Anal. Chem. 1980,52, 2235-2237
2235
Direct Ion-Selective Electrode Determination of Micromolar Boron as Tetrafluoroborate Janis Gulens‘ and Paul K. Leeson Atomic Energy of Canada Limited, General Chemistry Branch, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ lJ0, Canada
Boron is used in nuclear power reactors to control reactivity. In the Canadian power reactor CANDU (CANada, Deuterium, Uranium), boric acid is added to the heavy water moderator circuit. Heavy water moderator is both expensive and radioactive, thus a procedure for the determination of boron is required that is rapid and simple, yet is also sensitive and wide ranging (10”-10-2 M). The spectrophotometric determination of boron by curcumin is sensitive and accurate, but it is also complex and time consuming ( I ) . A direct current argon-plasma emission spectrometer has been used recently to measure boron in water over the range 0.02-250 mg/L (2 X 10”-2 X lo-’ M) (2); this method is accurate and precise but requires expensive instrumentation. Carlson and Paul (3) determined boron in water by reacting it with H F to produce fluoroborate and measuring the fluoroborate concentration with a fluoroborate-selective electrode. The rate of conversion of boric acid to fluoroborate increased with increasing temperature and increasing H F concentration. The conversion in 0.28 M H F was complete in 5 min at 60 “C but required 6 h at 24 O C ; however, the excess H F interfered with electrode response. The authors claimed that this interference limited the usefulness of their method for boron solutions more dilute than lo4 M. Fluoroborate levels as low as M could be measured in 0.028 M HF, but the time required to form fluoroborate in 0.028 M HF at 60 “C was in excess of 6 h. The authors recommended concentrating dilute boric acid solutions on Amberlite XE-243 resin and then converting it to fluoroborate on the resin using concentrated HF. After the excess H F was removed by washing, the fluoroborate was eluted from the column by a sodium hydroxide solution and determined with the fluoroborate electrode ( 3 ) . Recently, Hill and Lash ( 4 ) applied the above procedure of concentrating boron on Amberlite XE-243 resin and converting it t o fluoroborate on the resin, but they determined the fluoroborate eluted from the column by ion chromatography. Quantitative results were obtained for boron concentrations as low as 0.5 mg/L (5 X lo4 M) ( 4 ) . Since the reactor moderator does not contain anions that may interfere with the response of the fluoroborate electrode (31,we investigated the response of this electrode as a function of p H and fluoride concentration in an attempt to overcome the problems observed by Carlson and Paul ( 3 ) in applying this electrode to low level boron determinations. As a result, a procedure has been developed for the determination of boron as fluoroborate that is rapid, simple, sensitive, and accurate. Solutions containing levels of boron as low as 3 X lo4 M can be treated and analyzed directly without the need of preconcentration.
EXPERIMENTAL SECTION Orion Model 92-05 and Orion Model 93-05 fluoroborate electrodes were used. Their potential was measured relative to a standard calomel electrode with the latter separated from the sample solution by a 1.0 M NHIF bridge solution. The electrode potentials were measured in stirred solutions at room temperature. Reagent grade chemicals, except for electronic grade HF (J.T. Baker), plastic laboratory ware, and distilled, deionized water were used throughout. The electrodes were calibrated in the supporting electrolyte solutions by the standard addition procedure. Stock fluoroborate solutions were obtained either by dissolving the NaBF, salt in 0003-2700/80/0352-2235$01 .OO/O
distilled water or by reacting H3B03with H F (3). The stock fluoroborate solution was acidified with HF when required to minimize the problems of hydrolysis. Boron concentrations were also determined with a Jobin-Yvon inductively coupled argon plasma atomic emission spectrometer (ICP-AES), at the 249.7-nm B I line. The procedure for BF,- formation is as follows. Six milliliters of concentrated (29 M) HF was added to 30 mL of sample and allowed to react for 6 min at room temperature. The reaction was stopped by the addition of 14 mL of concentrated (14.5 M) aqueous ammonia. Caution: these reactions should be performed in a fume hood. The solution was cooled to room temperature, made up to volume (50 mL), and then analyzed with the fluoroborate-selective electrode.
RESULTS A N D DISCUSSION Effect of pH, F-. Initial experiments were performed by using the Orion Model 92-05 fluoroborate electrode. Its response to fluoroborate in a 0.28 M H F solution, Figure 1, agreed well with the response observed by Carlson and Paul (3);however, the limit of Nernstian response (58 mV slope) could be decreased from -3 X M t o -3 X M by performing the calibration in a 0.28 ‘M NaF solution (pH 7 ) . In addition, the sensitivity of the electrode to fluoroborate in the range 10” to 10” M fluoroborate was greater in the 0.28 M NaF solution than in the 0.28 M H F solution. The increased sensitivity of this electrode in the neutral solutions compared to acidic solutions prompted a study of its response to changes in pH, Figure 2. In the absence of fluoroborate, the electrode was very responsive to p H over the range p H 3-6.5. The response of the new Orion Model 93-05 fluoroborate electrode to fluoroborate ions was considerably better than that of the Model 92-05 electrode. The limit of Nerstian response in 0.28 M H F for the former electrode was -3 X 10” M, Figure 3, as compared to 3 X lo4 M for the latter electrode. Of more importance, the response of the Model 93-05 electrode was not affected by changes in the fluoride concentration, and the Model 93-05 electrode was more sensitive than the Model 92-05 electrode to fluoroborate in the concentration range 104-10-5 M in various supporting electrolytes. The change in potential of the Model 93-05 electrode in this range was -32 mV for 0.28 M NaF and 35 mV for 3.5 M NH,F-0.6 M NH3 (pH -8.4) solutions (Figure 3), while for the Model 92-05 electrode the potential change was only 20 mV in 0.28 M NaF and 14 mV in 2 M NaF (Figure 1). The Model 93-05 electrode was also responsive to pH changes, Figure 2, but showed hysteresis in its response over the range pH 6-8 at fluoroborate levels lower than 5 X lo4 M. Carlson and Paul reported that the response of the fluoroborate electrode was affected by excess H F at low boron concentrations (3). However, the response of this electrode to pH changes in the absence of fluoroborate ions, Figure 2, suggests that the electrode is responding to the presence of species such as HF2- which arise because of the following equilibria (5)
HF == H+ + F-
HF
K , = 6.7
+ F- F= HF2-
X
K z = 4.0
(1) (2)
As the fluoroborate concentration in solution is increased (Figure 2) and/or the H F concentration is decreased, interference from HF2- is decreased due to the greater selectivity 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2236
E rn:
1
Figure 1. Calibration response of Orion Model 92-05 fluoroborate electrode in various supporting electrolytes: A,0.28 M HF; 0 ,0.28 M NaF; 0 ,2 M NaF.
