frozen until analyzed. Laboratory tests were performed to determine the influence of sample storage on mercury recovery. Half-liter volumes of seawater were spiked with lg7Hg in the divalent state and frozen. After about one week of storage the samples were thawed at room temperature. Following this treatment, mercury activity was quantitatively recovered by the separation procedure described below.
H g IN SEAWATER
0
20
40
60
EO
100
ng kg
120
140
I60
180
500 1000 -
-
1500-
0 '
u w
Analytical Procedure
The samples, whose weights ranged from 300 to 900 grams, were analyzed by thermal neutron activation of mercury. The mercury was separated from sea salts by coprecipitation with copper sulfide from acid solution (Weiss and Crozier, 1972): The copper sulfide was dissolved in concentrated nitric acid, transferred to a 15-ml polyethylene vial and irradiated for 1 hr in a thermal neutron flux of 1.8 X 10l2crn-2 sec-l, in the Lazy Susan of the Triga Reactor at the Gulf Energy and Environment Service facility in San Diego. In experiments performed at 5OoCand a flux of lo1*neutron cm-* sec-', Bate (1971) demonstrated that mercury radioactivity is lost from plastic containers when irradiations of 12 to 65 hr are carried out. In our preliminary recovery experiments at the nanogram level, under the experimental conditions of the present study no significant loss occurred. After the irradiation, mercuric nitrate carrier was added to the samples and interfering nuclides were removed, The 77keV X-ray emitted in the decay of 65-hr 197Hg was measured with a NaI(T1) detector coupled to a 400-channel analyzer about one week after irradiation. All reagents used prior to irradiation contained negligible amounts of mercury. The limit of detection (defined as a count rate in excess of three standard deviations above the background count rate) is 4 ng. The error of analysis is 1 2 z . Results The mercury profiles appear in Figure 1. At Station V, mercury concentrations ranged from 12 to 27 ng/kg; the average was 19 ng/kg. The concentrations for Station IX ranged from 22 to 173 ngikg. The frequency and amplitude of the variations at this station were significantly greater than that at Station V. This difference perhaps is ascribable to varying particulate content which may be associated with a location closer to shore. A minimum and maximum in concentrations appear in both stations at 100 and 400 meters and the surface values in each are approximately the same. The mercury content for waters from Station IX were uniformly
2- 2000I
2500-
n 3000 -
35004000-
4500L
Figure 1. Vertical distribution of mercury in the eastern tropical Pacific Ocean
greater than for samples collected at Station V. A consistent increase or decrease in concentration from the top of the water column to the bottom was not apparent. The mercury concentrations found in the location farther out to sea agree closely with the 14 to 21 nglkg determined for waters from the English Channel (Burton and Leatherland, 1971) and with 30 ng/kg reported for the North Sea (Stock and Cucuel, 1934). On the other hand the values for the near-shore station approximate the 80 to 270 ngikg found in the RamaDo Deep (Hosohara, 1961). Temperature and salinity data indicate that the samples analyzed at each of the two stations were from the same water mass; yet, pronounced differences in their mercury content exist. The basis for these differences will be sought in future investigations. Literature Cited Ackefors, H., Loforth, G., Rosen, C. F., Oceanogr. Mar. Biol. Annu. Rec., 8, 203 (1970). Bate, L. C., Radiochem. Radioanal. Lett., 6,139 (1971). Burton, J . D., Leatherland, T. M., Nature, 231, 440 (1971). Hosohara, K., J. Chem. SOC.Jap. Pure Chem. Sect., 82, 1107 (1961). Stock, A . , Cucuel, F., Naturwissenschaften, 22, 390 (1934). Weiss, H. V., Crozier, T. E., Anal. Chim. Acta., 58,231 (1972). Receiced for reciew October 8,1971. AcceptedJanuary 10, 1972.
Modification of Technique for Determination of Aluminum in Water by Atomic Absorption Spectrophotometry Deh Y. Hsu and Wesley 0. Pipes1 Department of Civil Engineering, Northwestern University, Evanston, IL 60201
T
he determination of aluminum using an atomic absorption spectrophotometer is complicated by the formation of refractory aluminum oxides in the flame. Either nitrous oxide or air enriched with oxygen must be used to increase the flame temperature. Even at the high flame temperawre, the sensitivity of the aluminum determination
is not as good as the sensitivity obtained for other metals such as Cu, Fe, and Mg. Sachdev and West (1970) reported a simple solvent extraction procedure for preparation of samples for the determination of some bivalent and trivalent 1
To whom correspondence should be addressed. Volume 6, Number 7, July 1972 645
w The method developed by Sachdev and West (1970) for determination of trace metal ions by atomic absorption spectrophotometry was modified for the measurement of the concentration of aluminum in water. Benzene was used as the organic solvent for extraction instead of ethyl propionate. The lower solubility of benzene in water allows the use of a higher extraction ratio. The sensitivity of the aluminum determination was improved to 0.009 mg/l. per 1 % absorption, an elevenfold improvement on the previously published method.
