~~~~~
~
~
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~~~
Table IV. Comparison of Methods on Pure Chemical Compounds Nitrogen, % TheoAutoChemicals retical Analyzern Kjeldahl Ammonium persulfate 12.0 12.0 11.2 Tris hydroxy methyl amino methane 11.5 11.1 10.4 Ammonium thiocyanate 36.5 36.6 36.2 Ethylenediaminetetraacetate 7.46 7.10 7.29 Urea 46.6 46.9 46.1 Lysine monohydrochloride 15.3 14.9 14.2 Amino-naphthol sulfonic acid 5.86 5.38 5.49 Acetamide 23.7 22.9 23.3 a AutoAnalyzer values were obtained using ammonium sulfate in 10% sulfuric acid as standard.
method X samples interaction as error, gave relative standard deviations from 1.9 to 2.7, with the AutoAnalyzer results being significantly higher than those from the other two methods. These data indicate that all three methods have about the same precision, since the precisions are in satisfactory agreement with the results already shown. Apparently any grain can be accurately analyzed for nitrogen on the AutoAnalyzer if the sample of grain is presolubilized and if the standards consist of the same grain. By keeping the sample concentration in the 10 to 30 mg per 100 ml range, determinations are as accurate as those made by the Kjeldahl method. Relative standard deviations for both methods are usually around 2 %, but if heterogeneous materials and long-term variability are included, an average relative standard deviation of 3 would be a better estimate of precision. ACKNOWLEDGMENT
higher than those determined by Kjeldahl in five of the eight instances. These data indicate that the AutoAnalyzer was perhaps a little more accurate than the Kjeldahl method on these samples. I n the fourth comparison, nitrogen analyses were made on chemically modified corn flour, where the nitrogen content varied from 1.2 to 2.275 Single determinations were available by each of three methods: AutoAnalyzer, Kjeldahl, and micro-Kjeldahl. A two-way analysis of variance, using the
We thank Dr. W. F. Kwolek for advice and assistance with the statistical interpretation of the data and C. H. VanEtten for invaluable criticism and advice.
RECEIVED for review January 25, 1971. Accepted March 17, 1971. Mention of trade or company names is for identification only and does not imply endorsement by the Department of Agriculture.
Conducting Glass Electrode in a Thin-Layer Electrochemical Cell with Application to the Analysis of Neptunium R. C . Propst Savannah River Laboratory, E. I . du Pont de Nemours and Co., Aiken, S. C . 29801
A cylindrical cell for coulometry in thin layers of solution at a conducting glass electrode is described. Volumes were reproducible to &0.3%. Techniques are recommended to reduce edge diffusion and iR gradient efiects. Application of the method to determine neptunium in process samples is described. RECENTLY, A CONDUCTING GLASS (antimony-doped tin oxide) electrode was found to be well suited for the coulometric determination of actinides (1-3). However, coulometry in stirred solutions is slow, and rapid methods are desirable for routine control analyses. By combining the advantages of the wide workipg range of the conducting glass electrode (CGE) with the fast response of thin-layer cells (4), the analysis time for actinides is reduced to about one minute. For radioactive solutions, thin-layer cells should be compatible with containment equipment, rugged, and easily (1) R. C. Propst, ANAL.CHEM., 41,910 (1969). (2) R. C. Propst and M. L. Hyder, Nature, 221, 1141 (1969). (3) R. C. Propst and M. L. Hyder, J. Inorg. Nucl. Chem., 32, 2205 (1970). (4) C. R. Christensen and F. C. Anson, ANAL. CHEM., 35, 205 (1963).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
cleaned and refilled with fresh sample solution. Also, they should provide results reproducible to &0.5z. Several of the designs described in the literature offer excellent volume reproducibility; however, none were completely satisfactory for this purpose. The elegant micrometer-based designs (5-7) are difficult to decontaminate and clean, and the variable solution thickness feature is not required for routine applications. The sandwich-cell design (8) with parallel glass plates appeared suitable. When equipped with a side arm, this cell would provide flow-through containment (total enclosure from solution reservoir to waste container), and the incorporation of reactant-getter electrodes would eliminate edge effects (5). However, the spacer material could not be permanently bonded to the glass plates. Different spacer materials including glass solder and Teflon (Du Pont)-FEP were tried, but the seals were either initially defective or else failed after a short time. The rate of failure was aggravated ( 5 ) D. M. Oglesby, S. H. Omang, and C. N. Reilley, ANAL.CHEM., 37, 1312 (1965). (6) A. T. Hubbard and F. C. Anson, ibid., 40,615 (1968). (7) J. E. McClure and D. L. Maricle, ibid., 39,236 (1967). (8) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ibid., 1018 (1968).
by flexing of the side walls (1.6-mm borosilicate glass plate) of the cell when vacuum was applied to the side arm. The recent cylindrical designs (9, IO) have several desirable features; however, centering the electrode in the annulus can prove difficult, and the exposed tip of the wire electrode must be insulated when the bulk solution contains the electroactive species. This paper describes a uniform annulus cylindrical thinlayer cell (CTLC) with side arm that overcomes the spacer-seal deficiency of the sandwich design. Although reactant-getter electrodes (8) were sacrificed in converting from the parallelplate to the cylindrical form, the CTLC gives reproducible results, and the dip-type design can readily be adapted to most titration vessels.
