Determination of Traces of Hydrogen Sulfide in Sewer Gases A Convenient Field Method W. J. WILEY, Government Chemical Laboratory, Brisbane, Australia
I
from stout tubing circled the aspirator bottle and served the dual purpose of holding the absorption tube and acting as a cushion or the bottle in its case. This case was three-sided and stoutly constructed of wood, so as to be a neat fit for the bottle. The front was left open except for a small ledge to prevent the bottle sliding forward. The si hon tube ended with a length of rubber tubing which slipped &rough a hole in the base of the case. The height of the case was just sufficient to enable the aspirator to be lifted over the ledge for removal for refilling with water. Extensions t o two sides of the case enabled it to be placed on the ground without interfering with the action of the siphon. Thirty-three meters (100 feet) of plaited cord were attached to the case so that it could be lowered down a manhole. A case similar to that used for some types of portable pH colorimetric apparatus was used to hold the comparison tubes, a bottle of 5 per cent potassium hydroxide and one of potassium plumbate, and a stock of absorption tubes, as these were used for only one determination before being washed
;"; THE course of a recent investigation
of a sewage system it became necessary to make a large number of determinations of the concentration of hydrogen sulfide in the sewer gases. As the sewer was about 20 meters (60 feet) below ground level and determinations a t numerous different places were to be made on the same day, it was desirable that an apparatus be devised that would obviate the necessity of the manipulator's descending the manholes and would be strong and rapid in use. The concentrations of hydrogen sulfide to be measured varied from about 0.1 to 30 parts per million. A high degree of accuracy in t h e measurem e n t s was u n necessary. The apparatus and method described were found t o cover all the req u i r e m e n t s , and when used in the field gave results with an accuracy of a t l e a s t 1 0 per c e n t of t h e q u a n t i t y measured. The principle of t h e m e t h o d employed was the absorption of the hydrogen sulfide in potassium hydroxide and its colorimetric determination, using potasFIGURE 1 sium plumbate and a series of comparison tubes containing dye solutions prepared so as to correspond to various concentrations of the gas.
Standardization Owing to the rapidity with which the lead sulfide coloration fades, resort was had to more permanent color standards prepared from dye solutions. No single dye having the same tint as the lead sulfide was found, but a good match could readily be obtained by using a mixture of red, yellow, and blue dyes. Those selected were Ponceau 3 R, Chlorozol fast yellow, and Cotton blue (B. D. H.). No special virtue is claimed for these, other than that they were found reasonably stable to light and slight changes of pH in the solution and happened to be readily available. It was found that 30 drops of 0.1 per cent Chlorozol yellow, 5 drops of 0.1 per cent Ponceau 3 R, and 8 drops of 0.1 per cent Cotton blue diluted to 100 cc. gave a coloration, when compared in the absorption tubes of the apparatus, of similar tint and intensity to a solution containing 14 X 10-6 grams of hydrogen sulfide dissolved in potassium hydroxide and treated with 5 drops of potassium plumbate. At 25" C. and barometric pressure this would correspond to 10 parts per million of hydrogen sulfide if 1 liter of sewer gas had been aspirated through the absorption tube. In preparing the standards 25" C. was taken as typical of working conditions and no attempts were made to correct for changes of temperature and pressure. A series of standards was prepared in uniform tubes identical with that used for absorption of the hydrogen sulfide and of intensities corresponding to 0, 0.5, 1, 2, 4, 6, 8, 10, 12, 16, and 20 parts per million of hydrogen sulfide in the gas if 1 liter were aspirated. The color corresponding t o 0.5 part per million when the tube was examined longitudinally against a white base was quite definite. In preparing these standards comparison was made with sodium sulfide solutions carefully analyzed by titration with iodine and thiosulfate and added to 5 per cent potassium hydroxide, the color being developed with 5 drops of potassium plumbate in an identical manner to that used in an actual determination. The color of a dye standard was measured with a Lovibond tintometer and it was found that after one month in the diffused light of the laboratory it had not changed. However, the standards used were exposed to light only when a comparison was being made.
Apparatus The apparatus (Figure 1) consisted of a graduated 4-liter Winchester bottle which with a si hon acted as an aspirator. This drew the gases through an agsorption tube consisting of a 15 X 1.3 om. test tube with a mark etched at a height corresponding to 10 cc. This was filled to the mark with 5 per cent potassium hydroxide solution. The bubbling tube terminated in a bulb having three small orifices, so that the gas divided into three fine streams of bubbles. A glass tap regulated the rate of flow of gas through the absorption tube. A rubber band made 202
. . ANALYTICAL EDlTION
MAY 15, 1935
203
In order to be sure that the simple absorption tube used was effective, mixtures containing 5 and 15 parts per million of hyd:rogen sulfide in air were prepared in an apparatus similar to that described by Truesdale (1). These were aspirated at the rate of 1 liter in 10 minutes through the absorption tube. On adding potassium plumbate, the color developed was immediately matched with the previously prepared dye standards and found to correspond with a practically complete absorption of the hydrogen sulfide.
its contents, and the color was immediately compared with the dye standards. The total time involved, including packing and unpacking, was generally less than 15 minutes a t any particular place. When the concentration of hydrogen sulfide was lower or higher than corresponded to the standards, a larger or smaller quantity of the gas was aspirated through the apparatus.
