servations were recently reported by others (Chromatography Lipids, August 1967, Supelco, Inc., Bellefonte, Pa.). This interference was eliminated by using BSA reagent purchased in glass sealed ampoules. Glass vials stoppered with septum, as recommended in the present method, can be safely used as reaction vessels for preparation of derivatives; however, prolonged exposure of the derivative preparation to the septa should be avoided.
ACKNOWLEDGMENT
The author thanks W. F. Beach, 0. M. Garty, and J. W. Lewis for helpful discussions. The provision of pure samples oP the impurities by A. 6.Farnham and of twice recrystallized bisphenol A by P. Schnur is gratefully acknowledged. RECEIVED for review July 22, 1968. Accepted September 9, 1968.
irect Spectrophotometric and ethods for Determi on of Thiocyanat
tion Spectrometric
Richard S. Danchik and D. F. Bolta Department of Chemistry, W a y n e S t a t e University, Derroir, Mich. 48202
ABSORPTION SPECTROMETRIC METHODS for the determination of nonmetals are being investigated in this laboratory. This paper reports a study of indirect spectrophotometric and atomic absorption spectrometric methods for the determination of thiocyanate. These methods are based on the formation of a dithiocyanatodipyridine copper(I1) complex which is extractable with chloroform. Spacu (1-3) has studied the precipitation of copper(I1) as C U ( ~ ~ ) , ( S C Non) ~ both the micro and macro scales. The colorimetric determination of copper using this complex has been investigated (4-7). Colorimetric methods using the copper-pyridine reagent for the determination of thiocyanate have also been studied (8-II). In utilizing the quantitative formation and selective solvent extraction of the complex, thiocyanate can be determined indirectly by the determination of copper in the complex. The determination of copper by atomic absorption spectrometric method is very sensitive (12). The indirect atomic absorption spectrometric determination of thiocyanate has a sensitivity of 0.05 ppm and has the advantage of speed, high precision, and simplicity. EXPERIMENTAL
Absorbance measurements were made in 1.OOcm matched cells with a Cary Model 14 spectrophotometer. A reagent blank was used in the reference cell. The atomic absorption measurements were made with a Beckman Model 1301 Atomic Absorption Accessory, a Beckman Model DB prism spectrophotometer equipped with a Beckman potentiometric recorder, and a Techtron burner assembly. The hollow cathode tube was neon filled and supplied by Beckman. A Thomas shaking apparatus was used for solvent extractions. Apparatus.
G . Spacu, Bull. SOC.Stiinte Cluj,, 1, 284 (1922). [bid.,p 314. G . Spacu and J. Dick, Z . Anal. Chem., 71, 185 (1927). R. Biazzo, Ann. Chim. Applicata., 16,96 (1926). J. B. Hester, Chemist-Analyst, 25, 78 (1936). L. Chalk, Analyst, 55, 187 (1930). C. Bennoit, Ann. Chim. Anal. Chim. AppI., 12, 66 (1930). (8) J. Kruse and M. G. Mellon, ANAL.CHEM., 25, 446 (1953). (9) K. C. Bailey and D. F. Bailey, Proc. Roy. Irish Acad., 37B, l(1924). (10) T. Moeller and R. Zogg, ANAL.CHEM., 22,612 (1950). (11) Y. Y.Lur’e, Zauodskaya Lab., 11, 273 (1945). (12) Beckman Instruments, Inc., Flame Notes, 1, 88 (1966). Pre-
(1) (2) (3) (4) (5) (6) (7)
liminary Information Sheet.
Reagents. COPPBW(II) SOLUTION.Dissolve 0.252 gram of pure copper shot in dilute nitric acid and dilute to a liter with distilled water. One milliliter of this solution contains 0.252 mg of copper. STANDARDTHIOCYANATE SOLUTION. Weigh out 0.194 gram of potassium thiocyanate and dissolve in 1 liter of distilled water. Use standard silver nitrate as titrant and iron(II1) indicator to standardize the thiocyanate. One milliliter of the thiocyanate solution contains 0.116 mg of thiocyanate. All chemicals were reagent grade and all aqueous solutions were stored in polyethylene bottles. Procedure. Weigh or measure by volume an amount of sample containing no more than 800 pg of thiocyanate in a total volume not exceeding 10 ml. The sample should contain no more than 360 pg when the atomic absorption spectrometric method is used. Transfer the sample solution to a 125-ml separatory funnel, add 20 ml of the copper(I1) nitrate solution and mix thoroughly. Add 1 ml of pyridine and 20 ml of chloroform to the aqueous solution and shake for 6 minutes with an automatic shaker. After allowing 3 to 5 minutes for the layers to separate, collect the green organic layer in a polyethylene bottle. SPECTROPHOTOMETRIC METHOD. Measure the absorbance of the organic extract at 407 mp in a 1.000-cm cell using chloroform in the matched reference cell. Refer absorbance reading to a calibration graph. ATOMICABSORPTION SPECTROMETRIC METHOD. Adjust the current applied t o the hollow cathode tube at 10 mA. Adjust acetylene-air flame until a luminous flame is just observed. Use a slit width of 0.20 mm and adjust the monochromator setting until a maximum absorbance reading is obtained at the 324.7-mp wavelength. Aspirate a standard solution corresponding to 232 pg of thiocyanate in the chloroform layer and adjust the restrictor to obtain maximum absorbance. Aspirate standard and unknown solutions using the absorbance readout of the recorder. Prepare a calibration graph. In order to improve the sensitivity of the atomic absorption spectrometric i ~ ~ t h i o devaporate , the chloroform layer aImost to dryness and then dilute to 25 ml with ethyl acetate. Aspirate the unknown solution and standard solutions using the absorbance scale of the recorder. Prepare a calibration graph. RESULTS AND DISCUSSION
THIOCYANATE CONCENTRAFigure 1 shows the visible absorption spectrum of the organic solution containing the dithiocyanatodipyridine Spectrophotometric Method.