Tin oxide over Platinum
---
Grind to 8 m m Dia
EXPERIMENTAL Apparatus. Components parts for the CTLC are shown in Figure 1. The completed cell in the titration assembly is shown in Figure 2. Dimensions a and 6, Figure 1, were selected to give maximum sensitivity and reduce edge and iR gradient effects ( 8 ) . The cylinder and sleeve were fabricated from 0.392-in. 0.d. precision-ground and polished borosilicate glass rod and 0.394411, i.d. precision-bore borosilicate glass tubing, respectively. Between the cylinder and sleeve in the final assembly is a uniform ultrathin annulus only 25 microns wide. The loop at the top of the cylinder supports this piece in the furnace during the coating operations. The cylinder was cleaned, dried, and its lower 2 mm (dimension a, Figure 1) masked by painting with an aqueous dispersion of MgO. The MgO was air-dried, and the cylinder then painted from point c, Figure 1, to the loop with liquid bright platinum (Engelhard Industries, East Newark, N. J.). The platinum coating was also air-dried, and then fired in the tube furnace at 650 "C for 30 minutes. The tin oxide was applied as previously described (1). No attempt was made to limit the coverage of the tin oxide (except for the MgO mask). The overlap of the tin oxide onto the platinum provided excellent contact between the two coatings. After the cylinder had cooled to room temperature, the neck was painted from point d, to the loop with conductive silver paint (Silver 6216, Du Pont, Wilmington, Del.) to provide a scuffresistant surface to which an alligator clip could be attached. This coating was also air-dried and fired at 650 "C for 30 minutes. After cooling, the MgO mask was removed by gentle abrasion with scouring powder, and the cylinder was rinsed thoroughly with distilled water. Soluble tin species were then leached from the conducting glass electrode by immersing the lower 3/s in. of the cylinder in concentrated sulfuric acid for 24 hours. After a thorough rinsing in distilled water, the cylinder was dried and then mounted in a Jacobs chuck attached to a 25 rpm motor; the center portion (Figure 1) was heavily coated with epoxy cement (Armstrong Adhesive A-271, Armstrong Products Co., Warsaw, Ind.). The cylinder was rotated continuously until the resin hardened to ensure a smooth, uniform coating. The cement serves to insulate the tin oxide to platinum connection and defines the upper boundary of the conducting glass electrode. A coat of Apiezon "W" (Apiezon Products Ltd., England) in xylene applied over the epoxy cement protects this coating from chemical attack. Previous experience with Apiezon W has shown that this material is essentially chemically inert. Prior to assembly, the resistance of the conducting glass electrode and connecting films was determined by immersing the electrode in a pool of mercury and measuring the resistance between the mercury pool and the silver coating ( I ) .
Typical values for this resistance ranged from 100 to 200 ohms. The effect of this resistance will be discussed below. The completed cylinder was next inserted into the sleeve and the conducting glass electrode centered in the annulus by means of narrow strips of 0.8-mil shim stock. The upper part of the cell was then warmed, and the annulus above the side arm was filled with paraffin wax. When the wax had cooled, the shims were removed, and the cell was cleaned with chromic acid and rinsed thoroughly with distilled water. Because of small variations in dimension 6, the volumes of the completed cells ranged from 6.2 to 6.8 p1 (based on a 25-p annulus). The solution reservoir and electrodes (Figure 2) were similar to those previously described (11), but the diameter of the reservoir and Teflon cap was increased to 1.25 in. to accommodate the CTLC. The syringe control and valve assembly attached to the side arm served to flush the sample solution from the reservoir through the CTLC. The arrangement was similar to that described by Kissinger (12). The pinch-type bead valve at the side arm position was necessary to ensure that the solution in the thin layer remained immobile during the electrolysis.
(9) J. C. Sheaffer and D. G. Peters, ANAL.CHEM., 42,430 (1970). (10) L. P. Zajicek, J. Elecrrochem. Soc., 116, 80C (1969).
(11) R. C. Propst, ANAL.CHEM., 35, 958 (1963). (12) P. T. Kissinger and C. N. Reilley, [bid., 42, 12 (1970).
