signal will be due to Fe(I1) and not to another species. The intrinsic sensitivity of this chemiluminescence method is SO great that only very high concentrations of interfering species have any effect on the “practical” detection limit. Thousandfold excesses of Hg(II), Al(III), Pb(II), Cd(II), Zn(II), Ca(II), Ba(II), Ag(I), and Mg(I1) did not interfere with the light catalyzed by 2 X 10-*M Fe(I1); and 10-2M C1-, Br-, Soh-, and NOs- did not affect Fe(I1) peaks. Analyses. Table I compares iron concentrations determined by chemiluminescence with values found by other analytical methods. The orchard leaves, NBS Standard Reference Material 1571, were ashed with nitric and perchloric acids. For chemiluminescence analysis, the iron was reduced to Fe(I1) with excess bisulfite, and analysis was done by the method of standard additions, making three additions. The documentation for SRM 1571 gives an uncertified value of 270 pg/gram for soluble iron after nitric and perchloric acid ashing. There was upward curvature in the plot of peak height us. standard additions, which probably caused the extrapolation to give the low values for Fe determined by chemiluminescence. A phenanthroline method was used for colorimetric analysis of the ashed orchard leaves (16). (16) “Standard Methods for the Examination of Water and Wastewater,” 13th ed., American Public Health Association (1971).
The water samples were preserved with 1 ml concd H 2 S 0 4 per liter of sample and further acidified with 50 ml concd HC1 and 5 ml concd HNOI per liter. Aliquots of water sample were added to 500 ml of 10-aM bisulfite which reduced the iron to ferrous. The dilution factor was 500x for pond water, 250X for river water, and lOOx for tap water. N o color developed for any of the water samples when the phenanthroline method was tried on them without prior ashing or solvent extraction. ACKNOWLEDGMENT
The authors thank Tom Bennett of S E W for running the atomic absorption analyses and Bobby Carroll and Ray Hemphill of S E W for providing information on the water chemistry of iron. RECEIVED for review March 16, 1972. Accepted July 13, 1972. Use of trade names does not imply endorsement by the Environmental Protection Agency or the Southeast Water Laboratory. This work was supported in part through funds provided by the US. Public Health Service under a grant from the National Institutes of General Medical Sciences.
Gravimetric and Coulometric Analysis of BerylIium Samples Using 2-Methyl=&Quino1in01 J. R. Bacon and R. B. Ferguson Department of Chemistry and Geoiogy, Clemson University, Clemson, S.C. 29631 Two methods for determining beryllium have been developed. Both are based on the reagent 2-methyl8-quinolinol. Extensive use of masking agents makes the methods selective for beryllium. The interference from iron(lll), copper(ll), aluminum(lll), and many other ions commonly found in beryllium alloys and ores is eliminated. The gravimetric method i s accurate as indicated by the results obtained on a NBS beryllium-copper alloy. The NBS value is 1.770 i 0.0043% and our value was 1.761 =t0.0071%. The control of pH and the amount of excess reagent are important for accurate results. The coulometric method i s based on the coulometric bromination of the organic reagent. This quantitative bromination is an indirect determination of the beryllium. The two to one 2methyl-8-quinolinol-beryllium precipitate is formed as usual for effective separation. The precipitate is dissolved in acid and the organic portion i s coulometrically brominated. The coulometric results on the same NBS sample were 1.762 i 0.0079%.
