Fluoride Electrode as a Refereme in the Determination of Nitrate Ion Stanley E. Manahan’ Engineering Sciences Laboratory, Haruard University, Cambridge, Mass. 02138
To ACHIEVE OPTIMUM ACCURACY in the potentiometric determination of ion concentrations with ion-selective electrodes, the potential recorded in the unknown solution should be compared to the potential measured in a standard solution having exactly the same medium, in a concentration very close to that of the sought-for ion, and measured under identical conditions at the same time. Such conditions are approximated by a method variously known as standard addition, known increment (Model 94-16 Instruction Manual, Orion Research Inc., Cambridge, Mass.), or spiking ( I ) . The potential of the ion-selective electrode GS. a reference electrode is first measured in a very accurately known volume of unknown solution, a small accurately measured volume of relatively concentrated standard solution of the ion is added, the potential is recorded again after the addition of the standard and the initial concentration of the sought-for ion in the unknown solution is calculated from the potential shift. The method assumes that the unknown solution is not appreciably diluted by the addition of the small volume of standard and that the activity coefficient of the sought-for ion is the same before and after addition of the standard. Standard addition has been used (2) for the determination of fluoride in various media with the fluoride ion-selective electrode. At the 1 X 10-5Mconcentrationlevel the probable relative error was given as 10% and the relative standard deviation as ca. 5 %. The method applied to the determination of nitrate with the nitrate ion-selective electrode (using as a reference electrode several modifications of the calomel electrode) resulted in similar levels of precision and accuracy (3).
Three sources of error are still not totally eliminated by the conventional standard addition method. First, in the higher concentration regions in the absence of excess inert electrolyte, the addition of standard solution can change the activity coefficient noticeably, because a n approximately two-fold increase in ion concentration is required for optimum results. Second, the addition of volumes of standard so small that dilution is negligible, is inconvenient, inaccurate, and, at higher concentration levels, often impossible. Therefore, in most cases larger volumes of standard are added and the concentration determined must be corrected for dilution for each measurement. Third, the usual standard addition technique does not compensate for changes in liquid junction potential resulting from the addition of standard solution. Liquid junction potentials and the shortcomings of reference electrodes in general are one of the main sources of error in analysis with ion-selective electrodes (4). When common 1 Present address, Department of Chemistry, University of Missouri, Columbia, Mo. 65201
(1) Robert M. Garrels in “Glass Electrodes for Hydrogen and
Other Cations,” George Eisenrnan, Ed., Marcel Dekker, New York, N. Y . ,1967, pp 355-361. (2) Elizabeth W. Baumann, Anal. Chim. Acta, 42, 127 (1968). (3) Douglas J. Alexander and Stanley E. Manahan, unpublished data, University of Missouri, Columbia, Mo., 1968. (4) NewsletterlSpecific Ion Electrode Technology, 1, (4), 21 (1969). 128
0
commercial calomel electrodes-e.g., of the cracked bead junction type-are used with the nitrate ion-selective electrode, short term fluctuation of potential of the order of tenths of a millivolt may be observed. The filling solution of the singlejunction reference electrode (Orion Model 90-01) recommended for use with most ion-selective electrodes is compounded to provide a very stable junction potential, but contains a high concentration of nitrate, necessitating the substitution of other electrolytes which do not have such good junction characteristics. The high leakage rate of the Orion single-junction reference electrode may result in an appreciable rate of change of ionic strength and consequent potential drift in dilute solutions. The problems encountered in standard addition methods for the determination of concentrations of monovalent anions may be almost totally eliminated by the use of a cell without liquid junction in which the fluoride ion-selective electrode is the reference electrode. By the addition of a low concentration of fluoride ion to the solution to be determined, the potential of the fluoride electrode is poised, the liquid junction is eliminated, and dilution and activity changes cancel. The fluoride ion-selective electrode makes a n ideal reference electrode in that it has essentially no interferences and fluoride ion does not interfere appreciably with other electrodes used to measure monovalent anions. In combination with the nitrate ion-selective electrode the fluoride electrode provides a remarkably stable electrode system, preliminary experiments indicating a short-term fluctuation of less than ~k0.02mV. This paper reports results obtained in the standard addition determination of nitrate ion using the following cell:
1
fluoride electrode F-, N03-
I nitrate
1 electrode
EXPERIMENTAL
