timing be desired. 3) In changing the absorption solution, there is no longer any need to switch the toggle switch from the “titrate” to the “neutral” position; in the original instrument, the photo-cell would activate the titrant flow, resulting in flooding of the absorption vessel with the titrant, then requiring repeated rinsing. The addition described is easy, inexpensive, and allows many instruments still in use to be up-graded. The auto-
matic timing makes this spectrophotometric iodometric titration semi-automatic and requires adjustment of controls only a t the start of the determination. Received for review November 19, 1973. Accepted February 19, 1974. Mention of proprietary products is for identification only and does not constitute endorsement by the Department of Energy, Mines, and Resources.
Simple Hydrogen Sulfide Trap for the Gutzeit Arsenic Determination G. M. Crawford and Oscar Tavares El Paso City-County Health Department, El Paso, Texas 79907
The modified Gutzeit method ( I ) is probably the most common quantitative test for arsenic. This method presents several problems due to chemical interferences if the final reading is done colorimetrically. Hydrogen sulfide is most likely to cause interference. The usual method for removal of hydrogen sulfide is to pass the arsine gas mixture through a plug of glass wool that has been impregnated with lead acetate. This report describes a simple, more effective method of removing the hydrogen sulfide. The glass wool plug is replaced by a glass fiber disk impregnated with lead acetate. The arsenic procedure utilized by the authors involves the evolution of arsine from a solution that is also used for analysis of metals by atomic absorption. After treatment of the solution with concentrated hydrochloric acid, potassium iodide, and stannous chloride and the addition of zinc metal, then the arsine is bubbled through a pyridine solution of silver diethyldithiocarbamate. The resulting red complex is read on a colorimeter at 560 mu. The glass wool plug is ineffective in the complete removal of hydrogen sulfide. The actual preparation of the generators is quite troublesome just from the aspect of handling glass wool. A great deal of time is used to replace the glass wool after each sample run. The modification which is proposed involves the use of Gelman type A glass fiber filters, which are cut into disks using the metal die (3.75-cm diameter) made by our machine shop. These are then soaked in a solution of lead acetate (10%) for 3 to 4 hours and allowed to dry. A Millipore Field Monitor holds the disk in the generator assembly (see Figure l for dimensions of the apparatus). Tygon tubing has been found to be the best method of connecting the generator, field monitor, and bubbling tube. The results of a comparative study between the disk and the glass wool plug are presented in Table I. As can be seen, the results obtained with the disk were closer to the known concentrations of the standards than were the results using the glass plug. The large differences are due to the interferences of the unremoved hydrogen sulfide in the arsine. Upon completion of the reaction, the glass wool was still nearly white, whereas the disk exhibited the characteristic black color of lead sulfide. This modification helps simplify the method. With this modification and that proposed by Kopp ( 2 ) - i.e., replacement of the pyridine with a chloroform solution of 1-ephedrine-the Gutzeit arsenic determination should (1) V. Vask and Sedivee, Chem. Listy, 46, 341 (1952) (2) J. F. Kopp. Anal Chem., 45,1786 (1973).
MILL
Figure 1. Sulfidetrap for arsenic determination
Table I. Results of Comparison of Disk a n d Glass Wool Plug, 1 ppm S*- Added t o Each Sample ppm As as analyzed
pprn As standard
Disk
Plug
0 5 10 15 20 25
0.0 4.9 9.4 16.0 21.2 24.4
0.1 4.9 7.5 13.0 28.3 20.6
prove to be more reliable, as well as less disagreeable. Received for review October 24, 1973. Accepted January 22, 1974. ANALYTICAL CHEMISTRY, VOL. 46, NO, 8, JULY 1974
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