system as shown in Figure 2. T h e results of the Zn-TTA system are in good agreement with those in previous papers (12, 13). I t is reasonable that the ratio of the molar volume of the chelate to that of the ligand is about two, because two molecules of chelating reagent combine with a divalent metal ion. In the case of the Zn-STTA, Zn-TTA, and Co-TTA systems, the plots for polar solvents containing an oxygen atom are generally far apart from the lines for inert solvents as is shown in Figure 2. This may be due to t h e solvation effect of polar solvents (13). The difference of the lines for oxygen containing solvents from those for inert solvents increased in the order of Cu(I1) < Zn(I1) < Co(II), showing the magnitude of the residual coordination power of a metal ion with polar solvent (14). T h e solvation effect concerning oxygen containing solvents in the T T A system is relatively larger than t h a t in the S T T A system, as can be seen in Figure 2. This may be attributed to the enhancement of the stability of the metal S T T A chelates due to the strong covalent bonding between metal and sulfur atoms such as dsr-dsr back donation,
and the participation of solvation to the STTA chelates may become relatively smaller.
LITERATURE CITED (1j J. H. HiMebrand and R. L. Scott, "The Solubility of Non-Electrotytes", Dover Publications, New York, N.Y., 3rd ed., 1964. (2) K. Akiba, N. Suzuki, and T. Kanno, Bull. Chem. SOC. Jpn., 42, 2537 (1969). (3) H. A. Mottola and H. Freiser, Talanta, 13, 55 (1966); 14, 864 (1967). (4) T. Wakabayashi, Bul!. Chem. SOC.Jpn., 40, 2836 (1967). (5) S. Oki, Talanta, 18, 1233 (1971). (6) H. M. N. H. Irving and J. S. Smith, J. Inofg. Nucl. Chem., 30, 1873 (1968). (7) T. Omori, T. Wakabayashi, S. Oki, and hi. Suzuki, J . Inorq. Nucl. Chem., 26, 2265 (1964). (8) T. Honjo and T. Kiba, Bull. Chem. SOC.Jpn., 45, 185 (1972). (9) E. Uhlemann and H. Muller, 2.Chem.. 8, 185 (1968). (10) T. Honjo, S . Yashima, and T. Kiba, Bull. Chem. Soc. Jpn., 46, 3772 (1973). (1 1) T. Honjo, M. Horiuchi, and T. Kiba, Bull. Chem. Soc. Jpn., 47, 1176 (1974). (12) N. Suzuki, K. Akiba, and H. Asano, Anal. Chim. Acta., 52, 115 (1970). (13) N. Suzuki and K. Akiba, J . Inorg. Nucl. Chem., 33, 1897 (1971). (14) T. Sekine and D. Dyrssen, J . Inorg. rVucr. Chem., 26, 1727 (1964).
RECEI\-ED for review April 26, 1977. Accepted August 23,1977.
Determination of Trace Levels of Nitrates by an Extraction-Photometric Method Philip Baca and Henry Freiser" Department of Chemistry, University of Arizona, Tucson, Arizona 8572 1
Trace levels of nitrates may be quantitativeiy and selectively extracted as an ion pair with crystal violet using chlorobenzene. The results in the range of 60-720 ppb agree within a standard deviation of 28 ppb. The interference of bromide was substantially reduced by the use of hydrazine. It was surprising to learn that oxidation of bromide to tribromide under the mild extraction conditions will occur unless a reducing agent is added.
T h e most widely accepted methods currently available for analysis of nitrate ion generally involve either nitration of a phenol derivative, oxidation of an organic reagent, or reduction t o nitrite followed by production of a n azo dye ( 1 ) . These methods frequently give poor reproducibility because of the difficulty of maintaining proper control of the chemical reaction involved. Results may be affected directly by substances whose behavior is similar to that of nitrate or indirectly by substances which affect the reactivity of the reagents or the stability of the products, as well as by substances that react with nitrate under t h e reaction conditions used. Ion pair formation is another useful basis for nitrate analysis. The nitrate ion because of its size and relatively weak hydration readily forms extractable ion pair complexes with large organic cations. Although ions of similar charge and geometry interfere, very few such interferences occur in atmospheric particulate samples. Yamamoto et al. ( 2 ) introduced a procedure based on the extraction of the ion pair complex between nitrate ion and the cation of crystal violet into chlorobenzene a t pH 5-7, which has a working range of 0.24 to 1.20 ppm nitrate. When we examined the suitability of the method for analysis of nitrate in atmospheric particulate matter, we found it possible to
substantially improve both the sensitivity and selectivity of the crystal violet method.