Apparatus. Potential measurements were taken with a Corning Model 12 expanded scale p H meter (100 mV full scale). The nitrate electrode (Orion Model 92-07) was plugged into the measuring electrode input of the p H meter and the fluoride ion-selective electrode (Orion Model 94-09) was connected to the reference input. Because both electrodes have a relatively low resistance compared to the glass electrode, the above mode presented no difficulties in potential measurements. The solutions were contained in a 150-ml beaker and were stirred at a relatively slow rate (to avoid turbulence and bubble formation) with a magnetic stirrer. The standard solution was added from a 10-ml microburet, the tip of which dipped below the solution surface. Reagents. The nitrate stock solutions were prepared from dried reagent grade KN08. Stock fluoride solutions were prepared from reagent grade NaF. Procedure. The unknown nitrate solutions were spiked with NaF so that the concentration of fluoride was close to, but did not exceed, the concentration of nitrate. The spiking was done by adding solid N a F or a stock solution of NaF. In the latter case, of course, it was necessary to account for dilution of the unknown nitrate solution. The potential of a n exactly measured aliquot of the fluoride-spiked nitrate
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
Recovery Data for Nitrate Solutions Using Standard Addition with the Fluoride Ion-Selective Electrode as a Reference Electrode at 25 f 1 "C. Calculations Are Based on a Nernstian Slope of 59.1 mV Relative Relative standard [NO3-] added [F-I [NO3-] found error" deviationu +0.9 0.68 1.0 x 10-2 1.009 x 1.Ooo x 10-2 +0.8 0.58 0.504 x 0.500 x lo-* 0.5 X -0.2 0.65 1 . 0 x 10-3 0.998 X 1.000 x 10-3 +I .o 0.51 0 . 5 x 10-3 0.505 X 0.500 x 10-3 0.994 X 10-4 -0.6 0.65 1 . 0 x 10-4 1.000 x 10-4 -0.6 0.69 0.497 X 0.500 x 10-4 1 . 0 x 10-4 Based on 12-15 analyses at each concentration level.
Table I.
solution was measured and recorded as El. Ordinarily a 100-ml aliquot was taken and the electrode system allowed to equilibrate for 5 minutes with stirring before E1 was measured. Standard nitrate solution was added from the microburet until a potential shift of ca. 10 mV was obtained. The volume, U, of standard was noted and the new potential, E2, read approximately 30 seconds after the addition was complete. The addition was repeated for potential shifts from E, of ca. 15 mV and ca. 20 mV. The concentration of standard nitrate was such that after the final addition the original nitrate solution had not been diluted by more than 10%. The unknown nitrate ion concentration was calculated from Equation 5 or from a plot of A cs. Z - 1.
RESULTS AND DISCUSSION The potential of the nitrate-fluoride electrode system in the initial solution is described by the Nernst equation as follows:
where:
[NO3-] = the initial nitrate ion concentration = the initial fluoride ion concentration [F-] = the initial activity coefficient of nitrate ion Yn = the initial activity coefficient of fluoride ion y, = a constant depending upon the choice of Ea electrodes
After the addition of a volume, 0, of a standard nitrate solution of concentration, S, to the initial volume, V , of unknown solution, both the original fluoride and nitrate concentrations are lowered by the factor V/(V U) and Nernst relationship becomes
+
"
ion the ion the
v+v
E1
- E2
=
Z = antilog
- E2 2.3RT 5
E1 ~
(4)
the value of [NO3-] is given by the following equation: (5) The results of the analysis of nitrate solutions at several concentration levels are given in Table I. The accuracy and precision of the method described in this paper represent a substantial improvement over previously reported methods utilizing ion-selective electrodes. The data were deliberately taken under average laboratory conditions without special precautions to regulate temperature and stirring rate in order to get results to be expected with equipment commonly available in the analytical laboratory. Therefore, the full potential of the method has not been realized and, with careful temperature control, highly uniform stirring and potential measurements to =kO.Ol mV, even higher accuracy and precision should be attainable. The method could be applied to other monovalent anions and is particularly adaptable to automated analysis. Additional evidence that the ratio -yn/-y, remains essentially constant was obtained by simply diluting a solution containing nitrate and fluoride ions and observing any potential change. The concentration ratio of course remains constant and any change in potential should result from a change in the ratio of activity coefficients. No discernible change in potential was observed with dilution by water up to 20% in each concentration range analyzed, additional evidence that the basic premise of the method is sound. ACKNOWLEDGMENT
+
where Su/(V u ) is the contribution to the nitrate ion concentration from added nitrate standard and the activity coefficients are designated as y' to indicate that they have changed somewhat. Assuming, however, that the ratios of the activity coefficients do not change, the difference in potential, El - E2, is given by
+
2.3RT [NOs-IV SU __ [N03-]V 5 log 2.3RT -log 5
where A = ScjV, the concentration increment of nitrate assuming no dilution. If the term, Z , is defined as
+
[NOS-] A [NO8-]
(3)
Laboratory space and equipment were provided by the Harvard University Division of Engineering and Applied Physics Environmental Sciences and Engineering Research Laboratories. RECEIVED for review July 15, 1969. Accepted November 5, 1969. Research partially supported by the United States Department of the Interior, Office of Water Research, Matching Grant B-023-MO. The author was the recipient of a summer salary grant from the University of Missouri Graduate School and Research Council.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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