THEDETERMINATION OF BERYLLIUM in alloys and ores usually involves the determination of a minor constituent. The actual analysis is, therefore, frequently troubled by interference problems or involved prior separation procedures. The accepted method of analysis (1) involves the precipitation of the beryllium as the hydroxide after the prior separation of most of the other metallic ions present. The accuracy of this (1) “ASTM Methods for Chemical Analysis of Metals,” American Society for Testing Materials, Baltimore, Md., 1968.
method has been questioned by the National Bureau of Standards (2). J. Das and his coworkers (3-7) have investigated several organic reagents for beryllium. Motojima (8) developed a sensitive method for the determination of beryllium using 2-methyl-8-quinolino1 or 2-methyl-8-hydroxyquinoline. He showed that beryllium was precipitated by this reagent at pH values between 7.8 and 9.3. In the method many interferences were found, including copper, aluminum, and iron which are commonly found in beryllium alloys and ores. Work in this laboratory has involved the improvement of the method originally suggested by Motojima (8). The development of a coulometric method of analysis has also been undertaken. The use of several masking agents has improved the selectivity of Motojima’s method. Combinations of EDTA, sodium potassium tartrate, potassium cyanide, and ascorbic acid have been used to successfully mask most interferences while allowing the complete precipitation of the beryllium. (2) 0.Menis, Technical Nore, 424, National Bureau of Standards, Washington, D. C., 1968, (3) A. K. Sarkar and J. Das, ANAL.CHEM., 39, 1608 (1967). (4) J. Das and S . Bauerjee, 2. Anal. Chem., 184, 110 (1961). (5) Ibid., 188, 109 (1962). (6) Ibid., 189, 183 (1962). (7) J. Das and S . C. Shome, Anal. Chim. Acta, 24, 37 (1961). (8) K. Motqjima, Bull. Chem. SOC.Jap., 29, 29 (1956).
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Table I. Effect of Excess Reagent 2-Methyl-8-Quinolinol Excess 2-methyl8-quinolinol, Wt of precipi- Be recovery, Be taken, mg mg/ml tate, grams= %" 0.18 0.2347 88.0 7.387 0.69 0.2488 93.3 7.387 0.80 0.2666 100.0 7.387 0.91 0.2670 100.2 7.387 1.27 0.2660 99.8 7.387 99.9 7.387 1.55 0.2665 102.5 7.387 1.64 0.2734 a Each number in these columns is an average of two determinations. Table 11. Effect of Excess EDTA G.2M EDTA Be taken, mg added, ml Be found, mg Recovery, 2 6.235 1 .OO 6.265 100.4 6.213 99.6 6.235 5.00 6.176 99.1 6.235 10.00 100.2 20.00 6.247 6.235 Interference Studya Masking Be found, Error, agent mg mg 7.350 -0.037 Na2EDTA 7.375 Na2EDTA -0,011 Na2EDTA +0.001 7.388 7.388 Na2EDTA +0.001 Na2EDTA -0.006 7.381 $0.010 7.397 Na2EDTA, Fe3+-447 CN- AScorbic acid +O. 059 Ni 2+-470 Na?EDTA 7.446 7.387 +O ,066 Na2EDTA 7.321 7.387 Co2+-471 -0.015 Na2EDTA 7.372 Pb'+-1 ,650 7.387 Na2EDTA 7.402 +0.015 Cd2+-895 7.387 7.365 -0.022 Zn2+-523 7.387 Na2EDTA 7.374 -0.013 NazEDTA Bi3+-l,670 7.387 +O, 074 7.461 NalEDTA Mn2+-439 7.387 7.438 Na2EDTA +O ,052 7.387 Hg2+-1,600 7.387 Total interUO22+ ference Total inter7.387 Cr 3+ ference Total interF 17.387 ference Total interPOP 7.387 ference The metals were added as chlorides, nitrates, acetates, perchlorates, and sulfates which caused no interference. Be taken, mg 7.387 7.387 7.387 7.387 7.387 7.387
Table 111. Foreign ion, mg CU'+-508 A13+-216 Mg2+-195 Ca 2+-320 Ba 2+-1, 100
A coulometric method of analysis for beryllium has been developed based on constant current coulometric bromination. The beryllium is precipitated with 2-methyl-8-quinolinol as before, the precipitate is dissolved in acid, and the organic reagent can be quantitatively brominated. The end point for the bromination is detected using a n amperometric (dead stop) method. EXPERIMENTAL
Apparatus. Polarographic data were obtained with a chopper-stabilized Heath Company polarograph used in the three-electrode configuration. All pH measurements were obtained with a Leeds and Northrup Model 7401 meter. The constant current coulometer used was a Model 7960 coulometer by Leeds and Northrup. In the bromination reactions, the coulometer cathode was separated from the 2150
Table IV. Comparison of Gravimetric and Radiochemical Analysis Weight of Be added 7.387 mg 0,26613 mg Weight of complex found Weight of Be found 7.371 mg % Recovery (gravimetric) 99.79 Count rate of 1.ooO ml of standard 346,688 cpm Count rate of 1.OOO ml of filtrate 29.94 cpm 6.316 cpm Count rate of 211 ml of filtrate % Recovery (radiochemical) 99.77% Table V. Results of Coulometric Bromination of Beryllium Complex Be complex taken, meq Be found, meq Be recovery, 11.586 11,597" 100.09a 81.101 81.137 100.04 These values are the average of 10 determinations.