Determination
The author’s thanks are due to J. B. Henderson, Queensland Government Analyst, for permission to publish this description.
The determination on the field was simple and rapid. The siphon was started and the sampling apparatus immediately lowered down the manhole to within 30 cm. (1 foot) of the sewa’ge’ ilfterlo minutes’ when liter Of the gas had been aspirated, it was brought to the surface, the absorption tube was removed, 5 drops of potassium plumbate were added to
Acknowledgment
Literature Cited (1) Truesdale, E. C., IND.ENG.CHEN.,Anal. Ed., 2,299 (1930). R E C B I V ~i\laroh D 15, 1935.
A Direct-Reading pH Meter for Glass, Quinhy drone, and Hydrogen Electrodes ALLAN HEMINGWAY, University of Minnesota Medical School, Minneapolis, Minn.
T”
ass electrode is rapidly replacing the hydrogen and quinhydrone electrodes. It reaches equilibrium very g1 rapidly, there being no drift due to “poisoning.” It is not affected by oxidizing-reducing substances, and is applvcable through a wide p H range of 0 to 11.5. There is no removal of dissolved gases, such as carbon dioxide, due to the bubbling of hydrogen through the solution whose p H is to be measured. One difficulty with the glass electrode has been that the high resistant membrane makes balancing of the opposing e. m. f . from a potentiometer a difficult operation, since the high resistance of the glass reduces the balancing current to a small value. Another difficulty is that when a continuous current is drawn from a glass-electrode system the membrane polarizes, changing its equilibrium potential. In order to eliminate these difficulties several schemes (1) have been suggested. One which possesses certain inherent advantages is the method, first suggested for glass electrodes by Morton (a), in which the variable known e. m. f . from a potentiometer, the glass-electrode assembly, and a highgrade condenser are in series, the potentiometer e. m. f . opposing the cell e. m. f. in the usual way. The condenser is charged to the difference of potential between the potentiometer and the glass electrode. By means of a suitable tapping key the condenser is discharged through a voltage amplifier with a ballistic galvanometer in the plate circuit of the last tube. When the voltage of the glass-electrode assembly is balanced by an equal e. m. f. from the potentiometer, the ballistic discharge is zero. A small residual e. m. f., due to grid current, can be made negligibly small by a suitable choice of tube, grid resistance, and grid condenser (3) Improvements of this new circuit over one previously described (I) consist in the use of alternating current vacuum tubes whereby storage batteries are eliminated. Vacuum tubes of higher amplification are used, with the result that an inexpensive portable microammeter replaces the type R galvanometer. The temperature-correction device, whereby the potentiometer is converted into a p H meter for any temperature, and the accessory potentiometer as well as the con-
nections for quinhydrone and hydrogen have necessitated complete redesigning and revising of the circuit.
Construction of Meter The ballistic principle has been utilized in a direct-reading pH meter designed for rapid and simplified measurements (Figure 1). The upper circuit is a b e d s & Northrup potentiometer of the student type. The variable voltage is obtained from a variable decade of nine 40-ohm coils and a &ohm slide wire. When balanced as a simple potentiometer with the standard cell, there is a 100-millivolt potential drop across each 40 ohms. The wiring of the potentiometer was changed to the modified form as shown. The glass electrode assembly, represented diagrammatically, contains a calomel half-cell attached to the main potentiometer. A salt bridge connects to a vessel which contains the standard buffer or the solution whose pH is to be measured. The inside of the glass membrane contains a quinhydrone half-cell which makes contact with a highly insulated tapping key. An accesa supplies a variable voltsory potentiometer containing switch S age by means of a coarse (1000-ohm) and a fine (100-ohm) adjustment. A microammeter (zero center) with a suitable shunt can be thrown to three different positions, Q, X,and G, by double pole triple-throw switch &. The lower circuit is a voltage amplifier system using alternating current tubes for filament supply. It was found advisable to use a dry-cell B-battery supply for steadiest conditions. The resistors and condensers should be of good grade material (Shallcross resistors are used) and it is recommended that the 0.1-microfarad condenser in the grid circuit of the first tube be mica t o eliminate absorbed charge. The resistance values given are those for an older type of potentiometer. If another type of potentiometer is used, the shunt and series resistances can be calculated from the formulas Series resistance = R(l
- RT) KT
ParallelIresistance = 1 Y Z R where R is the total resistance of coils and slide wire from which the variable voltage is obtained. K Tis the voltage per
