4 .The potential of the calomel electrode and the potential of the fluoride electrode, both us. the glass sodium electrode. are shown in Figure 2 as a function of acetonitrile concentration. T h e potential of the fluoride electrode shifts -44 mV in going from aqueous solution t o 4 M acetonitrile. Acetonitrile affects the silver/sulfide electrode potential by complexing with silver ion. T h a t there is no effect of acetonitrile on the electrode p e r se is confirmed by identical results obtained with a silver metal electrode (2). T h e plot obtained from the cell without liquid junction (Figure 1) deviates considerably from t h a t taken in the cell with liquid junction. Furthermore, with increasing level of complexing agent, the negative shift in potential does not increase as would be expected. This discrepancy must be attributed to the reference electrode used. T h e deviation in potential cannot be explained upon the basis of a change in activity coefficient of fluoride ion. I t can be estimated (2) from the Debye-Huckel theory, t h a t the maximum change in activity coefficient of the fluoride ion in going from pure water to a medium 3.75M in acetonitrile would cause a potential change of only approximately 0.1 mV. If the discrepancy is due to a n asymmetry potential across the lanthanum fluoride membrane, this potential should be manifested as a difference in solubility of lant h a n u m fluoride in water as compared to a water-acetonitrile mixture. T h e solubility of lanthanum fluoride in water and in 4.OM acetonitrile can be evaluated by the titration of La3- with F- in water and in 4.OM acetonitrile. T h e titration curves are shown in Figure 3. T h e titration curve is sharper in 4.0M acetonitrile indicating a lower solubility product. Equivalence points, evaluated by the Gran method were used for calculation of K , by the Nernst equation. T h e average value calculated for K, of freshly precipitated lanthanum fluoride in aqueous solution at 0.100 molar ionic strength is 4.8 f 0.6 X 10-l8. This value compares favorably with the value of 1.2 x 10 l8 a t a n ionic strength of 0.03M reported by Lingane ( 6 ) . T h e average value calculated for K , of LaF3 in 4.OM (2) Kathieen Stelting. Ph.D. Thesis, University of Missouri-Columbia, 1973 (3) G. Rechnitz and S. Zarnochnick, Taianta, 11, 979 (1964). ( 4 ) R. Lanier, J. Phys. Chem.. 69, 2697 (1965). (5) Advan. A n a / . Chem. instrum.. 4, 302 (1965). (6) James J. Lingane. A n a / . Chem.. 40, 935 (1968).
I
l
1.20
1.60
l
l
l
l
0.80 0.40 log [CHSCN ]
l
l
l
l
l
0.00 -0.40
-
Figure 1. Potential of t h e silver/sulfide electrode vs. t h e calomel electrode ( D ) and vs. the fluoride electrode ( 0 ) in a medium 1.00 X 10-3F in Ag(l), 1.00 X 10-4F in NaF, and 0.100M in NaCIO4 upon addition of acetonitrile
40r
Figure 2. Potential of the calomel electrode ( D ) and fluoride electrode ( 0 ) vs. sodium glass electrode upon addition of acetonitrile
-80
0
I O 20 30 40 50 60 ml I . O O X I O - ~ La(NO3I3 added
Figure 3. Titration of 3.00 X 1 0 - 3 M NaF with 1.00 X 10-3M La(N03)3 in water ( D ) and 4.OM acetonitrile ( 0 ) .Both media
are 0.100M in NaCIO4 ANALYTICAL CHEMISTRY, VOL. 46, NO. 4 , A P R I L 1974
593
acetonitrile under identical conditions is 4.3 f 0.5 x lo-*". T h u s lanthanum fluoride is appreciably less soluble in 4.OM acetonitrile than in water. The difference in solubility product between the aqueous and partially organic media may be used to explain the shift in potential of the fluoride electrode when used as a reference. T h e equation for fluoride electrode response a t 25 "C may be written as
E = E,
- 0.0592 log uF-
=
E,*
+ 0.0592 3 1%
ULn
~
-
Since a~~ 3 + and U F - are related through the solubility product constant for lanthanum fluoride,
K.