bulk of the solution by a fine borosilicate sintered glass disk. The radiochemical data were obtained using a Model 8725 single channel analyzer manufactured by Nuclear Chicago Corporation. A well type NaI (Tl) scintillation crystal was used with the single channel analyzer. Chemicals. The beryllium for standard solutions was 99.9+ % pure and was obtained from Spex Industries. The 2-methyl-8-quinolinol was purchased from Aldrich Chemical Company and was purified by triple recrystallization from 60 ethanol. The radioactive beryllium-7 was obtained from New England Nuclear Corporation and had a purchased activity of 250 microcuries. Distilled-deionized water was used throughout. All other chemicals were used as received and were of reagent grade. Solutions. A beryllium standard solution containing 4.617 mglml was prepared by dissolving 2.3085 grams of beryllium in a few milliliters of hydrochloric acid and diluting to 500.0 ml with water. Other beryllium solutions were prepared from this solution by dilution. The 2-methyl-8-quinolinol was prepared by dissolving 2.00 grams in 5 ml of glacial acetic acid and diluting to 100 ml with water. The pH 9.1 buffer was prepared by dissolving 160 grams of ammonium chloride and 126 ml of concentrated ammonium hydroxide in water and diluting t o a liter. Other solutions were prepared by dissolving the appropriate salts in water. Gravimetric Procedure. The recommended procedure for the gravimetric analysis of beryllium with 2-methyl-8-quinolinol was as follows: The sample containing a t least 2 mg of beryllium was dissolved in 5 ml of 1:l nitric acid. The sample was diluted to about 20 ml. Five milliliters of 0.5M sodium potassium tartrate and enough 0.2M EDTA to complex all of the foreign ions with about 10% excess were added. The p H was adjusted to between 8 and 9 with ammonium hydroxide. If the sample contained iron, 2.0 grams of sodium cyanide and 0.5 gram of ascorbic acid were added. The sample was heated to 90 "C and 20 ml of pH 9.1 buffer was added. The temperature was maintained at about 90 "C and the reagent solution containing 2.0 % 2-methyl-8-quinolinol in 5 acetic acid was added slowly with stirring until the concentration of excess reagent was between 0.80 and 1.5 mg'ml in the final solution. The precipitate was digested at 90 "C for 1 hour and allowed to cool overnight before filtering through fine porosity sintered glass crucibles. The precipitate was washed with 1 % ammonium hydroxide and dried at 105 "C. A study was made to determine what excess of 2-methyl-8quinolinol was necessary for complete precipitate. NO
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
Table VI. Analysis of No. C1122 NBS Alloy +C U R R E N T 6 . 4 3 mA
Q
CHAN G E D T O 0.643 mPI
i ID
T
I
,-l1.600 1
a
+EPIV. I
I
