caused by the lowering of the temperature of the flame. I n spite of this, magnesium can be determined with a ten times higher sensitivity with the ultrasonic nebulizer, and-as the solution is almost quantitatively introduced into the flame-the consumption of sample is only a small fraction of that obtained with the pneumatic nebulizer, Different from the results reported by Dunken et al. ( 2 ) ) our results show a larger increase of emission with the earth metals than with the alkali metal, As the other conditions in the flame are the same in our experiments, the difference between the metals must be a function of the finer dispersion alone. It can probably be explained by the fact that the further dispersion, uhich occurs in the flame as a consequence of the heat and of chemical reactions, plays a more important rBle in the case
of the alkali metal with its and its compounds’ lower melting and boiling points, than in the case of the earth metals, for which the degree of dispersion before the entering of the flame is a more deciding factor. The authors hope that the described arrangement will open the way for the convenient routine analytical use of ultrasonic nebulization not only in flame photometry but also in flame absorption spectrophotometry, where it is desirable to have a high concentration of the sample in the flame. The experiments will be continued with a stronger ultrasonic nebulizer which is being manufactured by Mivab, and attempts are being made to design a more convenient device for the administration of the solution, e.g. through a peristaltic pump or a similar arrangement.
ACKNOWLEDGMENT
The authors are indebted to Dr. P. Herzog, Department of Anaesthesia, Thoracic Clinic, Karolinska Sjukhujet, Stockholm, Sweden, for placing the ultrasonic nebulizer at their disposal. LITERATURE CITED
(1) Dunken, H., Pforr, G., Mikkeleit, W., 2. Chem. 4, 237 (1964). (2) Dunken, H., Pforr, G., Mikkeleit, W., Geller, K., Ibid., 3, 196 (1963). (3) Herzog, Paul, Sorlander, 0. P., Engstrom, C. G., Acta Anaesthesiol. Scand. 8, 79 (1964). (4) West, C. D., Hume, D. K., A N ~ L . CHEY.3 6 , 412 (1964).
~F-OLFGASGJ. KIRSTEN GOTE 0 . B. BERTILSSON
Department of Agricultural Chemistry I Royal Agricultural College of Sweden Uppsala 7 , Sweden
Spectrophotometric Determination of Nickel in Sea Water with Quinoxaline-2,3-DithioI SIR: I n a study of the bio-geo-chemical circulation of nickel in Hawaiian waters i t was decided to adopt the colorimetric method described by Laevastu and Thompson (5) for the determination of this element in sea water based on the well known nickel dimethylglyoxime complex. The method, found to be extremely sensitive to slight modifications in the procedures, was studied in detail, modified, and improved. T o attain a maximum nickel precipitation of 90% from a 2-liter sample of sea water, a minimum of seven days was required for the aging of the precipitate following addition of sodium carbonate. h salt effect was described which required either a calibration curve prepared in sea water or an application of a salt factor with use of a distilled water calibration curve. .\ procedure was recommended by Forster and Zeitlin (3) by which a precision of 0.7y0relative standard deviation is achieved by proper control of experimental conditions. The availability, however, of a relatively new organic chelating agent, quinoxaline-2,3-dithiol (QXDT), for nickel and other transition metals (1, 2, 6 ) suggested the possibility of its incorporation with the modified procedure. The present paper describes a procedure for the determination of nickel in sea n-ater with quinoxaline-2,3-dithiol which possesses a number of advantages over the dimethylglyoxime method, yet retains its precision: the analysis is simplified, requires less time, and fewer reagents; increased sensitivity of the
complex, E520 mp = 5.0 X lo3 for the Ki-QXDT and mp = 2.2 X lo3for Ni-DMG ( d ) , makes possible a reduction in sample size from 2 liters to 1 liter; there is increased stability of the nickel quinoxaline complex. EXPERIMENTAL
Apparatus. Spectrophotometer, Beckman D U matched quartz cells of 10.0-em. light p a t h length. p H meter, Beckman Model H-2 with glass, as the indicating and calomel as the reference electrodes. Filters, millipore, H A (0.45 micron) with diameter of 47 mm. Separatory funnels, Squibb 125-m1. borosilicate glass. Reagents. All reagents were made from analytical grade chemicals dissolved in doubly distilled deionized water. Quinoxaline - 2,3 - dithiol reagent, 0.023V (E. Kodak). Ammoniacal solutions were prepared by dissolving 0.39 gram of reagent in 100 ml. of concentrated ammonia and filtering through a fritted-glass crucible. The solution should be clear red-brown and stable for one week if stored in a dark bottle in the cold. Ammonia concentrated reagent. Sodium carbonate. Fifty grams of anhydrous salt dissolved and diluted to 1liter of solution. Hydrochloric acid. One part concentrated acid diluted with one part water. Sodium citrate. 10% aqueous solution. Chloroform. Reagent grade.
