of D, for GC, and high pressures will be more feasible in LC because of the reduced danger of explosion. Difficulties encountered in using very fine packing will naturally require attention. Coninsiderations similar t o these-Le., creasing analysis speed with a limited pressure drop-have been discussed for GC by Knox (8). The foregoing treatment has indicated the magnitude of the parameters needed to make LC fully analogous to GC. I t is found that such analogous conditions put LC at a distinct disadvantage; the time required for analysis is increased by a factor of from lo2 to lo6. It is thus natural to inquire if analogous conditions are really the most desirable conditions. We have indicated, for instance, that the mass transfer coefficient for the stationary phase, C,, should be considerably larger in LC than in GC for analogous conditions. I n GC this term is generally large enough to be very significant in limiting resolution and analysis speed. It would be equally
harmful in LC under analogous conditions. It is probable that C. can be reduced well below its GC analog in most LC systems; in some cases the actual value of C, should be comparable, which means that the reduced transfer parameter, R, is -lo5 times less in LC than analogous conditions would demand. If C, can thus be rendered negligible in LC, higher reduced velocities are probably feasible without the severe loss of resolution expected for such an increase in GC. This would improve analysis speed in LC considerably. The brief treatment given here indicates that column parameters can undoubtedly be found that will make column performance in LC fully analogous to those in GC. This path may be profitable in acquiring all possible advantages of the GC technique. However, some of the disadvantages of LC should be avoided by the intentional departure from the GC analog.
LITERATURE CITED
(1) Pa! Nogare, S., Juvet:,R. S., “GasLiqmd Chromatography, Interscience, New York. 1962. (2) DeFord,’D. D., Loyd, R. J., Ayers, B. 0., ANAL.CHEM.35,426 (1963). (3) Giddings, J. C., Ibid., 35, 439 (1963). (4)Ibid., p. 1338. (5) . . Giddings, - . J. C., J . Chromataa., ” . in press.
(6) Hagdahl, L., in “Chromatography,” p. i 8 . E. Heftmann, ed., Reinhold, New York. ~1961.- - ~ (7)Karr, C., Jr., Childers, E. E., Warner, W. c.,ANAL.CHEM.35,1290 (1963). (8) . . Knox, J. H., J . Chem. SOC. 1961, I
R. H., Purnell, J. H., ANAL. CHEM.35,430 (1963). (10) Snyder, L. R., Ibid., 33, 1527. (1961). (11) Snyder, L. R., in “Advances in Anal. Chem. and Instr.,” C. N. Reilley, ed., Vol. 3 (in press), Interscience, New York, 1964. J. CALVIN GIDDINQS Department of Chemistry University of Utah Salt Lake City 12, Utah (9e’€%:ett,
WORK su ported by a research grant from the Rational Science Foundation.
Direct Coulometric Titrations with Hypobromite Ion SIR: The coulometric titration of ammonia with electrolytically generated hypobromite ion developed by Arcand and Swift (1) has been very useful. Recently, an extension of the method to organic nitrogen compounds was made (2). The technique used by Arcand and Swift involved generation of BrO- in basic solution until beyond the titration end point, acidification of the solution, and determination of the excess bromine either by a cuprous ion back-titration or a further generation of bromine with an amperometric titration plot. A simple titration to a fixed current level in
Table 1.
of
Direct Coulometric Titration
basic solution could not be made because the indicator electrode currents were not sufficiently stable. Both of the methods for end point detection developed by Arcand and Swift suffer from two major faults: an extra step, and possible source of error, is added to the method and the oxidized species may react with the bromine in acid solution. The latter item, in particular, is important since it limits the applicability of the method. Therefore, a study was made to see if the end point detection might be simplified. EXPERIMENTAL
Reagents. A buffer containing 10 grams of sodium tetraborate decahydrate and 500 grams of potassium bromide per liter was used for all
titrations. All other chemicals were reagent or U.S.P. grade and were used without further purification. Apparatus. Two different coulometers were used for the study. One consisted of a Sargent Constant Current Power Supply (No. 30974) and a Simpson 50-pa. meter with a 1.5volt battery and variable resistor for the indicator system. The other coulometer used a Regatron Constant Current Power Supply, Model C 612-A (Electronic Measurements Co., Inc.), Model S-10 Precision Timer (Standard Electric Time Co.) and a 10-0-10 pa. meter (Assembly Products, Inc.) with a 1.5-volt battery and variable resistor for the indicator system. A platinum anode and isolated platinum cathode (Leeds and Northrup Co.) and two platinum wire detecting electrodes comprised the titration cell assembly. A
10.2 Micromoles of Ammonium Chloride with Hypobromite Ion4
Micromoles Run 1
2
3
4 5
6
7 8
9
lob
Micromoles “IC1 consumed found 30.9 10.3 30.9 10.3 30.9 10.3 30.9 10.3 30.7 10.2 30.7 10.2 31.0 10.3 31.9 10.6 30.9 10.3 31.1 10.4 Mean 10.3 Br
u
fO.ll
Titrations carried out at 19.3 ma. current. * Titration carried out at 9.65 ma. current. a
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ANALYTICAL CHEMISTRY
Figure 1. Representative plot for direct coulometric titration of ammonia with hypobromite ion
small voltage (0.15 voli,) was impressed across the detecting ele1:trodes. Procedure. A suitable-sized sample was transferred to a 100-ml. beaker, 10 ml. of buffer solution were added, and the solutic n was diluted to 50 ml. with distilled water. The p H a t final dilution was 8.9. A stirrer and the electrode assembly were dipped into the solution and the initial current in the G etection system recorded. Bromine (hypobromite) was generated and the current in the detection system recorded at suitable electrolysis intervals. The current-electrolysis time data were then plotted and an extrapolation to the end point was made. A blank titraticn was carried out in the same manner arid the time subtracted from the srimple titration. Calculations based or Faraday’s law were then made to obtain the amount of reactive species in the cell. RESULTS AND DI!ICUSSION