Figure 1. Amperometric end point for the coulometric bromination sodium cyanide and no ascorbic acid were added since no iron a a s present. The rest of the procedure was the same as that previously given. The results are given in Table I. Since EDTA was the main masking agent used and the only one likely to form a complex with beryllium (2), a study was carried out to determine if an excess of EDTA affected the beryllium results. The results are given in Table 11. An interference study was undertaken. In each case the foreign metal ions were added in approximately a 10 :1 molar excess. This required SO ml of 0.2M EDTA to ensure an excess of the masking agent. The remainder of the procedure was as previously given. The results are given in Table 111. A radio tracer study was carried out to test for completeness of the precipitation by an independent method. A sample of beryllium was spiked with 'Be and the amount left unprecipitated was determined by a gravimetric method and also by counting the radioactivity of the filtrate. The results are given in Table IV. Electrochemical Investigations. Corsini and Graham (9) found that 2-methyl-8-quinolinol could be determined polarographically in a 1M NaOH electrolyte. This electrolyte would dissolve the Be 2-methyl-8-quinolinol precipitate and the polarographic determination of the 2-methyl-8quinolinol would be an indirect determination of the beryllium. The beryllium was precipitated and washed as usual. This precipitate was dissolved in 1 M NaOH and diluted to 50 ml with this electrolyte. A portion was transferred to a polarographic cell and deaerated with nitrogen. The polarogram was obtained and the diffusion current measured. The half wave potential was - 1.67 V us. SCE. The calibration curve had a slope of 0.32 pA/ppm Be and intercept or blank of 0.2 MAfor no Be added. The method was not used extensively because of the limited accuracy. Carson (10) has shown that 8-quinolinol can be quantitatively brominated by a coulometric titration in solutions that were 0.1M in potassium bromide and 0.001M in HC1. For 2-methyl-8-quinolinol the results were better if the acid strength was increased to 0.14M in nitric acid. This acid was found sufficient to dissolve the beryllium 2-methyl-8quinolinol precipitate. In the coulometric procedure, the beryllium was precipitated and washed as before. The precipitate was dissolved in 0.14M nitric acid. Fifty milliliters of 0.1M potassium bromide was added to the solution and the coulometric titration was carried out with amperometric end-point detection. The actual end point was obtained by a graphical method as shown in Figure 1. Coulometric results are given in Table V. (9) S . Corsini. and R. P. Graham. C m . J . Chem..41, 1936 (1963). ( I O ) W . N. Carson Jr., ANAL.C H E ~22, I . , 1565 (1950).
Alloy taken, gram
Precipitate found, gram
0.44613 0.43353 0,42996 0.41732 0.44700 0.42953 0.44127 0.43174
0.28561 0.27457 0.27275 0.26561 0.28543 0.27204 0.28023 0.27358
Z Be Z Be gravimetric" coulometric* 1.773 1.755 1.757 1.763 1,769 1.754 1.759 1.755
1.779 1.757 1.758 1.759 1.769 1.759 1.760 1.756
Average of gravimetric determinations 1.761 & 0.0071 Z. Average of coulometric determinations 1.762 i 0.0079z.
