Direct Determination of Normality of Electrolyte Solutions Paul C. Mangelsdorf, Jr. Department of Physics, Swarthmore College, Swarthmore, PA 1908 1
.
Wen M. Chang Department of Chemistry, Woods Hole Oceanographic lnstitution, Woods Hole, MA 02543
Despite the fundamental role of the concept of normality in electrochemistry, it is not often used as an experimental observable. An examination of C h e m i c a l A b s t r a c t s reveals only four entries under normality during the 65-year period 1907-1972, none of these of much experimental significance ( 1 - 4 ) . It may well be that this dearth of references means that normality is not much needed as an experimental variable, but the possibility exists that the lack of an established measurement technique has also been a factor. Inasmuch as we have been making direct normality determinations on mixed electrolyte solutions (e.g., sea water cognates) routinely for the past year, we venture to make our method known.
METHOD The principle of the method is very simple. A solution of a standard electrolyte flows through a cation exchange column and an anion exchange column in series (though a mixed bed column seems to work as well). Into this stream is injected a sample of an arbitrary electrolyte solution whose normality is to be determined. The first peak to emerge from the dual column combination will consist of the standard electrolyte a t the normality of the unknown (the Cheshire cat lags behind, but the smile emerges), so that the normality determination reduces to a concentration determination on a standard solution of known composition. Any quantitative detector that reliably senses the concentration of the standard solution can be used to monitor the output. The precision of the determination can be made as great as one likes by choosing a standard solution of normality close to that of the unknown and using a sufficiently sensitive detector. Because the original normality difference emerges imposed on the standard electrolyte solution, the ultimate limit of detectability of this normality difference is set by the noise level of the detector when a uniform stream of the standard solution is flowing through it.
THEORY The basic theory underlying this method has been presented previously in this journal (see reference 5 especially Equation 2, et seq.; also reference 6, equation 3.19). In any chromatographic process, composition changes in the flowing solution which do not require adjustment of the fixed bed composition must travel through the column with the speed of the solution. Conversely, during the first peak (or front), nothing can change in the solution that would affect the fixed bed composition. As an arbitrary electrolyte mixture follows a standard solution into, for example, the cation column, all the changes in anion composition, including total anion normality, must take place in the first front, whereas none of the changes in cation composition, except in total cation normality, can travel so fast. The solution following this first front will have the normality and anion composition of the unknown electrolyte, but will have a cation composition completely specified by the constraints
that the solution must be a t the new normality, but must remain at equilibrium with the fixed bed composition which was previously at equilibrium with the original standard. When this first front then proceeds into the anion column, the anion composition changes also get left behind, leaving the first front as a pure normality change. Minor qualification of those principles can be made to allow for non-ideality. If, for example, an anion is appreciably bound on the cation column, the method could fail; but, if the extent of binding is small, the adjustment of the column will occur quickly after the initial front passes, so that a useable zone of separation between cation alteration and anion alteration can still be defined. We have also encountered a number of interesting problems associated with buffering solutions and the sorption of H* and OH- ions. These problems seem tractable but have not yet been dealt with theoretically to our satisfaction.
EXPERIMENTAL T o obtain the data illustrated in Figure 1, we have used the following combination: Standard electrolyte: Sargasso Sea surface water, diluted to a n approximate salinity of 27.5%. Flow rate: 120 ml/minute. Sample volume injected: 0.2 ml. Column: 5-cm column, 3-mm i.d., packed with a 50/50 mixture of BioRad AG-50WX12 cation resin (minus 400 mesh) and Mallinckrodt CG-400 anion resin (200-400 mesh), probably somewhat more interbedded than uniformly mixed. Detector: Ion exchange membrane cell of the form
I anion 1 column 'cation standard Ag/AgCl electrode' s e a water' membrane 1 effluent'membrane s e a water, electrode Ag/AgCl standard
This cell has been thoroughly described elsewhere ( 7 ) . Membranes used here are AMFion A-104B anion membranes and C-103C cation membranes formerly manufactured by American Machine and Foundry Company. Detector sensing volume: -0.2 ml. Output signals were amplified and recorded on a chart recorder. With this particular combination, the chromatographic record after the first peak was poorly resolved, confused, and useless, with the anion and cation responses superimposed on one another. More recently, however, we have been using a scheme in which the effluent from an anion column passes through a sampling loop before entering the detector, so that the initial peak can be separately shunted off to a cation column for both normality analysis and cation analysis. The subsequent peaks emerging from the anion column continue through the detector to yield an undisturbed anion chromatogram.