EXPERIMENTAL Apparatus. Absorbances were measured with a Gilford 2400 spectrophotometer using matched 1-cm path length glass cells. Reagents. Crystal violet chloride (Matheson Coleman and Bell) was used as received. Hydrazine sulfate (Fisher Scientific Company), potassium monobasic phosphate and chlorobenzene (Mallinckrodt Chemical Works) were reagent grade. Deionized water was used. Procedure. The following reagents were added to 50-mL volumetric flasks: 10.0 mL crystal violet chloride (1.3 X M aqueous solution), 4.0 mL potassium inonobasic phosphate (pH 6.0),5.0 mL of 1.4 X lo-' M hydrazine sulfate, and 1.0-12.0 mL of 3.00 ppm nitrate (prepared with KN03) working solution. Aliquots of 10.0 mL were transferred from each flask to separatory funnels, 10.0 mL of chlorobenzene were added to each funnel, and the funnels were shaken for 2 min. Absorbances of the organic phase were measured at 595 nm against a chlorobenzene reference. RESULTS A N D DISCUSSION A number of anions, including perchlorate, chlorate, and halides compete with nitrate for pairing with cations. Of these, only bromide ion and, to a lesser extent, chloride ion, were expected to present interference problems since t h e other interfering ions are not usually !present in atmospheric particulate samples. In the course of a bromide interference study, absorbances were observed to continually increase with repeated shaking of the extraction vessels (Table I) or merely on prolonged contact (Table 11) of organic phases with aqueous phases containing bromide ion. In contrast, absorbances remained constant when bromide was not added to the standard solutions. T h e results are consistent with atmospheric oxidation of bromide to tribromide, a large and poorly solvated ion which ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2249
Table 1. Effect of Repeated Shaking of Extraction Vessels on Br- Interference Set A
Sample No.
Concn NO,', PPm
Concn Br-, ppm
Absorbance
Absorbance after second shaking of extraction vessela
1
0.00
2
0.48 0.48 0.48 0.48 0.48
0.00 0.00 0.32 0.64 0.96 1.28
0.463 0.577 0.578 0.590 0.604 0.616
0.464 0.576 0.580 0.608 0.610 0.622
3 4
5 6 a
Absorbance after third shaking of extraction vessel 0.464 0.578 0.585 0.611 0.614 0.631
Extraction vessels were shaken for 5 min each time.
Table 11. Effect of Prolonged Contact of Phases on Br- Interference Set B
Sample N0 .
Concn NO,~.-, PPm
Concn Br-. PPm
Absorbance of organic phase
Absorbance of organic phase after 36 hours in contact with aqueous phase
0.00
0.00 0.00
0.464 0.578 0.579 0.591 0.606 0.613
0.464 0.577 0.582 0.595 0.612 0.622
0.48 0.48 0.48 0.48 0.48
0.32 0.64 0.96 1.28
Table IV. Crystal Violet Method for Nitrate Calibration Run
Table 111. Effect of Hydrazine Sulfate on Br- Interference
Sample No.
Concn hydrazine sulfate, mol/L
1 2
0.0 0.0
3 4
8
4.0 x 10-5 8.0 x 8.0 x 10-5 1.2 x 0.0 8.0 x
9
0.0
5
6 7
io
8.0 x 10-5
Concn BrPPm
Concn
0.00
0.00 0.00
1.28 1.28 1.28 1.60 1.60 0.00 1.28 0.00 1.28
NO, -,
PPm
0.00 0.00 0.00 0.00 0.36 0.36 0.48 0.48
Absorbance 0.476 0.508 0.501 0.476 0.489 0.479 0.599 0.605 0.630 0.632
tion No.
Absorbance reading Obsd Calcda 0.507 0.534 0.543 0.566 0.587 0.613 0.708 0.740 0.760
0.499 0.524 0.549 0.574 0.598 0.623 0.697 0.722 0.772
Nitrate concentration, PPb Taken Calcda 60 120 180 240 300 360 540 600 7 20
79 144 166 221 272 336 566 643 692
Error, ppb
+19 + 24 - 14 - 19 -
28
- 24
+ 26 +43 - 28
S = 28 ppb
has a greater tendency to form an extractable ion pair than does the smaller bromide ion. In this event, use of a reducing agent should correct the problem. Hydrazine sulfate, a reducing agent which was considered as likely not to interfere (e.g., by reducing the dye to the leuco form), was therefore added to the calibration standards to determine its effect on the extent of interference in solutions containing bromide ion. The data in Table I11 indicate a decrease in the extent of interference. In the presence of the reducing agent, variation of absorbance with nitrate concentration was approximately constant regardless of whether or not bromide ion was added to the standards. Because addition of hydrazine sulfate similarly improved selectivity with respect to chloride, the concentration of crystal violet chloride reagent was increased in an attempt to achieve higher sensitivity to nitrate. Linear and reproducible calibration curves with working range 0.06-0.72 ppm nitrate were obtained when the crystal violet concentration was 1.3 X M and the hydrazine sulfate concentration in the standards M. From a typical calibration run, illustrated was 1.4X in Table IV, it may be seen that the method may be successfully applied to nitrate solutions from 60-720 ppb NO3 with a standard deviation of 28 ppb. 2250
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
a Calculated from least squares equation: A = (412.5 i 12)CNO + 474.6 t 7.4 where A is absorbance and CNO, is expressgd in ppb.
CONCLUSION Modification of the crystal violet procedure to include hydrazine sulfate reagent and an increased concentration of crystal violet chloride reagent has resulted in greater sensitivity and selectivity. T h e method is reliable and simple enough to use routinely. Finally, the surprising ease of oxidation of bromide to tribromide suggests that much more care in dealing with samples containing bromide (and of course iodide) need be exercised. LITERATURE CITED (1) M. J. Taras, "Colorimetric Determination of Nonmetals", D. F. Boltz, Ed., Interscience, New York, N.Y., 1958, pp 135-147. (2) Y. Yamamoto, S. Ushikawa. and K . Akabori, Bull. Chem. SOC. Jpn.. 37, 1718 (1964).
RECEIVED for review July 25,1977. Accepted August 15, 1977. This work was supported by a grant from the Environmental Protection Agency.