Analysis of an Actual Sample. A sample of C1122 Beryllium Copper alloy was obtained from the National Bureau of Standards. The sample was cut into shaving using a clean lathe and it was washed in acetone. Approximately 0.4 gram of the alloy was dissolved in 5 ml of 1:1 nitric acid. Following dissolution the sample was diluted to 20 ml, SO ml of 0.2M EDTA and 5 ml of 0.SM sodium potassium tartarate was added. The pH was adjusted to about 8.5 with ammonium hydroxide. The solution was heated to 80-90 "C. Next, 2.0 grams of potassium cyanide was added slowly with stirring followed by 0.5 gram of ascorbic acid. With the solution still hot, 25 ml of pH 9.1 buffer was added and the sample was diluted to about 175 ml. The 2 z 2-methyl-8-quinolinol solution was added slowly with stirring, 25 ml being required for each sample. The precipitate was digested at 90 "C for 1 hour and cooled overnight. The precipitate was collected on weighed filtering crucibles and washed with about 100 ml of 1:100 ammonium hydroxide. The precipitates were dried at 130 O C and the precipitate weights determined. The above precipitates were dissolved in SO ml of 0.14M H N 0 3 and diluted to 100.0 ml. A 1.00-ml aliquot was added to 50 ml of 0.1M potassium bromide. The samples were coulometrically brominated to a dead stop end point. The gravimetric and coulometric results are given in Table VI. RESULTS AND DISCUSSION
The quantitative determination of beryllium by two methods has been developed. The gravimetric method is selective when the proper masking agents are used. The EDTA at pH 9.1 is effective in masking most of the foreign ions normally found with beryllium. The pH was maintained at 9.1 near the maximum suggested by Motojima (8) to give EDTA and the other masking agents maximum complexing ability. The sodium potassium tartarate was added to prevent hydroxide formation at pH 9.1. The masking agents for iron, potassium cyanide and ascorbic acid, served to reduce the iron(II1) to iron(I1) and stabilize the iron(I1) as the cyanide complex. This prevented the formation of the very stable iron(III)-2-methyl-8-quinolino1precipitated. The excess reagent study indicates that the 2-methyl-8quinolinol must be present in excess but not too great an lexcess. The workable range is from 0.80 to 1.55 m g h l excess in the final solution. One must therefore have a ireasonable knowledge of the beryllium level expected. The coulometric results were in good agreement with the gravimetric results. The coulometric method should be much inore sensitive than the gravimetric method. This was not found to be the case. The precipitate formed slowly and an apparent nucleation problem seemed to limit the use with
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lower amounts of beryllium. The limit was about 2 mg of beryllium in about 100 ml of precipitating solution. The use of smaller volumes would probably lead to difficulties with real samples, since the other dissolved metals would be more concentrated . The analysis of the NBS alloy yielded results that were in good agreement with the published value of 1.770 f 0.0043 %.
Our values were 1.761 + 0.0071 by the gravimetric method and 1.762 0.0079% by the coulometric method, The precision was reasonably good as indicated by the standard deviations.
*
RECEIVED for review March 27,1972. Accepted July 20,1972.
Application of Coulostatic Charge Injection Techniques to Improve Potentiostat Risetimes James E. Davis and Nicholas Winograd Department of Chemistry, Purdue University, West Lafayette, Ind. 47907 The design of a potentiostat is given which is capable of applying large amounts of peak power (300 watts at 1 psec) to an electrochemical cell. The design incorporates the use of a coulostat to initially charge the double layer while the potentiostat supplies the relatively small current needed to sustain any electrode processes. For systems with rather high resistances and large capacitances, time improvements of more than an order of magnitude are indicated with this device. Current-time data are presented on the oxidation of ferrocene-carboxylic acid in acetonitrile which agree with the theoretical values to 10 psec. In addition, internal reflection spectroelectrochemical experiments at optically transparent electrodes follow predicted behavior to less than 4 psec. This time improvement will greatly extend the use of potentiostatic methods for fast reaction rate studies to highly resistive nonaqueous solvent systems.
THEINVESTIGATION of rapid kinetic reactions associated with heterogeneous electron transfer processes has long been an important application of potential step techniques ( I ) . Many workers have presented detailed analyses of constructed instruments with special emphasis on frequency response, compensation for unwanted solution resistances (2-5), and cell geometry (6, 7). With the advent of cheap, readily available high performance operational amplifiers, potentiostats capable of applying controlled voltage steps in times on the order of 1 psec are presently being designed. Nearly all of these studies have been concerned with “small signal” conditions, that is instruments which are neither limited by current, voltage, nor slew. These conditions generally require very small applied voltage steps (
