New Spectrophotometric Reagent Specific for the Determination of Copper Rostam H. Maghssoudi and Ahmad B. Fawzi Department of Pharmaceutical Chemistry, College of Pharmacy, Tehran University, Tehran, Iran
Many colorimetric methods have been described for the determination of trace amounts of copper, and those based on bathocuproine ( I ) (2,9-dimethyl-4,7-diphenyl-1,10phenanthroline), neocuproine ( 2 ) (2,9-dimethyl-l,lO-phenanthroline), and 2,2'-diquinoline ( 3 ) are considered to be specific for copper(1). The use of either diethyl dithiocarbamate or dithizone in the spectrophotometric micro- and macrodetermination of copper did not prove to be entirely satisfactory. The former is not sufficiently specific because it is not burdened by the presence of commonly present ions, such as iron, nickel, and cobalt. The later is not satisfactorily selective in its action a t a wide range of p H values, and the color of the reagent itself interferes with the measurements. For this reason Irving et al. ( 4 ) gave a new but more complicated procedure for the use of dithizone for the determination of copper. Most of the other colorimetric methods (5-7) that have been proposed for the determination of copper are of slight importance from the practical point of view. This paper introduces 6-phenyl-2,3-dihydro-as-triazine3-thione(PDTT) as a new colorimetric reagent specific for the determination of copper(111, and describes systematic studies on the solvent extraction of Cu(I1)-PDTT complex. P D T T was first prepared and reported by Iraj Lalezari (8). Then it was used for the gravimetric determination of thallium ( 9 ) ,and for the spectrophotometric determination of mercury(I1) (10).
"1
0.8
WAVELENGTH, nm
EXPERIMENTAL Apparatus. The absorbances were measured by a Carl-Zeiss spectrophotometer, PMQ I1 type, with 1-cm quartz cells. A Pye-Unicam model SP 90A atomic absorption spectrophotometer was used for atomic absorbance measurements. The p H of the solutions were measured by using a Beckman H3 type pH meter. All measurements were carried out a t room temperature (20 f 2 "C). Reagent and Chemicals. A stock solution of copper nitrate, containing 0.7608 mg Cu/ml solution, was prepared by dissolving the pure metal in 5 ml concentrated nitric acid, followed by heating until almost dry to dispel the gases, and dilution to volume. This solution was standardized by the atomic absorption method vs. a standard sample of the National Bureau of Standards (United States Department of Commerce, Washington 25, D.C.). The solutions of lower concentration were prepared by volumetric dilution of the stock solution with distilled water. A solution of 0.001M P D T T was prepared by adding a slight excess of concentrated sodium hydroxide solution to the weighed amount of P D T T , and diluting with distilled water to the right volume. This solution is stable up to 5 hours. T o study the effect of other ions on the Cu(11)-PDTT complex, solutions containing 10 mg/ml of the desired ion were prepared. All other reagents used were of Analar grade chemicals. General Procedure. Pipet a sample of sufficient size to contain 25-100 kg of copper(I1) into a 50-ml separatory funnel. Add 1 ml of 4N tartaric acid, 3 ml of freshly prepared 0.001M P D T T , followed by 5 ml of chloroform, shake vigorously, and then add 1 ml of 4N sodium hydroxide solution. Continue shaking to extract all of the red color formed in the aqueous layer. Remove the chloroform layer in a 10-ml volumetric flask. Then continue the extraction with 3 and 2 ml of chloroform, respectively. Adjust the volume of the flask, and finally measure the absorbance of the solution vs. a similarly prepared reagent blank at 500 nm. 1694
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Figure 1. Absorption spectra of chloroform solutions of various concentrations of Cu(II)-PDTT complex obtained following the recommended analytical procedure
RESULTS AND DISCUSSION Absorption Curve. Figure 1 shows the absorption spectra of a series of chloroform solutions containing different amounts of the complex. The maximum absorption of the chloroform solution of Cu(I1)-PDTT complex was a t 500 nm. Determination of the Composition of the Chelate. The stoichiometry of the compound involved in the formation of the complex was ascertained by the following methods. T h e Mole Ratio Method. The method of Yoe and Jones ( 1 1 ) was applied to aqueous solutions containing standard copper solutions a t a fixed concentration of 1 X lO-*M and different amounts of standard PDTT solutions. Absorbances were measured a t 500 nm after 10 minutes. Figure 2 shows that the rapid increase of the absorbance occurs as the concentration of P D T T increases until a molar ratio [Cu(II)]:[PDTT] = 1:2. For higher concentrations of PDTT, the absorbance remains constant, suggesting the formation of 1:2 complex. T h e Continuous Variation Method (12, 1 3 ) . The total concentration of the mixtures [Cu(II) PDTT] was kept constant, first a t 5 X 10-4M and then a t 3 X lO-*M. The procedure was performed a t pH 14, with the initial use of
+
0.6
1
I
0.6
1
W
u z
U m
E
0
ln m U
2.0
4.0
6.0
0.2
0.4
0.6
0.8
1.0
I
~PDTTI/tCu**I
[Cu2+1/( tCu"I+[PDTTI)
Figure 2. Variation of the absorbance at 500 nrn. [Cu2+]
= 1.0 X
i o - 4 ~
Table I. Extraction of Copper(I1)-PDTT Complex with Chloroform as a Function of pH
Flgure 3. Variation of the absorbance at 500 nrn. Concentration of Cu2+ and PDTT: ( 0 )5.0 X 10-4M; (0)3.0 X IOe4M
Table 11. Effect of Foreign Ionsa Ila\r,mun.
