ure 11. As expected, the increase of injection size of the 10-2 iron(I1) adduct results in an almost linear increase in peak height. The data obtained by GC analysis for the injection of from 1 to 5 pl of a solution which was 0.01M in iron(I1) and 0.1M T B P in cyclohexane, are given in Table VI. The standard deviation for each point was obtained from 10 independent measurements. However, two calibration curves which were obtained by injecting extracts having TBP:iron(II) ratios of 5 : l and l O : l , respectively, differ considerably in slope; the lower the excess of TBP in the extract, the lower the slope. Apparently, the GC peak height is very sensitive to the TBP:Fe(II) ratios and a 1 : l ratio, no peak was obtained, just a pronounced tailing after the TBP peak. By increasing the ratio TBP: iron(11), the peak height corresponding to the iron(II) adduct progressively increases. The apparent reason for this is thermal dissociation: Fe(HFA)2(TBP)2 == Fe(HFA)*
+ 2TBP
(7)
which is partially supressed by using a higher excess of TBP in extracts. This assumption was checked by sealing extracts containing different excesses of TBP in glass tubes and heating them to 200 "C. The color of the extracts changed from violet to dark red, the color of the chelate, Fe(HFA)2. The color change was more pronounced in extracts which contained lower excesses of TBP. After cooling, the color of the extract changed back to violet, and the visible and infrared spectra show again the presence of the iron(I1) adduct; however, the concentration of adduct had decreased. The decrease was more pronounced a t lower TBP:iron(II) ratios in the extracts. The TGA data have shown that the iron(II1) adduct is volatile as well, however, its GC behavior seems to be even more complicated. The iron(1II) adduct undergoes
thermal reduction, and as a result of this, as well as of a number of other nonpredictable reactions in the gas phase, the GC peaks were not reproducible. However, it is noteworthy that the retention time was the same as for the iron(I1) adduct, which confirms the assumption that under the GC conditions, the iron(II1) adduct undergoes thermal reduction. The rare earth adducts which were obtained in the same extraction system show a remarkable thermal stability, which makes possible their successful GC determination and separation even when no excess TBP was employed ( 2 ) .This may be explained as the consequence of stereochemical factors, as the average ionic radius for trivalent rare earth ions equals 1.05 A, compared with 0.76 and 0.64 A for iron(I1) and iron(II1) ions. A larger radius might explain the higher thermal stability of rare earth adducts. It seems possible that the adducts of other transition metals formed in the same extraction system could show a similar thermal instability as a consequence of steric crowding of the bulky ligands and donor molecules which surround the small central cation. The GC method, which appears to be a very promising method for the determination of low quantities of metallic species, necessarily reflects the coordination properties of analyzed compounds, and our present knowledge shows that the choice of appropriate ligands and donors can improve the thermal and GC characteristics of ternary complexes. Therefore, further studies of related systems, which will give new information on the coordination and thermal properties of ternary complexes, seem to merit attention.
ACKNOWLEDGMENT Purification of reagents by J. Richard was appreciated. Received for review November 4, 1971. Accepted January 24, 1973.