TION.
Ion Nickel(I1) Mercury(1I) Iron(I1) Silver(1) Iodide Nitrite Permanganate Vanadate Iodate
Table I. Interfering Diverse Ions Spectrophotometric Atomic absorption method, method, permissible permissible amt., pg amt., pg 20 20
20 20
5
5 20 200 200
20 100 250 25 200
...
...
80 500
copper(I1) complex. The spectrum is characterized by the absorbance maximum at 407 mp. Conformity to Beer’s law was observed for 2.0 to 40.0 pprn of thiocyanate. The optimum concentration range is approximately 15 to 40 ppm of thiocyanate, corresponding to the 0.2 to 0.6 absorbance range. The molar absorptivity is calculated to be 790 1. mole-lcm-l. A reagent blank was found to have zero absorbance; therefore, chloroform was used in the reference cell. At less than 2 pprn, a negative deviation from Beer’s law was observed, probably caused by unfavorable equilibrium conditions. DIVERSE IONS.A study was made to determine the permissible amounts of various ions that may be present without interfering with the determination of ppm of thiocyanate. No attempt was made to determine the effect of the addition of more than 5000 pg of a specific ion, because this concentration is rather large compared to the concentration of thiocyanate being determined. Errors three times the relative standard deviation were considered negligible. Noninterfering ions include aluminum, ammonium, arsenate, bromide, carbonate, chloride, chromate, chlorate, cobalt(II), iodate, iron(III), lead(II), magnesium, molybdate, nitrate, perchlorate, phosphates, potassium, sodium, and zinc(I1). The tolerances of interfering ions are listed in Table I, column two. PRECISION. An estimate of the precision of the spectrophotometric method was ascertained from the results of eight samples each containing 29.04 ppm of thiocyanate. These samples gave a mean absorbance value 0.414 at 407 mp. The standard deviation was 0.002 absorbance unit, or a relative standard deviation of 0.40%. In a series of determinations, about 25 minutes is required for each determination. EXTRACTTION OF COMPLEX.After testing various organic solvents, chloroform was found to be the most suitable extractant of the complex. STABILITY OF COMPLEX.In a study of the stability of the complex in the chloroform layer, the absorbance remained unchanged for at least 2 hours. After standing for 7 days, EL decrease in absorbance and a shift of the band to 401 mp was observed. Atomic Absorption Spectrometric Method. THIOCYANATE CONCENTRATION. The indirect atomic absorption spectrometric method is more sensitive than the indirect visible spectrophotometric method for thiocyanate. The calibration graph is linear up to 18.0 ppm of thiocyanate using chloroform as the solvent. The optimum concentration range is approximately 7 to 18 ppm of thiocyanate, corre-
m
ANALYTICAL CHEMISTRY
‘0 WAVELENGTH, m u
Figure 1. Absorption spectra for dithiocyanatodipyridine copper(I1) 1. 11.6 ppm of thiocyanate 2. 23.2 ppm of thiocyanate 3. 34.8 ppm of thiocyanate
sponding to the 0.13 to 0.36 absorbance range. The reagent blank gave a reading of zero absorbance. The sensitivity is 0.2 ppm of thiocyanate. The most sensitive resonance line for copper is at 324.7 mp (12). PRECISION. An estimate of the precision of the indirect atomic absorption spectrometric method was ascertained from the results of eight samples each containing 11.6 ppm of thiocyanate. These samples gave a mean absorbance value of 0.254. The standard deviation was 0.005 absorbance unit, or arelative standard deviation of 1.87%. DIVERSE IONS. The effect of a number of diverse ions on the determination of 11.6 ppm of thiocyanate was studied. Those ions which did not interfere at the 5000-pg level include: arsenate, bromide, carbonate, chloride, and nitrate. Minor interferences are chlorate at 3000 pg; sulfate, perchlorate, molybdate, and phosphate at 2000 pg, and chromate and tungstate at 1000 pga The interfering ions are listed in Table I, column 3. Modified AAS Method. An increase in sensitivity for the atomic absorption spectrometric method is obtained by evaporating the chloroform extract and diluting the residue to a volume with ethyl acetate. A calibration graph is linear 0 to 2.5 ppm of thiocyanate. The optimum concentration range is approximately 0.5 to 2.0 ppm of thiocyanate. The sensitivity limit is 0.05 ppm of thiocyanate. A reagent blank gave a small, reproducible absorbance reading which is probably due to the very slight solubility of copper(I1) nitrate in the chloroform. RECEIVED for review July 26, 1968. Accepted September 14, 1968. Presented at Pittsburgh Conference, Cleveland, Ohio, February 1968.
