Determination of Potassium by Means of the Cotlove Chloridometer William P. Ferren Department of Chemistry. Wagner College Science Center, Staten Island,
N.Y. 70307
John A. Cucco Stillwell and Gladding Jesting Laboratories, New York, N.Y . 70006
The determination of potassium has been carried out by numerous analytical procedures (1-6). Gravimetric methods are accurate but involved while volumetric and spectrophotometric methods are usually indirect and also tedious. The technique most widely used is flame photometry which suffers by interference from sodium (7). A potentiometric method (8) has been described which eliminates the error caused by the presence of sodium but it employs a titration and a back titration. Coulometric procedures have been described by Patriarch and Lingane and others (9-12) for the determination of potassium. While these methods are excellent in regards to precision and accuracy, the equipment used is atypical of the commercial instruments usually found in a clinical laboratory. In contrast, the Cotlove chloridometer is found almost universally in any clinical or hospital laboratory and, in point of fact, is one of few coulometers so widely employed. Our coulometric procedure uses a chloridometer (designed specifically for the determination of chloride in blood serum) without any modification or change in instrument design but with the proper prior application of the tetraphenylborate ion precipitation technique which adapts the chloridometer for use as a “potassiumometer.” The resultant convenience, low detection limit, automatic shut-off, and readout are typical of the methodolgy required for multielement testing as described by Laessig (13). A similar adaptation of the chloridometer has been described by Ferren and Fortinash ( 1 4 ) in the determination of organic chloride as opposed to ionic chloride.
EXPERIMENTAL A p p a r a t u s . A Buchler Digital Chloridometer Model 4-2500 was used. Reagents. Potassium nitrate. sodium tetraphenylboron; nitric acid. acetic acid, ethyl alcohol (all reagent grade) were used. All of t h e above compounds are stable and can be stored for a long period of time with t h e exception of sodium tetraphenylboron. As a solid, tetraphenylboron can be kept for several months and as a solution for approximately one or two weeks or until the solution H. Tollert, “Analytik des Kalium,” Enke, Stuttgart. Germany 1962. S. Kallman, “Treatise on Analytical Chemistry,” I . M . Kolthoff and P. J . Elving, Ed., Vol. 1 . Part I I , Wiley-lnterscience, New York,
N . Y . . 1961. E. N . Archibald. W. G . Wilcox, and B. G . Buckley, J. Amer. Chem. SOC..30, 747 (1908) G . F. Smith and T F. Ross, J . Amer. Chem. SOC.,47. 1020 (1925). M . Kohler, Z. Anal. Chem.. 138,9 (1953).
“Official Methods of Analysis of the A.O.A.C..“ Lindo-Gladding Method, 10th ed., 1965, p 20. I . M . Kolthoff. E . 8. Sandell. E. J . Meehan, and S. Bruckenstein. “Quantitative Chemical Anaiysis.” 4th ed., Macmillan, London, 1969. T . S. Prokopov. Anal. Chem., 43, 793. (1971). G. J . Patriarche and J. J . Lingane, Anal. Chem.. 39, 168 (1967) S. Suzuli, Jap. Anal., 10, 837 (1961) G . D. Christian, J . Electroanal. Chem., 11, 94 (1966). G . J. Patriache and J . J. Lingane. Anal. Chim. Acta.. 37, 455 (1967). R. H. Laessig, Ana/. Chem., 43 (8),18A (1971). W . P. Ferren and W. M . Fortinash., J. A s s . Offic. Anal. Chem.. 56, in press.