Dimethylglyoxime reagent. One gram dissolved in 100 ml. of ethyl alcohol. Standard nickel solutions. A solution containing 0.85 wg.-at./ml. (53.5 p.p.ni.) was prepared by weighing 0.3365 gram of nickel ammonium sulfate hexahydrate and diluting to a liter with water. This will remain stable for months. For use in preparing standards, 5.0 ml. of the above solution is pipetted into a 100-ml. volumetric flask and diluted to the mark with water. This solution, 0.0425 pg.-at./ml. (2.4 p.p.m.), is unstable and must be made u p daily. Aliquots, 1 to 5 ml., when diluted to 50 ml. as in the recommended procedure, provide concentrations of nickel ranging froin 0.85 pg.-at./nil. (53.5 p.p.b.) to 8.5 pg.-at./liter (535 p.p.b.). Procedure, Collection, and Concentration. Collect sea water samples in 1-liter plastic containers charged with 50 ml. of sodium carbonate. Mix t h e precipitate thoroughly and allow to settle for a minimum of 7 days (a rolling ship is a n ideal agitator). Separation. On shore, filter t h e precipitate through a n HA-millipore filter, wash twice with distilled water, and dissolve with a minimum of hydrochloric acid (about 15 ml.). Add sodium citrate (15 ml.) and evaporate t h e resulting solution at low temperature t o a volume of about 25 ml. T h e extraction of nickel from other dissolved metal ions is accomplished after adjusting the p H of t h e solution t o 8.0 with ammonia. Add 2.0 ml. of D M G reagent, mix well, cool in t h e refrigerator to room temperature, and add 3 ml. of chloroform. Shake in a separatory funnel for 2 minutes and let VOL. 38, NO. 4, APRIL 1966
649
5
0.94
Table I.
Stock solution added to 1 liter of sea water, ml. 0 (Blank)
Recovery of Nickel from Sea Water Spiked with Nickel
Absorbance at 520 mp
1.0 2.5 5.0
0.143 0.275 0.466 0.775
Concentration of nickel, p.p.b. Calcd. Found 3.0 5.5 9.3 15.5
...
5.2 9.0 15.3
0.7 Oei
Yield, 76 ... 95 96 97
0.5 4
0.3
Table II. Precision“ of QXDT Determination of Nickel in Distilled Water at 520 mp
Kickel concn., p.p.b. 2.5 6.25 12.5
Rel. std. Alean absorbance* dev., % 0.185 f 0.003 0.466 i 0.005 0.858 0.008
1.9 1.3 1.0
a
Precision calculated on the basis of
b
Absorbance values are less blank.
stand for 2 minutes. Reniove the lower chloroform layer. Repeat the extraction Lvith a second portion of chloroform and conibine with the first extract. Wash the combined chloroform extracts twice with 5-ml. portions of dilute ammonia (one part in 49 parts water) to remove the nickel from the ammonia washings. To complete the removal of nickel, extract the ammonia solution once niore with 3 ml. of chloroform, shake, and allow to stand as above. Add the chloroform layer to the previous chloroform extracts to give a total of approximately 9-nil. volume. T o remove the nickel from the organic phase, carry out two extractions with two 5-ml. portions of hydrochloric acid. I n all separations, avoid the white foam a t the interphase of the two liquids. Color Development. T o the above acid solution (10 nil. plus rinsing volume) add 10 ml. of concentrated ammonia with a pipet, smirl, add 3.0 nil. ( 1 0 . 1 ml.) of QXDT reagent, dilute to 50 ml. in a volumetric flask, and let stand for 30 minutes. Transfer the solution to 10.0-cni. cells and measure the absorbance a t 520 mp against a reagent blank. After subtraction of the blank, read the concentration of the nickel in the sample from the calibration curve (below). Sea Water Calibration Curve. T h e curve is prepared b y spiking 1-liter sea water samples with aliquots of standard nickel solution ranging from 0 to 5.0 ml. (0 to 12.5 p.p.b. of Xi). The spiked samples are carried through the total process of precipitation with sodium carbonate-aging the precipitate for the same length of time accorded the samples to be analyzed, and then through filtration, extraction, and color development. When the absorbance minus the blank is plotted against concentration of nickel, a linear plot with a slope of 0.0026 absorbance unit/p.p.b. 650
ANALYTICAL CHEMISTRY
