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20 minute
Figure 3. Decomposition of hydrogen peroxide (0)Curve obtained with the proposed gas-measuring device, ( x ) curve obtained with a traditional gas buret
tion) can be determined in a separate experiment in the following way. The gas-measuring device is connected to an automatic piston buret and a sensitive manometer is also fitted into the system. The piston buret is then moved at a definite rate, and the pressure excess is read off on the manometer. Since the frictional force is proportional to the rate of movement of the plunger, it is understandable that different excess pressures develop for dif-
ferent rates of change of the gas volume. For the buret made in our case, in the range of rate of gas admission of 0.08-0.27 ml sec-1, the pressure excess varied between 46 and 66 mm of water, or 3.4 and 5.0 mm of mercury. It follows from this that the excess pressure correction varies with the rate of change of volume. Since the data obtained with this device are usually subjected to computer processing, however, the correction is not accompanied by any particular inconvenience. The sensitivity of the device can be varied by selection of the diameter of the measuring tube and the distance of the electrode pairs, while the total volume to be determined can be varied by the length of the PVC tube. The range of reaction rate which can be determined automatically depends in practice on the chart speed and the response time of the recording potentiometer. It should be noted that the recording also gives direct information on the rate of reaction. The number of peaks in unit distance (in unit time) is proportional to the rate of reaction. The suitability of the developed gas-measuring device for the study of hydrogen peroxide decomposition is shown in Figure 3. For purposes of comparison. the experimental points obtained with a traditional gas buret are also reported. Received for review February 14, 1973. Accepted March 28,1973.
/-Ephedrine in Chloroform as a Solvent for Silver Diethyldithiocarbamate in the Determination of Arsenic John F. Kopp U.S. Environmental Protection Agency, National Environmental Research Center, Analytical Quality Control Laboratory, Cincinnati, Ohio 45268
The most widely used colorimetric technique for the separation and determination of arsenic in water samples is evolution as arsine from a hydrochloric acid solution followed by a spectrophotometric measurement using silver diethyldithiocarbamate (SDDC) as the color forming reagent ( I ) . Pyridine, customarily used as the solvent for SDDC, has a very disagreeable odor and is often objectionable to the analyst and his immediate associates. Recently, it has been reported that chloroform is a satisfactory solvent for the SDDC when an organic base is present. Bode and Hachmann ( 2 ) describe the use of l-ephedrine in chloroform as particularly suitable. This solution is less expensive and without disagreeable odor.
EXPERIMENTAL Apparatus. The arsine generator and absorber tube have been described previously ( I ) . I t has been observed, however, that a commercially available generator assembly (Fisher, Cat. KO.1405) with more uniform physical dimensions will provide an increase in precision. A Perkin-Elmer, double-beam spectrophotometer with 1-cm cells was used for the absorbance measurements. Reagents. The solvent for the SDDC was prepared by dissolving 0.41 g of 2-ephedrine (Aldrich Chemical Company, Cat. No. ( 1 ) "Standard Methods for the Examination of Water and Wastewater," 13th ed, 1971, p 62. (2) H. Bode and K . Hachmann, 2. A n a / . Chem., 224,261 (1967).
1786
13,491-0) in 200 ml of chloroform. Silver diethyldithiocarbamate, 0.625 g, was then added and the volume adjusted to 250 ml with additional chloroform. The reagent was filtered and stored in a brown bottle. A 1570 KI solution, a 40% SnC12.2HzO in concentrated HCI, and a 1070 Pb (CzH302)2-3HzO solution are also required. The stock arsenic solution was prepared by dissolving 1.320 g of arsenic trioxide, As203, in 10 ml of distilled water containing 4 g of NaOH. Approximately 100 ml of distilled water was added and the solution acidified with "03. The final volume was adjusted to 1000 ml with distilled water; 1.00 ml = 1.00 mg of As. An intermediate arsenic solution was prepared by diluting the stock solution 1:100 and a working arsenic solution was prepared by diluting the intermediate solution 1:10. Procedure. Because of the possible presence of organically bound arsenic, a digestion step must be included to ensure conversion of the arsenic to a n inorganic form. This is necessary as only inorganic arsenic compounds will form the hydride. Concentration of the sample and oxidation of organic matter is accornplished by evaporation with "03 and H2S04. To a suitable aliquot of sample containing from 1 to 10 fig As, add 7 ml of 1 : l HzS04 and 5 ml of concentrated "03 and evaporate to SO3 fumes. Cool, add about 25 ml of distilled water, and again evaporate to SOa fumes t o expel oxides of nitrogen. Maintain an excess of " 0 3 until the organic matter is destroyed. Do not allow the solution t o darken while organic matter is being destroyed because arsenic is likely to be lost. Transfer the sulfuric acid-sample concentrate to the generator, add 25 ml of distilled water and cool. Prepare the scrubber and absorber tube by impregnating the glass wool in the scrubber
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
Table I. Recovery of Arsenic from Demineralized and River Water Based on Seven Replicates of Each Demineralized River water,Q water, pg/l. r!3/l.