Although the indic,itor currents in basic solution were not stable enough for titration to a fixed current level, it was thought that a plot of the indicator current as a function of electrolysis time might be suitable for analytical purposes. Figure 1 shows a typical curve for the titration of ammoniuri chloride. The current prior to the cnd point of the
titration was essentially constant and then increased in a nonlinear fashion after the end point. However, an extrapolation to the end point was easily and reproducibly made. Since the current during the titration was constant, it was necessary to record only one or, a t most, two points prior to the end point. A blank titrat.ion was carried out in the same manner and the time was subtracted from the titration of the unknown. At a generation current of 19.3 ma., the blank titration was 3.5 seconds with the sample titration ca. 158 seconds. Table I lists the data for 10 determinations of ammonium chloride. The standard deviation for the 10 analyses was * O . l l with a mean of 10.3 microThe moles of ammonium chloride. experimentally-found mean deviated from the true value by 0.1 micromole. Table I1 shows the results for various concentrations of sulfamic acid. Included in the data are two sets of replicate analyses. In one set of replicate analyses, the standard deviation was zt0.04 with a mean of 5.01 micromoles and in the other it was hO.09 with a mean of 10.63 micromoles. The former mean deviated from the true value by 0.03 micromoles and the latter by 0.03 micromoles also.
Table II. Direct Coulometric Titration of Sulfamic Acid with Hypobromite Ion Micromoles Generating Micromoles current, XHsSO, NHISOI ma. added found 4.98 4.98 4.98 4.98 4.98 10.66 10.66 10.66 10.66 10.66 14.22 10.66 7.11 3.56
9.65 9.65 9.65 9.65 9.65 22.0 22.0 22.0 22.0 22.0 19.3 19.3 19.3 19.3
5.03 5.06 5.00 4.97 4.97 10.55 10.75 10.58 10.58 10.71 14.18 10.68 7.48 3.80
LITERATURE CITED
(1) Arcand, G. M., Swift, E. H., ANAL.
CHEM.28,440 (1956). (2) Knvis, A. F., Microchem. J. 5, 559 (1961).
ALANF. KEUVIS GEORGE R. SWPP EUGENE S. GAZDA Central Analytical Laboratories Olin Mathieson Chemical Corp. New Haven, Conn.
Coulometric Titration of Ammonia with Hypobromite Using Direct Amperometric End Point Detection SIR: Willard and Cake (6) and Kolthoff and Stenge. (2) have developed volumetric riethods for the determination of ammonia by oxidation to nitrogen wii;h hypobromite. Since then Kolthoff, Stiicks, and Morren (3) have described zn amperometric end point detection ir the titration of 6 X lo-‘ to 4 X 10-6M ammonia in sodium bicarbonate solution. They employed a rotating platinum wire electrode at +0.2 volt. us. S.C.E. as the indicating electrode. Laitinen and Woerner (4) employed a similar technique in titrating ammonia with hypochlorite. ilrcand and Swift (1) developed a coulometric titration of 14 to 230 pg. of ammonia using electrogenerated bromine in a supporting electrolyte of pH 8.5. I n alkaline medium, bromine disproportionates to kiromide and hypobromite ions. Two platinum foil indicator electrodes with 150 mv. impressed between them were used for the arnperometflic end point. Arcand and Swift found that the indicator current resulting from a given quantity of bromine in alkaline solu-
tion was not sufficiently reproducible for use as a means of determining the end point. Therefore, they used the current in the alkaline solution only to indicate when a slight excess of bromine had been generated. Then an indirect method was employed to find the end point; the solution was acidified with perchloric acid and additional bromine was generated a t 1-second intervals. The current readings were plotted against the generation time and a straight line resulted. A blank was run in the same manner and an arbitrary current reading in the acid solution was chosen as the end point current. The authors have experienced difficulty in trying to employ the method of Arcand and Swift in the titration of ammonia solutions obtained from biological samples. We found that the indirect end point determinlttion was not reproducible. The current readings due to the excess bromine in acid solution appeared to be very dependent on the amount of excess bromine generated before the solution was acidified. In a blank showing a difference of only 1.7 seconds to generate excess bromine at
4.825 ma. before acidification, the generation-time readings differed by 7 seconds to reach the same current reading in acid solution. illso, a given current reading depended upon strict reproduction of the stirring rate and volume. The authors observed the same difficulty experienced by Arcand and Swift in trying to generate to an arbitrary current value directly in alkaline solution. However, in preliminary titrations we noted that the generation time needed for the current readings to increase in alkaline solution appeared to correspond closely to the end point value and was reproducible. Therefore, a direct, conventional end point This detection was investigated. method has been employed to determine as little as 1.4 pg. of ammonia. EXPERIMENTAL
Reagent grade chemicals were used without further purification. Solutions of ammonium sulfate were prepared by dissolving Mallinckrodt “Analytical Reagent” ammonium sulfate in an appropriate volume of water. The VOL. 35, NO. 13. DECEMBER 1963
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