Tin oxide'
!-Dimension 2 mm
a
Figure 1. Parts for CI'LC
Figure 2. CTLC titration assembly
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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eliminate loading effects. Coulomb-time curves were recorded with a F. L. Moseley Model 135 x-Yrecorder. Fast processes were recorded with a Tektronix Corp. Model 564 storage oscilloscope. RESULTS AND DISCUSSION
4
0.3
0
5
IO
15
20 2 5
30 35 40
Electrolysis Time, seconds Figure 3. iR gradient studies 10-2MK3Fe(CN)G1MNa2S04
Table I. Thin-Layer iR Gradients in CTLC By Chronoamperometry of K3Fe(CN)glM N a S 0 4 Solutions at -0.34 V us. MSE iR gradient ( t = 0), mV K3Fe(CN)6,M i ( t = 0), pA
x x x 5 x 1x 5 1 2
10-4 10-3 10-3 10-3 10-2
170
200 250 300 350
110 120 140 150 170
Table 11. Component Resistances of CTLC in 1M NazS04 Observed, Calculated, Configuration ohms ohms A = CTLC (as assembled) 500 ... B = CTLC, dimension a = 0 230 ... C = CGE without sleeve 185 17ga (A-B) = Thin-layer dimension a 270 300 (B-C) = Thin layer across CGE 45 ... (A-C) = Total thin layer 315 328 Value determined during construction of CTLC. 5
All potentials were measured with respect to the mercury : mercurous sulfate (1M H2S04)electrode (MSE). This electrode had a potential of 0.415 V (including junction potentials) us. the saturated calomel electrode (SCE) in 1M Na2S04. For a typical titration, the reservoir and electrodes were first rinsed with sample solution (as in polarography), and a portion of the rinse solution was flushed through the CTLC. The vessel and electrodes were then drained, and fresh solution was added to the vessel. The sample in the cavity of the CTLC could be changed as desired by flushing additional solution from the reservoir through the CTLC. Reagents. A neptunium stock solution (17.45 mg/ml in 1N H N 0 3 ) was prepared by dissolving the oxide and was standardized by alpha counting and controlled-potential coulometry (13). All other solutions were prepared from reagent-grade chemicals and demineralized, laboratory distilled water. Solutions in the reservoir were continuously purged with helium which had been bubbled through distilled water. Instrumentation. The electrochemical instruments based on operational amplifiers used conventional circuitry. Voltage followers were provided at all high impedance inputs to (13) R. W. Stromatt, ANAL.CHEM., 32, 134 (1960).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
Although this thin-layer cell was developed for the coulometric determination of actinides, thin-layer techniques have other important applications. Therefore the electrochemical response of the CTLC was investigated in detail. Radioactive solutions were not employed in the electrochemical response studies; actinide studies were limited to the coulometry of neptunium in 1M sulfate because this system adequately demonstrates the wide working range of the conducting glass electrode. iR Gradient Effects. Serious iR gradient effects have been reported for thin-layer cells that incorporate a thin layer of solution between the electrode and the bulk solution to minimize edge diffusion (8). This study shows that the thin layer (dimension a, Figure 1) at the entrance to the CTLC accounted for 86% of the initial thin-layer iR gradient. These gradients, which ranged from 100 to 200 mV for stepcoulometric titrations, produced chronoamperometric response curves identical in shape to those previously reported and attributed to the effect (8). Values for the iR gradient across a thin-layer cell have not been reported. In this study, the iR gradients in the thin film of solution were measured by means of two reference electrodes (Figure 2): one inside the solution reservoir at the entrance to the CTLC, and one in a reservoir connnected to the side arm of the CTLC to monitor the potential of the upper edge of the CGE. Etched glass beads at locations A and B, Figure 2, ensured a conducting path between the MSE in the side arm position and the upper edge of the CGE. These beads also functioned as pinch-type valves. Open circuit potentials of the CGE as measured us. the MSE in either location were within + 5 mV. Typical iR gradients during chronoamperometric studies of K3Fe(CN)Gsolutions at -0.34 V us. MSE (in solution reservoir) are given in Table I; current- and gradient-time curves are shown in Figure 3 (gradient: E us. MSE in solution reservoir minus E VS. MSE in side arm). The values for Table I were obtained by extrapolating the current- and gradient-time curves to t = 0 and thus do not include doublelayer charging effects. These results show that the iR gradients in the CTLC were significant even at low concentrations. At 5 X IO-'M, the initial electrolysis potential at the upper edge of the CGE would only exceed the half-wave potential for the Fe(CNh3-Fe(CN)64- couple (-0.155 V cs. MSE) by 75 mV. The potential at the lower edge of the CGE could not be estimated from these data because of the unknown contribution by the thin layer of solution between the bottom edge of the CGE and the entrance to the CTLC. To estimate the iR gradients in the CTLC, resistance measurements were made between the CGE and a l-in. diameter cylindrical gold foil electrode in 1M Na2S04solution with a conductivity bridge. A cylindrical geometry was used, and the resistance of the gold electrode and bulk solution was negligible (