RESULTS In Figure 1,the detector peak height in millivolts is plotted vs. the logarithm (to the base 10) of the sample normality. The important observation is that the response is really quite independent of the electrolyte composition. While NaCl might be considered t o be an approximation to sea water-though a rather poor one, in our opinion-none of the other salts could be easily mistaken for sea salt. NH4* ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
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CONCLUSIONS
t
w
LOGlo
Y O
NORMLLIiV
-
4-
Detector signal plotted vs. logarithm, to the base 10, of solution normality in equivalents per liter Figure 1.
Solutions tested were NaCl ( O ) ,KCi (A),NaN03
(m),
NH&l (V),and MgC12
(e)
There are two general areas in which this method of normality analysis is apt to prove useful: 1) Analysis of mixed electrolyte solutions of all sorts, sea water and blood being two important examples. Total normality observed directly has some clear advantages over total normality deduced by analyzing and summing all cations or all anions, especially if those sums appear to disagree. 2) Precision titer determinations where gravimetric methods are not suitable. When drying and weighing are difficult or impossible, as in the case of HCl or MgC12, a normality determination made by close comparison with some gravimetrically controlled standard could easily be arranged to be much more precise than any titration method. Although this method is contingently original with us, we make no claim for ultimate originality. A method so simple and straightforward is certain to have been noted by others and used by others. Only our observation that high precision is possible if the normality difference between sample and standard is made small enough, is likely to be a t all novel. We would welcome any information helping to establish true priority of the dual column method.
and NOS- are found in the ocean only in trace amounts. Note also that the cation valence does not seem to matter (we could not easily test divalent anions because solutions of most of them form precipitates on contact with sea LITERATURE CITED water). We did test a number of other solutions, mostly acids, for (1) J. Lee, Educ. Chem., 2 (5), 229-35 (1965); CA, 63 15514 g. (2) J. G. Stark, Educ. Chem., 3 (2), 70-6 (1966); CA, 65, 4620 g. which the agreement was not so good but, in all those cases, (3) S. S. Chissick and, E. R. Judd. Educ. Chem., 4 (2), 104-5 (1967); CA, 67, 17672 b. it was evident that the initial peak was being so closely fol(4) W. Farrar. Educ. Chem., 4 (6), 277-9 (1967); CA, 68,3566 e. lowed by another peak as to markedly alter the peak (5) P.C.V.Mangelsdorf, Jr. Anal. Chem., 38, 1540-1544 (1966). height. Presumably the ideality of the response could be (6) F. Helfferich and G. Klein, "Multicomponent Chromatography". Marcel Oekker. New York, 1970. improved considerably by adjustment of experimental pa(7) P. C. Mangelsdorf, Jr., and T. R. S. Wilson, J. Phys. Chem., 75, 1418rameters. Figure l reflects our first and only attempt a t 1425 (1971). setting up an analytical combination solely for normality RECEIVEDfor review November 14, 1974. Accepted March analysis. Incidentally, the point where the experimental line in 26, 1975. This investigation has been supported a t the Figure 1 intercepts the zero voltage axis represents the nor- Woods Hole Oceanographic Institution by AEC Research mality of our standard sea water thus determined. We esti- Contract No. AT(11-1)-3119. This is contribution number 3224 from the Woods Hole Oceanographic Institution. mate this intercept to be 1.673, the logarithm of 0.471 N .
Differential Pulse Polarographic Determination of Digoxin and Digitoxin K. M. Kadish and V. R. Spiehler Department of Chemistry, California State University-Fullerton, Fullerton, Calif. 92634
The cardiac glycosides digoxin and digitoxin (the active components of digitalis) are a t present two of the most commonly prescribed medications in this country. Despite their high use [over two million people were on digitalis in 1974 ( I ) ] , they are not ideal drugs. Even though they are among the more valuable therapeutic agents employed in modern medicine, they are said to be the most dangerous drugs presently in use. I t has been estimated that a therapeutic dose of digitalis is equivalent to 50% of a lethal dose (2). Because of this narrow safety margin between a beneficial and a harmful dose of digitalis and the fact that tolerance levels for the drug vary with the individual, it is important that a sensitive, accurate analytical method for 1714
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both digoxin and digitoxin be available. In addition, this same quantitative test should be both rapid and inexpensive. Both digoxin and digitoxin have been determined by spectrophotometric (3-6), fluorometric (7), and gas chromatographic techniques (8-12). Biochemical assays involving the use of radioimmunoassay techniques (RIA) have also been developed for both digoxin (13) and digitoxin (14).In this paper, we have described the use of differential pulse polarography for the electrochemical determination of both digoxin and digitoxin. Previous studies (15-18) have indicated that digitoxin is reducible a t a dropping mercury electrode. However, quantitation was limited to