of nickel is obtained passing through the origin. X typical plot is shown in Figure lb. The use of this curve avoids the need for a salt factor, which is salinity dependent, as well as the need for correction due to changes in the absorbance of the Xi-QXDT resulting from the reagents used throughout the process. If the blank is included, the concentration of the nickel originally present in the sea water may be obtained from the extrapolated abcissa intercept-i.e, Figure la. The salt factor iq represented by the difference in absorbances of nickel coniplexes of standards developed in sea water compared to those in distilled water (Figure 1, b and c ) and is treated in detail elsewhere (3). Transmittance Spectrum. The transmittance spectrum of a 5.231 aqueous ammoniacal solution of the nickel quinoxaline dithiol complex, as found by Skoog, Lai, and Furst ( 7 ) , has 3 well defined maximum absorbance a t 520 nip. Because the reagent absorbs a t this wavelength, a blank was used in all determinations. Color Stability. The complex developed from standard nickel solution is considerably more stable than t h a t of the nickel dimethylglyosime (3). Reagent Variables. . l h f l l O S I A CONCESTRBTIOS. T h e concentration of ammonia in t h e medium used in the development of the Ni complex is crucial in agreement with the finding of Skoog, Lai, and Furst ( 7 ) . -15.2M complexation medium with respect to ammonia which is obtained by the addition of 10 ml. of concentrated ammonia in the color development step (see procedure) is recommended because excellent reproducibility has been obtained with this concentration. REAGEKTSTABILITY. The recommended period of effectiveness, 7 2 hours, as found by Skoog ( 7 ) , may be extended to one week by keeping the filtered reagent in a brown bottle and storing it in the refrigerator. RE.4GENT COSCENTRATION. Unnecessary excess of quinoxaline dithiol must be avoided because of the high absorbance of the reagent at 520 mp. T o ensure an adequate blank correction, the reagent must be added with an accuracy of 1 0 . 1 ml. RECOVERIES. The previously constructed sea water calibration curve (Figure l ) , prepared from water off Koko Head on the northeast shore of Oahu (11/1/65), was used as reference to determine the effectiveness of the procedure in terms of the recovery of nickel from sea water. Freshly collected sea water samples (11/2/65) were spiked with varying aliquots of standard nickel solution (0 to 5 ml.). These
00 0
25
50 75 100 125 Nickel Concentration (ppb)
150
Figure 1 . Calibration curve for nickel in sea water developed with quinoxaline-2,3-dithiol a. Absorbances of sea water solutions b. Absorbances of sea water less blank C. Absorbances of distilled water less blank
were carried through the total process, the absorbances were measured, and the nickel concentrations were read from the calibration curves. The results are shown in Table I. The blank (3.0 p.13.b.) is due to the nickel present in the sea water. The overall recovery was 947, over the range of concentrations used. This same figure could be obtained by comparing the slope of the newly collected spiked sea water (0.127 absorbance unitlml.) with the slope of the spiked sea water used as a standard (0.134 absorbance unit/ml.). PRECISIOS. Six replicate determinations of the color development with quinoxaline dithiol were carried out in distilled water containing various amounts of nickel stock solution. The results are shown in Table 11. The precision of the method a t 3 concentration of 2.6 p.p.b. of nickel, normal for water in the Hawaiian area, is 1.9% relative standard deviation. LITERATURE CITED
(1) Ayres, G. H., Annand, R. R., ANAL. CHEM.35, 33 (1963). (2) Burke, R. IT., Yoe, J. H., Ibid., 34, 1378 (1962). (3) Forster. W. O., Zeitlin, H., Ana!. Chim. Acta. 34 (in press). (4) IUPAC “Spectrophotometric Absorp-
tion Data,” Butterworths,
London,
1963. (5) Laevastu, T., Thompson, T. G., J . Consei! 21, 125-43 (1956). (6) llorrison, D. C., Furst, A,, J . Org. Chem. 21, 470 (1956). (7) Skoog, D. A., Lai, lIing-Gon, Furst, A., ANAL.CHEW30, 365 (1958).
KILLIAM FORSTER HARRY ZEITLIN
Department of Chemistry and Hawaii Institute of Geophysics University of Hawaii Honolulu, Hawaii 96822 Contribution S o . 132 of the Hawaii Institute of Geophysics.