NOTES
Potentiometric Determination of Boron in Aluminum Oxide-Boron Carbide Using an Ion Specific Electrode H. E. Wilde Nuclear Laboratories, Combustion Division, Combustion Engineering. Inc., Windsor, Conn. 06095
The determination of boron in materials has traditionally depended upon the successful separation of boric acid from interfering elements. The techniques for this separation are usually long and tedious ( 1 ) . Recently, advances in liquid ion exchange membrane electrodes have made it possible to potentiometrically determine boron as tetrafluoroborate in agricultural samples. This technique of boron determination compares favorably in accuracy and sensitivity with the older technique and, in addition, it is much more convenient and less time consuming (2, 3). In particular, the potentiometric technique is free of the in( 1 ) H. Blurnentha1,Anai. Chem., 23,992 (1951). (2) R. M . Carlson and J. L. Paul, Anal. Chem., 40, 1292 (1968). (3) R . M. Carlson and J. L. Paul, SoilSci. 108, No. 4 (1969).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
terferences that plague the other methods. We have recently extended this technique t o boron determination in crystalline solids. This article discusses the procedure and results for boron determination in aluminum oxide-boron carbide (A1203 BIG) matrices, which are important materials as burnable poisons (consumable neutron absorbers) in nuclear reactors. One major difference exists between boron determination in A1203. B4C and the previously described determination (2, 3) involving agricultural samples. Whereas interfering elements in the agricultural or water samples form soluble salts and can be separated from tetrafluoroborate by using boron specific resins, similar separation is not feasible when the samples are crystalline solids which form insoluble salts that interfere physically with the ac-
40 35
30 z I-
25
if
> 0
2 = 20
u
4
15 10
5
0.'2
0'
014
0.6
1.0
0.8
1.2
1.4
6
Figure 2. Accuracy of boron determinations as a function of boron content 0 Specific ion electrode: A mass spectrographic lab No. 1 : spectrographic lab No. 2
11 110
130
,
150
170
190
mass
0
210
MILLIV 0LT S Figure 1. Typical standard curve for boron in A1203 B4C electrode potential vs. fluoroborate ion concentration (as boron). Hydrochloric acid dissolution of fusion melt
50 40
tion of the specific ion-exchange resins. Fortunately, there is no need to use the resins because tetrafluoroborate is formed during dissolution of the melt and the insoluble salts formed do not interfere with the potentiometric determination.
30
20
EXPERIMENTAL Preparation of Sample Solution. The A1203. B4C matrices are first ground into powder of about 300-mesh size. About 0.8 gram of the powder is placed into a 100-ml platinum beaker. Granular anhydrous sodium carbonate, 15 grams, and sodium fluoride, 10 grams, are then thoroughly mixed with the powder. The platinum beaker is then covered and the contents are fused at 950 "C for 1 hour. After 1 hour, the cooled beaker and its contents are placed in a 600-ml Nalgene beaker and leached with 200 ml of 1:l H:C1; the leach containing the solution plus solids is then transferred to a 500-ml Nalgene volumetric flask from which a 50-ml aliquot is taken and placed in a 100-ml Nalgene beaker. By monitoring with a p H meter, the p H of the aliquot is adjusted to 4.5 with 6M NaOH. Gpon transferring to a 100-ml Nalgene volumetric flask and diluting to volume, the solution is ready for potentiometric determination. Preparation of Standard Solution. Take 0.8 gram of pure A1203 and follow the fusion and leach procedure described above. Dilute the leach to 500 ml. Weigh out 0.7279 gram of KBF4 (See below for preparation), transfer to a 250-ml volumetric flask, dissolve, and dilute to volume with distilled water (1 ml = 250 Fg boron). Transfer 50 ml of the aluminum oxide fusion leach to each of five 100-ml Nalgene beakers. Add 1-,2-, 4 - , lo-, and 20-ml aliquots of the standard boron solution. As with the sample solution, adjust the pH to 4.6. The standard solution is now ready for potentiometric measurements. Preparation of Reagent KBF4. The lack of high purity fluoroborate reagent suitable for use in the standard solution made it necessary to prepare this reagent in the laboratory. Pure potassium tetrafluoroborate can be prepared as follows: Add 2000 ml of distilled water to a 3000-1d polyethylene container. Add 62 grams
p/M I
B 10
9 8
7 6 5
4
3
2
1
10
130
150
170
190
MILLIVOLTS Figure 3. Typical standard curve for electrode potential vs. fluoroborate ion concentration (as boron). Phosphoric acid dissolution of fusion melt ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
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Table I. Replicate Electrode Analyses of NBS Borosilicate Standarda (3.96% B) N BS
3.87 3.96 3.91 3.95 3.93 3.93 3.91 3.94 31.40
(X
- X)
-0.06 0.00 -0.02 f0.02 0.00 0.00 -0.02 +0.01
N = 8. X = 31.40/8 - 3.93:S2 = 0.0049/7 = 0.0007. S = 0.0265. Relative standard deviation = S / X = 0.67%.