Copper(I1)- PDTT compleu e m - c t e d
PH
1.o
2 .o 4 .O 8 .O
14.0
in CHCl3,
18.85 35.23
100 .oo 100 .oo 100 .oo
Ions a d d e d
Sources
Hg?' Hg' Ag'
HgNO?. 3HzO -%NO,
Fe2+
FeSO,
C02'
COC1, 6H?O
D, distribution ratio
0.46
1.09 m
cc co
tartaric acid. The absorbance measurements were made at 500 nm. Typical observations are shown in Figure 3 where it is seen that the maxima occurs at 0.33 mole fraction of copper indicating that the composition of the complex is 1:2. Beer's Law, Range, a n d Precision. Three standard series of 7 samples, ranging in concentration from 2.1 to 12.3 bg/ml of cooper(II), were treated by the recommended procedure. Conformity to Beer's law was obtained in each case. Optimum concentration range for measurement a t 500 nm and 1.00-cm optical path is about 2.5 to 10.5 bglml of copper(I1). The absorptivity a t 500 nm is 4.976 X lo3 liter mole-' cm-l. The relative standard deviation of the calculated absorptivities of the 2 1 samples in the three standard series is 2.7%. The relative standard deviation of the calculated absorptivities of the 15 samples in the optimum concentration range is 1.3%. Effect of pH. T o prevent the precipitation of most cations, including copper, present in the form of hydroxides in the alkaline medium, tartaric acid was added to the solution prior to the addition of PDTT. Addition of tartaric acid forms copper tartarate salts, which eventually complex with the PDTT. 'Following the above procedure, the pH could be raised to as high as 14 with complete extraction of the complex. Table I shows the extraction of Cu(11)-PDTT complex in chloroform at pH 1 to 14. The results indicate that the extraction is complete only between pH 4 to 14. Stability of t h e Colored Complex. The chloroform solution of Cu(I1)-PDTT complex is completely stable as a function of time. The absorbance of the complex solution
a
H@1,
aniomt, u 9
1000 1000 600 20000
100
Cu(I1) = 61.0pg; pH 14; PDTT = 0.001.V
in chloroform was measured a t elapsed intervals of 0, %, %, 1, 24,48, 72, and 120 hours, and was stable. Solutions of the colored complex in chloroform have been kept in stored tubes in the diffuse light of the laboratory for one year. These solutions showed no change in the intensity when compared with freshly prepared solutions. Sensitivity of t h e Reaction. As little as 5 wg/ml of copper could be detected visually as "red color" before the chloroform extraction. However, the visual detection could be improved to as low as 0.5 pg/ml, if the complex was extracted into 0.2 ml of chloroform. Effect of Foreign Ions. In determining the effect of foreign ions, a known volume of standard copper(I1) solutions containing various concentrations of the ion in question was taken. Copper(I1) was determined as previously described. Thallium, nickel, magnesium, manganese, aluminum, barium, strontium, calcium, platinum, molybdenum, antimony, bismuth, lithium, potassium, sodium, tin, chromium, iron(III), cadmium, lead, zinc, gold, fluoride, chloride, and phosphate, were tested for their effect on the Cu(I1)P D T T complex, and were found to be completely without interferences a t any concentration. Among these ions, zinc, aluminum and antimony form white precipitates in alkaline medium which could be dissolved by raising the pH to 14 with the addition of sodium hydroxide solution. Magnesium, manganese, chromium, calcium, molybdenum, bismuth, and barium form precipitates in alkaline medium which could be prevented by raising the p H to 9 after the addition of tartaric acid. ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
1695
Only iron(II), cobalt, silver, mercury(1) and (11) show serious interferences in high concentrations. However, P D T T dissolved in chloroform could be used to prevent the interference of iron(I1) and cobalt. Addition of silver, mercury(1) and (11) ions to the Cu(I1)-PDTT complex decomposes the complex giving the corresponding complexes. T o prevent this interference by these ions, dithizone was added to form stable complexes with silver, mercury(1) and (11) in acidic solution, which could be extracted with chloroform with no effect on the copper ion content. Then, the copper ion content was determined by the described procedure. However, the above recommendations could be omitted if the concentration of the foreign ions is within the limit shown in Table 11.
ACKNOWLEDGMENT We thank Ali Shafiee for his valuable advice.