= ( ~ 1 -)(aF-)' , ~ ~
(3)
the equation for electrode response may be expressed as follows:
E = E , *+
0.059 K. log 3 OF-)
7
~
(4)
This equation separates the E , term normally referred to in describing fluoride electrode response into a membrane solubility term (expressed as 0.0592/3 log K , and E,*,a term representing all other contributions to the "constant" term. In aqueous solution, the potential measured for a given fluoride concentration therefore can be given by
E,,
=
E,," - 0.0592 log
CZF-
0.0592 +_ _ log K , 3
0.0592 +log K , , 3
(6)
where K,,o is the solubility product constant of the lanthanum fluoride membrane in the partially organic solvent mixture. Thus, the shift in potential, JEobs,at a given, constant fluoride concentration on transfer from aqueous to partially nonaqueous solution is given by
(1)
where E is the potential in volts, E, is a constant for a specific electrode system in a specific medium, and uF- is the activity of the fluoride ion. However, the electrode also responds to lanthanum ion, and the potential may be expressed as the following:
E
E,, = E , * - 0.0592 log aF-
(5)
where Ka.wis the solubility product constant for the lanthanum fluoride membrane in water. Likewise, potentials measured in a partially organic medium may be expressed as
For 4.OM acetonitrile, potential changes resulting from changes in LaF3 solubility were determined by substitutand Ks,w = 4.8 X into ing Kh,o = 4.3 X Equation 7. The potential shift predicted on the basis of solubility differences is -40.4 mV. The shift actually observed was -44 mV. Therefore, the shift observed a t the fluoride electrode on addition of acetonitrile may be explained in terms of changes in the solubility of the LaF3 membrane. Since the K , values from which the potential shift was predicted are for freshly precipitated lanthanum fluoride rather than large, well-aged crystals, it is doubtful that any particular significance can be attached to differences of less than f5 mV. The fluoride electrode in a cell without liquid junction has been found less suitable than the calomel electrode with liquid junction as a reference electrode for complexation studies in systems where addition of an organic ligand appreciably alters the properties of the solvent. The relatively large negative shift in potential of the fluoride reference electrode in going from a n aqueous medium 0.100M in acetonitrile to a similar medium 4.OM in acetonitrile can be explained entirely upon the basis of decreased solubility of the lanthanum fluoride electrode membrane in the partially organic medium. Received for review August 20, 1973. Accepted November 7, 1973. This research was supported in part by the United States Department of the Interior Office of Water Resources Research Allotment Grant A-049-Mo. K . M.Stelting gratefully acknowledges support through X.D.E.A. Title IV Fellowship funds and a n American Chemical Society Analytical Division Summer Fellowship sponsored by Carle Instruments, Inc.
Quantitative Spectrometric Determination Specific for Mannose Ralph W. Scott and Jesse Green Forest Products Laboratory, Forest Service. U.S. Department of Agriculture, Madison. Wis. 53705
Specific quantitative methods for single sugars in sugar mixtures generally require a preliminary separation of the sugars or the application of specific enzymes such as glucose oxidase for the measurement of glucose. Photometric methods are of interest because they are sensitive and simple, but their use is often limited by a lack of specificity. Maksimenko et al. ( I ) have recently described a method t h a t deals with the specificity problem in mannose-glucose mixtures by using the different sensitivities of mannose and glucose in the phenol-sulfuric acid reaction. (1) 0. A . Maksimenko. L. A . Zyukova, N . S. Andreev, and R. M . Fedorovich, Zh. A n a / Khim., 26, 2467 (1971).
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 4, A P R I L 1974
In the procedure described here, use is made both of the dehydration of sugars to furans in concentrated sulfuric acid and of a rather specific change in the dehydration of mannose caused by chloride and boric acid. This sensitivity to chloride and to boric acid permits the determination of mannose in plant materials without preliminary separations of sugars.
EXPERIMENTAL Apparatus. Concentrated sulfuric acid and 72% sulfuric acid were dispensed by glass hand-pumped dispensers (5-ml Repipet, Labindustries. 1802 Second St.. Berkeley. Calif. 94710) with caps to fit 9-lb sulfuric acid reagent bottles.