Table II. Boron Analysis in AI203.B4C Round-Robina Individual deterAccuracy, Std dev. % % Laboratory minations 1.24 -5 1.4 A (specific ion electrode) 1.27 1.26 1.28 0 1.05 -19 1.9 1.07 1.09 C 1.05 -17 5.4 1.07 1.16 D 1.22 -9 1 .o 1.19 a
of HsB03 and dissolve. Add 7 5 grams of KC1 and dissolve. Add 70 ml of 48% HF. Allow to stand about three hours to precipitate the majority of the KBF4. After three hours, decant off most of the liquid and discard. Filter the KBF4 using a 180-ml polypropylene Buckner funnel with a Whatman N o . 2 filter paper or equivalent. Wash the KBF4 thoroughly with distilled water, using about 3 or 4 100-ml portions. The amount lost by washing is not significant. Wash the K B F l with about 300-ml of methyl alcohol. Transfer to a drying dish and place in a drying oven for 1 hour a t 100 “F. Measurements. Electrode potential measurements were made with a n Orion 801 Digital p H / m V Meter. An Orion fluoroborate ion specific electrode, Model 92-05, was used as the indicator electrode in conjunction with the Orion single junction reference electrode. Model 90-01.
RESULTS AND DISCUSSIONS During the fusion of the sample solution, the reaction of aluminum oxide with the fluoride results in the formation of the insoluble Na3AlF6. Upon leaching the melt with HC1, the boron present reacts with F- to form BF4-. Enough fluoride, including a slight excess, must be present to allow both reactions to take place. The Na3AlF6 is, of course, present as a precipitate in the standard solution, in order to have the samples and standards similar in background. Electrode Response. Figure 1 shows a typical calibration curve and the electrode response as a function of boron concentration. Response is masked by excess hydrochloric acid at the lowest concentration of boron. The slope of the linear portion of the curve is 54 mV per decade of concentration. To avoid errors in measurement due to hydrolysis of the fluoroborate, standard solutions should be prepared fresh daily. Sample Analysis. As a consequence of the lack of reliable A1203. B4C material, preliminary studies of boron re-
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8,. JULY 1973
Material used for analysis was crushing and blending of 20 sintered
Ai203.B4C pellets containing 1.32% boron with an uncertainty of f 3 % .
covery were done with NBS borosilicate glass. Table I shows the results of a series of determinations. As can be seen, the accuracy and precision of this potentiometric technique is quite good. Samples of A1203 B4C were analyzed in a “roundrobin.” Each laboratory used its own method of analysis. The results were compared with the specific ion electrode method and are listed in Table 11. The “round-robin” results in Table I1 sufficiently illustrate the accuracy and precision of the technique discussed here. Figure 2 shows graphically the accuracy of the specific ion electrode us. mass spectrographic techniques by which boron is determined by isotopic dilution. In this procedure, a known weight of enriched 1OB spike is mixed with a known weight of sample. After appropriate dissolution and separation chemistry, the loB/l1B ratio is measured by surface ionization mass spectrometry. From this ratio and a knowledge of the lOB/llB ratio in both spike and sample, the quantity of boron in the sample can be accurately determined. Future Investigations. It should be pointed out that the use of hydrochloric acid as the leach tends to decrease the sensitivity of the method. Other means of melt leach are being investigated to increase the accuracy and sensitivity. Figure 3 shows a calibration curve using &PO4 as the leach media. Application of the method to other boron-containing materials is also being investigated. 9
Received for review November 2, 1972. Accepted February 7, 1973.
