Colorimetric Determination of Cyanide in Stack Gas and Waste Water F. B. FISHER AND J. S. B R O W S Cnion Oil Co. of California, Brea, CaliJ. The work reported in this paper was developed as a result of a general dissatisfaction with the accuracy and reproducibility of several colorimetric procedures used for the control evaluation of cyanidein refinery waste effluents. A method, which is based on the reduction of sodium picrate by cyanide to form a colored product, is presented for the quantitative determination of cyanogen and hydrogen cyanide in refinery stack gases and of those cyanides in refiner) waste waters which are readily decomposed by strong acids. The determination is sensitive to 1 p.p.m.
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N A preliminary literature search for a suitable method sensitive to about 1 p.p.m., for the determination of small amounts
of cyanide in refinery waste effluents, several colorimetric methods were considered. The thiocyanate (5, 16) and Prussian blue (3, 4,16) methods, which are specific for cyanide and of ample sensitivity, were deemed unsatisfactory because of the instability of the developed color. The phenolphthalin (6) and o-cresolphthalin (8) methods are extremely sensitive, but appeared t o be difficult t o control, and erratic results may be caused by minute amounts of foreign oxidizing materials. .I review of several alkali picrate procedures (1, 3, 15-15) indicated that the picrate method would have several advantages over other colorimetric methods. The developed color appeared to be stable, the procedures seemed simple, the reagents were easily prepared and stable, and a sensitivity of 1 p.p.m. \vas olaimed. It was felt that these advantages warranted experimental investigation of the adaptability of the alkali picrate method to the specific problem. None of the picrate methods reviewed proved suitable to this problem; however, the work of Smith (16) and Sinclair (IS)provided an excellent background from which to develop a satisfactory procedure. Investigations have confirmed most of their conclusions regarding reaction rates, reagent concentrations, color stability, and the like, and have extended the knowledge of the importance of various pertinent variables. In general the alkali picrate procedures involve the distillation of hydrocyanic acid from the acidified sample into a sodium carbonate solution t o which excess picric acid is subsequently added. The alkali picrate is reduced by the cyanide t o form a colored reduction product which is compared with t'he color produced by standard cyanide solutions. Two major difficulties appeared during the investigation.
and accurate to within 2 7 of ~ the correct value; the developed color is stable and the deterniination is not affected bj the other substances normally encountered in refinerq wastes. The paper presents a comprehensite discussion of the variables affecting the picrate-cyanide reaction, such as reaction rate, reagent concentration, color stabilit?, and interfering substances. The presence of sulfide or sulfur dioxide does not interfere Mith the determination. 'The procedure has been used for more than a )ear for refiner? control.
hydrogen cyanide in stack gases and alkali cyanidea or other aciddecomposable cyanide salts in waste waters. APPARATUS
Spectrophotonieter. AIonochromatic, variable xave length. h Beckman llodel DU quartz spectrophotometer was used in this investigation. Modified Redemann micro-distillation apparatus (9). This apparatus is diagrammed in Figure 1. REAGENTS
Picric acid, 1% by weight in distilled water. (Analj tical grade picric acid as usually packaged contains 10% water to prevent accidental detonation. This water should be taken into account in weighing the wet acid.) Sodium carbonate, 1.0 A[, 0.5 A I , and 0.25 31. Standard Potassium C anide Solution. Reagent grade potassium cyanide is assayelby the silver nitrate method (11)and an amount of potassium cyanide equivalent to 1000 mg. of hydrocyanic acid is reighed into a 1-liter volumetric flask and filled 10mm
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1. It was difficult to make an accurate comparison of' the colored reaction product v i t h the blank because of the intense color of the yellow picrate solution. 2. Volatile reducing agents other than cyanide interfered with the determination.