i i:L
1
L
1BFq-I
Figure 3. Calibration response of Orion Model 93-05 fluoroborate electrode in various supporting electrolytes: A, 0.28 M HF; 0 ,0.28 M NaF; X , 3.5 M NH,F-0.6 M NH,.
1
I
1
1
I
CP
Figure 2. Effect of pH on the response of fluoroborate electrodes: 0, Model 92-05 electrode in 0.28 M NaF; X , Model 93-05 electrode in
1 M NH,F containing 0 M BF,-; 0 ,Model 93-05 electrode in 1 M NH,F containing 1 X M BF4-; 0 , Model 93-05 electrode in 1 M NH,F containing 5 X lo-' M BF,-.
of the electrode for fluoroborate ions; thus, Carlson and Paul observed that there is less interference in a 0.028 M H F than in a 0.28 M H F solution and that changes in pH from 3 to 11 had no effect on the electrode potential in or M fluoroborate solutions ( 3 ) . The cause of the hysteresis observed a t low fluoroborate concentrations in Figure 2 is not clear, but it appears to be related to the formation of a thin translucent film that forms on the electrode membrane at pH 9; as the p H of the solution is decreased, the film dissolves but can be made to reappear by increasing the solution pH. Carlson and Paul concluded that the amount of fluoride in the final solution affected the sensitivity of the electrode a t the lowest levels of boron ( 3 ) and thus recommended washing the column with water to remove the excess H F before eluting the fluoroborate from the column with sodium hydroxide. Our results, Figure 3, show that there is no difference in the electrode response to fluoroborate between a 0.28 M NaF and a 3.5 M NH4F-0.6 M NH3 solution, apart from the expected shift in electrode potential due to changes in the ionic strength of the solution. Carlson and Paul noted this influence of fluoride concentration since they were working in acidic solutions where the amount of fluoride eluted from the column would affect the concentration of HF2present and thereby affect the electrode response.
The Model 93-05 electrode has a modular sensing element, while for the Model 92-05 electrode the sensing element must be hand-assembled. The improved response of the former electrode, Figure 1 vs. Figure 3, is presumably due to the fact that the size, thickness, and geometry of the sensing element have been optimized and are fixed, while for the latter electrode these factors are variable and probably not optimized since the electrode is continually disassembled and reassembled. All further results presented were obtained with the Model 93-05 electrode. BF4-Formation. Since the electrode sensitivity to fluoroborate was not affected by the fluoride concentration at pH 7-9 (Figure 3), a concentrated H F solution was used to increase the rate of conversion of boric acid to fluoroborate. The sensitivity of the electrode response to pH and the presence of the hysteresis (Figure 2) indicated that the fluoroborate measurements should be performed at pH 8-9; concentrated aqueous ammonia was used to neutralize the excess H F and also to provide pH buffering capacity. Concentrated sodium hydroxide was not used due to its lack of buffering ability and due to problems arising from the lower solubility of sodium fluoride relative to ammonium fluoride. The conditions (see Experimental Section) for the quantitative conversion of boric acid to fluoroborate were determined within the constraints that the procedure should be wide ranging and as rapid and simple as possible. The conversion of boric acid solutions ranging in concentration from lo4 to M was complete within 6 min of mixing in a 4.8 M H F solution at room temperature (23 "C). The volume of the reagents used was a compromise between the rate of the fluoroborate formation and the effect of sample dilution on the electrode response. Hill and Lash ( 4 ) reported that the conversion of a 0.4 M boric acid solution in 6.8 M H F was complete in 10 min but that the conversion rate decreased in more dilute solutions to the extent that in a M boron solution the conversion time was prohibitive ( 4 ) . We observed that the conversion
Anal. Chem. 1980, 52,2237-2238
Table I. Precision of Boron Determinationsa [H3 BO 3 3 , M x lo6
3.3
= 9.
b
3C
lo6
M X
3.4 8.4 84
8.3 83 a
s,c
x,b
Mx
= xxi/n.
c s =
lo6
0.2 0.2 1
[ x ( ~ x- i ) 2 / ( n - 1)11'*
Table 11. Comparison of Boron Determinations sample a D-1 D-2
D-3 D-4 D-5
D-6 D-7 D-8 R-1 R-2 R- 3 R-4 R-5
amt B added, M x lo6
amt B found,b M X l o 6 by ICP-AES
by electrode