metal ions using atomic absorption spectrophotometry. In their method, diphenylthiocarbazone, 8-quinolinol, and acetylacetone were used as chelating agents, and ethyl propionate was used as the organic solvent for the extractions. The sensitivity which they obtained for the aluminum determination was 0.1 mg/l. per 1 % absorption, and the range of the linear calibration curve was from 0.0 to 10.0 mg/l. Al. A sensitivity better than 0.1 mg/l. per 1 absorption was required for the determination of residual aluminum in solution in contact with an Al(OH)3floc. When the procedure of Sachdev and West (1970) was tried, it was found that the sensitivity could not be improved further by increasing the volume of aqueous sample per unit volume of organic solvent simply because the solubility of ethyl propionate in water (2.69 m1/100 ml) is too high. For this investigation the volume of aqueous sample per unit volume of organic solvent is called the extraction ratio. It was decided to try using benzene for the extraction in place of ethyl propionate because of the very low solubility of benzene in water (0.08 m1/100 ml). The purpose of this study was to compare benzene to ethyl propionate as the organic solvent for extraction of aluminum from water using the same chelating agents suggested by Sachdev and West in an attempt to improve the sensitivity of the determination.
A hollow cathode lamp served as the light source and the wavelength used was 3092.7A. Samples were aspirated into a nitrous oxide-acetylene flame system. Reagents. The analytical grade reagents, diphenylthiocarbazone, 8-quinolinol, acetylacetone, benzene, ethyl propionate, acetic acid, ammonium acetate, ammonium hydroxide, and potassium aluminum sulfate were used. Deionized water was used for preparing standard solutions. Standard A1 Solution. Potassium aluminum sulfate was dissolved in deionized water to prepare the standard solution. The aluminum concentration in the stock solution was 1000 mg/l. and all the standard solutions were prepared by appropriate dilutions of the stock solution with deionized water. Extraction Solution. Diphenylthiocarbazone, 0.1 gram; 8-quinolinol, 0.75 gram; and acetylacetone, 20 ml, were dissolved in benzene and the volume was made up to 100 ml. The other extraction solution was prepared with the same components but ethyl propionate was used instead of benzene. Operating Conditions. The operational parameters were EO adjusted that optimum sensitivity could be obtained. These parameters were: current in the hollow cathode lamp, 16 mA; burner height, 10 (scale unit of the instrument); and the flow rate of nitrous oxide, 19 SCFH. The flow rate for acetylene was so adjusted that the red portion in the flame was about 3/4 in., as recommended by the manufacturer. It was 5.0 SCFH when benzene was used as the solvent and 10.0 SCFH when ethyl propionate was used as the solvent. In either case, the rate of sample aspiration was about 5 . 2 ml/min. Results The laboratory procedure consisted of three steps: determination of the optimum pH for extraction, determination of the sensitivity of the test at two different concentrations of
1.0
Experimental Equipment. A Jarrell-Ash atomic absorption spectrophotometer with a laminar flow, premixed burner was used.