clouds. In general, all sodium tetraphenylboron, solid or solution, should be kept refrigerated and in the dark to ensure maximum shelf life. Procedure. Samples analyzed were aqueous potassium nitrate solutions, which were added t o 4-ml portions of t h e standard chloridometer titration solution just prior to coulometric titration. The standard chloridometer titration solution is prepared by adding 6.4 ml of concentrated nitric acid a n d 100 ml of glacial acetic acid t o 900 ml of distilled water. The usual procedure is to add 4 drops of a gelatin-indicator-preservative mixture consisting of gelatin, thymol blue, and thymol in a weight ratio of 60:l:l which is prepared in advance by adding 6.2 grams of this dry mixture to 1 liter of hot distilled water. In our procedure, the standard chloridometer titration solution was modified as follows: (1) 1.8 grams of polyvinyl alcohol were dissolved in 100 ml of hot distilled water and cooled to room temperature. (2) T h e use of gelatin-indicatorpreservative was eliminated. (3) T h e potassium chloridometer titration solution was prepared by adding 6.4 ml of concentrated nitric acid to TOO ml of distilled water, with good agitation, and subsequently adding 100 ml of glacial acetic acid, 100 ml of the polyvinyl alcohol solution, and finally 100 ml of ethyl alcohol. Acid “blanks” were coulometrically titrated by the generation of‘ silver ions a t t h e chloridometer HIGH setting. After this preliminary treatment to remove any impurities t h a t might react with silver ions, a 0.1-ml sample of 0 . 2 M sodium tetraphenylboron was added to t h e acid vial by means of a n Eppendorf pipet a n d sodium tetraphenylboron content determined by operating the Buchler-Chloridometer at the HIGH setting. The digital readout presented after automatic monitoring of the end point is in terms of milliequivalents per liter of tetraphenylboron instead of the usual chloride ion measurement (See Table I). The sodium tetraphenylboron blank solution should not be permitted to stand for any length of time because as stated by Patriarche and Lingane (9) and others ( I 5 ) , a n acidic supporting electrolyte does involve some reaction between the acid “blank” and sodium tetraphenylboron. The above procedure was repeated except this time 0.1 ml of 0.2M sodium tetraphenylboron solution plun 0.1 ml of 0.1,M potassium nitrate solution was added to an empty chloridometer via! a n d 2 minute reaction time was allowed before adding the 4 ml of acid reagent. The digital readouts presented a t the end of this second series of coulometric titrations were one-half those of the first series as shown in Table I. T h e difference between the digital readout in t h e two series represents the potassium content of t h e second system. In a similar fashion, another series of ten measurements was carried out,using 0.1 ml of 0.2M sodium tetraphenylboron plus 0.3 ml of 0.05.44 potassium nitrate solution. 4 n additional series of another ten measurements was performed employing 0.1 mi of 0.1M sodium tetraphenylboron plus 0.1 ml of 0.05M potassium nitrate, using the chloridometer at the LOW setting. Results obtained are shown in Table 11.
RESULTS AND DISCUSSION The chloridometer as designed by Cotlove (16, 17) represents a type of coulometer designed for “the automatic, rapid, accurate and sensitive determination of chloride in biological samples.” The contribution of Cotlove was to (15) H. Flaschka and J. Barnard, J r . . Advan. Anal. Chem. Instrum.. 1 , 119601
(16) E. Coilove, H. V. Trantham, and R . L Bowman, J . Lab 50,358 (1958). (17) E. Cotlove and H. H . Nishi, Ciin. Chem.. 7 , 258 ( 1 9 6 1 ) .
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sence of Cotlove’s chloridometer is the coulometric generation of silver ions in a nitric acid-acetic acid medium with an amperometric indicator electrode system designed to automatically stop the generation of silver ions and record by means of digital read-out system the amount of chloride present in terms of milliequivalents per liter. The presence of excess free silver ions depolarizes the indicator electrodes and is evidence that all chloride initially present has been precipitated as silver chloride. One extension of chloridometer from the determination of free ionic chloride to organic chloride, such as organohalogen pesticides, has been reported by Czech (20) and Ferren and Fortinash (14). The same approach of appropriate prior chemical treatment and modification of the chloridometer titration solution was used by these investigators in an effort to extend coulometry into the area of “multielement chemical testing in clinical analytical chemistry,” as described by Laessig ( 1 3 ) . The limit of detection with the Model 4-2500 Buchler Chloridometer is about 1 milliequivalent per liter using a 10-microliter sample on the LOW setting where the generator current is approximately 5.6 milliamperes. In terms
Table I. Coulometric Determination of 0.1 ml of 0.2M Sodium Tetraphenylboron and a Mixture of 0.1 ml of 0.2M Sodium Tetraphenylboron plus 0.1 ml of 0.1M Potassium Nitratea Trial No.