Arsenic added Arsenic recovered
Mean, 3 Range Stand dev Coeff of variation a All
57
100
57 57
57 65
86 97 91 95
52
100
55 56
57.0 52-65
92 88 92.7 86-1 00
f4.0
f4.8
7.0
Table II. Recovery of Arsenic in the Presence of Other Metals Metals AS added, added, Sample P!3/1, rdl. Distilled AI 1000; Cd 80; Cr 400; 266 water Cu 300; Fe 700; Pb 300; M n 500; Zn 300 Distilled AI 600; Cd 20; Cr 90; 67 water Cu 80; Fe 400;Pb 80; M n 100; Zn 70 Distilled AI 40; Cd 1; Cr 7; 29 water Cu 8; Fe 20; Pb 40; Mn 10; Zn 7
Ohio River Ohio River STP effluent STP effluent
5.2
values are corrected for a residual arsenic content of 6 p g / l
with lead acetate solution. Do not make too wet because water will be carried over into the reagent solution. Pipet 4.0 ml of SDDC reagent into the absorber tube. Add 5 ml of KI Solution, 4 drops of SnClz reagent, and 2-3 g of arsenic-free zinc (20 mesh) to the sulfuric acid-sample concentrate and immediately connect the absorber tube to the generator. Immerse the generator in a cold water bath to delay the reaction. Allow the evolution to proceed for 1.5 hr to make sure that all arsine is released. Pour the solution from the absorber tube directly into a 1-cm cell and measure the absorbance of the solution at 520 nm using the reagent blank as the reference. Treat aliquots of the working arsenic standard containing 0, 1.0, 2.0, 5.0, and 10.0 p g As in the same manner and plot absorbance us. concentration of arsenic in the standard.
RESULTS AND DISCUSSION Absorbance curves for both SDDC in pyridine and in 1-ephedrine-chloroform are shown in Figure 1 and indicate a maximum at 520 nm for l-ephedrine. Examination of these absorbance curves indicates that l-ephedrine gives more background material than pyridine and also that the band is not as well developed. Comparison of the standards using both solvents showed almost identical curves over the same concentration range. To evaluate the precision and accuracy of SDDC with I-ephedrine in chloroform, a series of seven replicate determinations were made a t the 57 pg/l. level in demineralized water and at the 100 pg/l. level in an Ohio River composite. Results are shown in Table I. It should be noted that the test in demineralized water was carried out a t 2 pg per 35 ml sample volume and that the standard deviation shown in the left hand column of Table I represents A0.14 pg of arsenic in the sample aliquot used for analyses. The reagent was also investigated on actual samples as well as in the presence of other metals. Results on a typical river water, a sewage sample, and distilled waters containing low to high concentrations of seven metals are shown in Table 11. On the basis of these results, Tables I and 11, it was concluded that I-ephedrine in chloroform gave an accuracy within the acceptable range. It should be pointed out that the same interferences are to be expected and the same precautions observed with 1-ephedrine in chloroform as with pyridine. Several metals such as chromium, cobalt, copper, mercury, molybdenum, nickel, platinum, and silver interfere in the generation of arsine; however, the concentrations of these metals normally present in water samples do not constitute sig-
.3 W
'4
1
None None None None
AS
found, PSIl.
None 100
None 100
250
66
31 6 110