Benzene
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.-c
70
0
onate
0.5
k
!i OS2
t
8 0.1
-
T
0.5 0.05 0.5 0.05
n
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il 0.05
E"
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.-
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c
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0.5 rng Aluminum
0.02
c VI
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0.01
0.005
4.5
5.0
5.5
PH
Figure 1. Determination of optimum pH for extraction 646 Environmental Science & Technology
I
I
I
I
I
I
5
IO
20
50
100
200
Extraction ratio =
Volume of aqueous solution Volume of organic solvent
Figure 2. Sensitivity of aluminum determination at various extraction ratios
aluminum and different extraction ratios, and the preparation of the calibration curve. Two concentrations of aluminum were used to determine the optimum pH for the extraction procedure. The pH values were adjusted with acetic acid-ammonium acetate buffer system using a Fisher titrimeter. After pH adjustment, 50 ml of the aqueous solution was transferred to a separatory funnel and 5 ml of the solvent extraction solution was added. The separatory funnel was shaken and left quiescently for phase separation. After 3-5 hr, the organic layer was collected and aspirated into the flame of the atomic absorption unit. It was found that maximum absorption was reached at pH = 5, as shown in Figure 1. All the subsequent extractions were done at pH = 5 f 0.05. The sensitivity of the extraction procedure with benzene as the solvent was tested by diluting 0.05 and 0.5 mg of aluminum to 50, 100, 200, 250, 500, and 1000 ml, respectively, with deionized water and extraction of the aqueous solutions with 5 ml of the organic solvent. The results are plotted in Figure 2, together with two curves obtained with ethyl propionate as the organic solvent. The optimum pH value for ethyl propionate extraction was also determined to be 5.0. The maximum extraction ratio for this solvent, however, was limited to about 20. The cross marks on the curves show the point where phase separation starts to disappear. Nine points for a calibration curve were determined in triplicate by using an extraction ratio equal to 40 (200 ml aqueous sample and 5 ml benzene solution). Results were plotted in Figure 3 with the corresponding standard deviation of each point. A calibration curve obtained by using'ethyl propionate as the solvent at extraction ratio equal to 20 is also presented for comparison. Discussion
Solvent extraction is a very useful technique for concentrating the element to be measured and improving the sensitivity of the analysis. However, the sensitivity is limited by the solubility of the solvent. As shown in Figure 2, at 0.05 mg Al, with ethyl propionate as the solvent, at the maximum practicable extraction ratio the sensitivity of the measurement cannot be improved beyond 0.24 mg/l. per 1 % absorption. However, with the same amount of aluminum, the sensitivity of the measurement using benzene as the solvent can be improved to 0.009 mg/l. per 1 absorption simply by using a larger quantity of the aqueous sample. Phase separation with a volume of aqueous solution up to 1000 ml was always clear when benzene was used as the solvent. The use of 1000ml aqueous sample and 5-ml organic solvent is almost the maximum working limit owing to the mechanical problems of handling the separatory funnel and separating the two phases. Nevertheless, if an extraction ratio greater than 200 were used, the sensitivity of the measurement could be further improved. A larger quantity of aqueous sample can be used because of the lower solubility of benzene in water. Munro (1968) noticed that, because of the solubility of the solvent, the extraction ratio and the acidity of the aqueous sample should be kept the same as that used for the standard curve. The sensitivity of the measurement is also controlled by the extraction ratio. The advantage of using benzene as the solvent is not only due to its lower solubility, but also as a result of the improvement in the flame condition. This effect can be seen in Figure 2, where, at the extraction ratio, 20, the sensitivity with benzene as the solvent may be as high as three or four times that obtained with ethyl propionate as the solvent. This
I
-e
100
-
50
-
20
-
IO
-
i
I
I
I
I
I
I
I 2.0
I 5.0
Ethyl propionate, Extraction'iotio 20
0
C
.-
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n v)
a #
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0.5
0.I
-
I
QOIO.02
I 0.05
I 0.1
I 0.2
I 0.5
I
1.0
Concentration of Aluminum in aqueous solution, rng / I
Figure 3. Calibration curves for aluminum determination
improvement in flame condition could be due to the higher stability of the extracted aluminum in benzene because two and 8of the chelating agents-diphenylthiocarbazone quinolinol-are aromatic compounds and they should form more stable complexes in benzene than in ethyl propionate. From the two calibration curves for the aluminum determination in Figure 3, the effect of improvement by the use of benzene can be further realized. The precision, which indicates the stability of the flame when benzene is aspirated into it, is excellent as shown by the average standard deviation of 0.09 of the determined aluminum concentration. The chelation technique with ethyl propionate as the solvent gave a maximum sensitivity of 0.24 mg/l. per 1% absorption (Figure 2) in this study, as contrasted with 0.1 mg/l. per 1 absorption as previously reported. This difference could be due to the atomic absorption spectrophotometer employed and the difference in the flame conditions. If the same atomic absorption were employed under the same conditions, a sensitivity for aluminum determination much better than 0.009 mg/l. per 1% absorption could be expected if benzene were used as the organic solvent for the extraction and larger quantity of aqueous sample were available. In addition to the advantages mentioned above, other advantages of using benzene as the organic solvent are the smaller amount of acetylene required to support the flame and the lower cost of benzene than ethyl propionate. Since the extraction solution used in this work is not limited to aluminum, other metals such as Zn, Be, Co, Cu, Fe, Ni, could also be determined using benzene for extraction. Literature Cited Munro, D. C., Appl. Spectros., 22, 199-200 (1968). Sachdev, S. L., West, P. W., ENVIRON. Scr. TECHNOL., 4, 749-51 (1970). Receivedfor retiiew Nocember 1,1971. Accepted March 9,1972. Volume 6 , Number 7, July 1972 647