0.1 ml of 0 . 2 M sodium tetraphenylboron, mequiv/l.
0.1 ml of 0.2M sodium tetraphenylboron f 0.1 ml of 0.1M KN03, mequiv,’l.
199.1 204.0 199.8 199.6 200.6 201.5 199.6 200.8 203.0 200.3 Av = 200.83 Std dev = 1.58 9 5 % Conf. interval = (197.66-204.0)
102.4 100.2 98.2 102.0 100.6 99.1 102.0 101.4 101.8 102.4 Av = 101.01 Std dev = 1.45 95% Conf. interval = (96.6-105.4)
1 2 3 4
5 6 7 8 9 10
Each of the above volumes were measured out using a 1 00-pi syringe.
Table I I . Coulometric Determination of Mixture of 0.1 ml of 0.2M Sodium Tetraphenylboron plus 0.3 ml of 0.05M Potassium Nitrate; a System Containing 0.1 ml of 0.1 M Sodium Tetraphenylboron and a Mixture of 0.1 ml of 0.1M Sodium Tetraphenylboron plus 0.1 ml of 0.05M Potassium Nitratea Trial No
va
0 1 ml of 0 2M sodium tetraphenylboron f 0 3 ml of 0 05M KNO3 mequiv i
1 2 3
52 7 51 5 52 1
4 5 6 7 8 9 10
50 9 52 53 51 51
0 1 ml of 0 1M sodium
tetraphenylboron mequiv I 103 0 102 9 105 9 104 6 102 8 105 1 103 4 102 8 103 6 104 1 Av = 103 74 Std dev = 1 08 9 5 % Conf interval = (101 57-105 90)
8 1
0 3
50 8 51 4
Av = 51 76 Std dev = 0 85 95% Conf interval
= (50 9-53
46)
0 1 ml of 0 1M sodium tetraphenylboron f 0 1 mi of 0 05M KNO3 mequiv I
51 51 2 53 2 50 8 52 1 51 8 51 6 50 8 52 0 51 5
Av = 51 6 Std dev = 0 73 95% Conf interval = (50 14-53 06)
The 0 2M sodium tetraphenylboron used in the above determinations is from the same solution used for the determinations in Table I
take a chemical system as nonspecific as the precipitation of silver halides and design a chemical system and a coulometric instrument specific for the determination of chloride in biologic fluids. The universal acceptance of the chloridometer in hospital and clinical laboratories parallels the acceptance of the pH meter in almost all research and industrial chemical laboratories. While the p H meter design has more or less remained the same, the modification of the chemical system uia specific ion electrodes has revolutionized its area of application. Lingane wrote in 1958 ( 1 8 ) , “At long last . . . coulometric analysis has been added to the analytical chemist’s roster of tools.” Unfortunately no inexpensive coulometer has become commercially available except for the uni-purpose chloridometer. The chloridometer has not been applied to the coulometric determination of other electrolyte ions, such as potassium, prior to our method although it has numerous other potential applications with appropriate electrode and chemical system modifications (19). The es(18) J. J. Lingane,Ana/.Chem., 30, 1716 (1958). (19) W. P. Ferren, “Practical Coulometry,” Wagner College Science Center Press, Staten Island, N.Y. 1972.
of potassium, this corresponds to about 4 micrograms when using a 0.1-ml sample volume. Utilization of larger sample volumes might extend this limit even lower. In those situations where halogens are present in the potassium compound or the sample contains a halogen salt, such as sodium chloride for example, the chloride is determined by the standard Cotlove procedure prior to applying our method. During this investigation, potassium was determined in blood sera in the range 5 to 3 milliequivalents per liter after determining chloride in the range of 99to 105 milliequivalents per liter. The use of ethyl alcohol in our modified titration solution lowered the solubility of silver tetraphenylboron and yielded a sharper end point. A higher volume of ethyl alcohol interfered with the indicator electrode system because of changes in solution conductance. According to Prokopov ( 8 ) , mercury(I1) decomposes sodium tetraphenylboron. Further, only NH4+, A ~ T T1+, , R b + , and Cs+ ions form very slightly soluble precipicates with the tetraphenylborate ions and halogens: none of the common cat(20) F. P. Czech , J. Ass Ofiic. Ana/. Chem., 51, 568 (1968)
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ions and anions interfer with the potassium determination in dilute nitric acid medium. When the chloridometer is used in the HIGH range, the titration is carried out a t a relatively fast rate. For example, a lOO-mequiv/l. sample of sodium tetraphenylboron requires about 30 seconds titration time. With the Buchler Model 4-2500 Chloridometer set on the LOW range, this same 100 mequiv/l. sample requires about five minutes titration time. For most titrations, the LOW range setting is more accurate, but for our system because of the lack of stability of tetraphenylboron in acid medium, it is advantageous to use the fast rate of the HIGH range setting. Cation exchange between silver ions and potassium ions is avoided because of the rapidity of our procedure and thus the troublesome titration of potassium tetraphenylborate precipitate or back titration of excess silver ions is avoided.