LITERATURE CITED (1) G. F. Smith and D. H. Wilkins. AnalChem., 25, 510 (1953). (2) G. F. Smith and W. H. McCurdy, Anal. Chem., 24, 371 (1952). (3) J. G. Breckenridge. R. W. Lewis, and L. Quick, Can. J. Res., Sect. 6,17, 258 (1939). (4) H. Irving, G. Andrew, and E. J. Risdon, Nature (London), 161, 805 (1948). (5) J. R. Johnston and W. J. Holland, Mikro Chim. Acta, 1, 126 (1972). (6) M. C. Patel, J. R. Shah, and R. P. Patel, J. Prakt, Chem., 314, 181 (1972). (7) R. N. Virmani, 8. S. Gray, and R. P. Singh, lndian J. Chem., 10, 225 (1972). (8) I. Lalezari and H. Golgolab, J. Heterocycl. Chem., 7, 689 (1970). (9) M. Edrissi. A. Massoumi, and I. Lalezari, Talanta, 19, 814 (1972). (10) R . H. Maghssoudi and F. A. Shamsa, Anal. Chem.. 47, 550 (1975). (11) J. H. Yoe and A. L. Jones, lnd. Eng. Chem., Anal. Ed., 16, 111 (1944). (12) P. Job, Ann. Chim., 9, 113 (1928). (13) P. Job, Ann. Chim., 16, 97 (1936).
RECEIVEDfor review October 30, 1974. Accepted February 4, 1975.
Application of Cryogenic Infrared Spectrometry to the Identification of Petroleum Patricia F. Lynch, Sheng-yuh Tang, and Chris W. Brown Department of Chemistry, University of Rhode lsland, Kingston, Rl 0288 7
During the past two years, we have explored the applicability of infrared spectrometry in identifying crude and refined petroleum products (1-5), The sources of several actual oil spills involving both light ( 5 ) and heavy (2) fuels have been correctly identified, even after the oils had been on water for several weeks. In some cases, there were several suspect sources involved, and it was necessary to determine the correct source from as many as 14 oils. Others (6-15) have used infrared spectra measured a t room temperature to identify the source of petroleum but, to our knowledge, no one has reported on the low-temperature spectra of petroleum. Herein, we discuss the feasibility of using infrared fingerprints obtained a t 80 and 20 K to identify the source of petroleum.
EXPERIMENTAL Infrared spectra were measured on a Perkin-Elmer Model 521 infrared spectrometer. A typical low-temperature infrared cell shown in Figure 1 was used for the measurements a t 80 K. CsI substrate and outer windows were used on the cell. T o place a sample of oil in the cell, one outer window was removed, the cell was held in a horizontal position, and oil deposited on the CsI substrate (alternatively, the oil can be deposited on the substrate through a ground glass joint on the side of the cell). After depositing the sample, cold gaseous NZwas passed into the inner chamber to cool the substrate and sample; the outer chamber was flushed with dry N p gas to eliminate condensation of H20.After the sample solidified, the outer window was replaced, the outer chamber evacuated to -lo-” Torr, the cell turned upright, and the inner chamber filled with liquid N2.Time required to prepare the sample cell and measure the sample spectrum a t 80 K is approximately 45 minutes. Preparation of the sample and measurement of the spectrum a t room temperature requires 25 to 30 minutes. Spectra were also measured a t 20 K using a Cryotip cell (Air Products Inc.). The sample deposition procedure was similar to that used for the 80 K measurements, but more time consuming.
RESULTS AND DISCUSSION Infrared spectra in the 650-1200 cm-l fingerprint region of ( a ) a No. 2 fuel oil, ( b ) a crude oil, and ( c ) a No. 6 fuel oil measured a t room temperature and a t 80 K are shown in 1696
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Figure 2. The major differences between the low temperature spectra which are typical of all oils studied thus far are: i) the 720 and 725 cm-I bands appear as two distinct absorptions, increasing in intensity compared to the bands measured a t room temperature; ii) the 740 cm-l band decreases in intensity at 80 K; and iii) the 890 cm-’ band increases significantly in intensity compared to the band in room temperature spectra. The bands a t 720 and 725 cm-I are due to the in-phase CH:! rocking mode of long-chain n-paraffins. The increase in intensities and sharpness of these bands is most probably due to alignment of the n-paraffins into a polymeric crystalline structure such as in polyethylene. Instead of having a distribution of frequencies we observe only one band, which is split into two components by intermolecular interactions ( 1 6 ) . There are other changes in the low temperature spectra, but these are characteristic of each oil and cannot be generalized. The distinct differences in the fingerprint a t low temperature as compared to room temperature give an oil sample, in effect, two “fingerprints”. The reproducibility of the technique was tested by depositing the same oil sample in the low temperature cell several times and measuring its spectrum after each deposition. Previously, we have shown that the room temperature spectrum of,an unknown oil can be matched to that of the correct source by ratioing the absorptivities of the bands of oils from known sources with those of the unknown ( I ) . Identification is made to the known having the most ratios closest to the average ratio (currently, we are using a logscale). In the present study, we have used the same type of analysis on the low temperature spectra, i.e., we have matched unknown oils with the correct source oils by ratioing absorptivities. The comparisons were made using bands a t the same frequencies as those for the room temperature spectra. During October 1974, two major oil spills occurred and the low temperature techniques were tested in a “real spill”