Potassium, a cation of extreme importance in biological fluids and cells, can be determined by our procedure using a type of coulometer familiar to virtually all clinical chemists. Future efforts might be directed to improving this procedure for the analysis of potassium in blood sera. The increased concentration of potassium in immature cancer cells as compared with normal cells (21) should encourage others to modify and extend our procedure to the analysis of cellular material.
Received for review March 23, 1973. Accepted July 16, 1973.
(21) Kenneth Maclean, Biomagnetic Institute, New York, N.Y., personal communication, 1973.
I CORRESPONDENCE Precaution in Computer Simulation of DTA or DSC Curves Sir: Because a computer has no way of ascertaining the preciseness or completeness of its data or instructions, special care must be used in describing the system under analysis to avoid spurious effects. Spurious effects can easily lead to inaccurate conclusions or predictions. For this reason, critical examination of reports of computer simulation of DTA or DSC peaks is important. There is enough to be gained by successful description to justify prompt criticism so that other workers can pursue effective methods as well as avoid errors. Robinson and Scott ( 1 ) have derived a set of equations based upon the characteristics of a differential scanning calorimeter, prepared a relatively simple computer program to describe and use these equations, and have simulated DSC curves for phase diagram studies. Their computed curve for a naphthalene-azulene system showed a double peak. Since the experimental peak for some compositions also showed a double peak, they examined a more extensive range and found that “many of the computed curves showed more or less well defined double peaks.” This behavior is not immediately obvious or predictable from their set of equations, but the reason lies therein. Simplifying the terms used by Robinson and Scott, f, the fraction melted a t temperature, 7, of the sample is as illustrated in Figure 1. This is identical to Robinson and Scott’s
( 1 ) P. M. Robinson and H. G . Scott, Nature (London). 238. 14-15 (1972).
176
It is very apparent from the slope of the solidus in Figure 1 that in the early stages of a melting with great separation of the liquidus and solidus, f will increase very rapidly at the start. That is, a low slope for the solidus yields a high df/dT (or dfldr). The result is a sharp break away from the base line followed by an approach to steady state heating and melting; that is, a maximum followed first by decay, then by the normal rise in differential power. In the case shown by Robinson and Scott, the effect is accentuated by the existence of a horizontal section of solidus (eutectic). The quantity f ( T ) is not satisfactorily defined at the beginning of the peak (Figure 2) because at any increment of T above the eutectic temperature, y and z must suddenly assume very real values. The programming would see a pronounced discontinuity and call for an immediate increase in the observed quantity, the differential power. This is because it sees, suddenly. a non-zero (and not very small) value of f and, consequently, a near infinite value of dfldt. The sudden rise of computed temperature does not disappear immediately in the equation given, but it does tend to diminish because the computed df/d.r is small (though increasing). Eventually the spurious input is dissipated and the smooth program is followed as f 1. Because of the relative slopes of the liquidus and solidus for this illustrated composition, the quantity df/dr increases steadily as f 1 until the discontinuity at f = 1. when the df term again drops out. Consequently, a second peak is seen. These effects are in complete agreement with the computed peak, which is not in good agreement with their experimental curve. The double peak in the latter arises from another cause. Experimentally, the supplied power in a differential scanning calorimeter is not related directly to T , - T ~ as .
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