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isha GROUND'JOINT
The first of these difficulties was overcome through the elimination of the picrate blank by incorporating a monochromatic variable-wave-length spectrophotometer. The volatile reducing agents common in refinery waste effluents were studied, and their interference in the determination of cyanides was effectively eliminated. The elimination of these difficulties and a careful investigation of the reaction conditions became the basis for a method, sensitive to I p.p.m. and accurate to within 2% of the true value, for the quantitative determination of cyanogen or
' STEAM GENERATOR\
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Figure 1. 1440
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Ilodified Redemann Microstill
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 to volume with distilled water. A 100-ml. aliquot of the above solution, diluted to exactly 1 liter, will have an equivalent hydrocyanic acid concentration of 100 p.p.m. (Throughout this papw parts per million is equivalent to milligrams per liter of solution.) Lead acetate, saturated aqueous solution. PREPARATIOX OF STANDARD CURVE
-1liquota containing 10, 20, 50, and 100 ml. of the s t a n d a d cyanide solution are pipetted into separate 100-ml. volumetric flasks and diluted t.o volume with distilled water. Five-milliliter aliquots of these solutions are pipetted into dry 100-ml. volumetric flasks. -4blank is prepared by pipetting 5 ml. of distilled water into a 100-ml. volumetric flask. To each of the fivr flasks is added 5 ml. of 0.5 sodium carbonate followed by 5 ml. of the 1'; picric acid solution, making the total volume 15 nil. Inimediately following the addition of the picric acid, the flasks are immersed in boiling water for 5 =k 0.5 minutes. Both the heating time and the 15-ml. reaction volume are important factors in obtaining reproducible results. .ifter removal from the hot water bath, the flasks are immediately filled t o volume with distilled water to stop any further reaction, and cooled to room temperature in tap water. h final adjustment of volume is made and the optical densities of thp solutions are read against distilled water on the spectrophotonietei' a t 520 mp using a 0.02-mm. slit, ultraviolet phototube, and 1-cni. cells. These readings should be made ivithin 30 hours alter color development. The optical density reading of the blank E S . distilled water should b r very small (less than 0.004) and need not be taken into accoilnt in the preparation of the standard curve. If the blank reacling is high, it may be necessary to use a slightly longer wave length, in order to diminish the picrate absorption to a neg1igit)lv amount, The rquivalent hydrocyanic acid concentrations in the othei, four volumetric flasks are 0.5, 1.0, 2.5, and 5.0 p.p.m., respectivel?-. The graph of optical density E S . equivalent hydrocyanic acid concentration which is then prepared should follow Beer's law. Figure 2 illustrates the graph obtained with the instrument used in this laboratory. The developed color is stable for at Iewt 30 hours.
1441 petted into a dry 100-ml. volumetric flask, 5 ml. of the 1% picric acid solution is added, and the color is developed and read in the same manner as prescribed for the preparation of the standard curve. The equivalent hydrocyanic acid concentration is read from the standard curve and calculated back to the original sample. Duplicate results should agree within 2% of the amount reported. If the optical density is greater than that covered by the standard curve, a smaller aliquot of the distillate is transferred to a 100ml. volumetric flask and sufficient 0.25 M sodium carbonate is added to bring the volume to 10 ml. The 5 ml. of picric acid is then added and the procedure carried out as before. 0700
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0.500
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0.200
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SAMPLE ANALYSIS
Stack Gas Condensates and Absorber Solutions. Whrri eatahlished sampling techniques (12) are used, the cyanogen or hydrogcn cyanide from the stack gas will be collected in both the water condensate of the gas and the caustic absorber solutions in the sampling train. If these solutions are t o be analyzed separat,ely, the condensate sample should be made alkaline upon receipt t o avoid cyanide loss. Prior to analysis, a portion of the sample is checked for the preseuce of sulfitee or sulfides by a qualitative test. If the sample i3 free of these interfering substances, the following procedure is used: The alkaline sample, of sufficient size to contain from 1.0 to 5 nig. of cyanide, is measured into the distillation flask (Figure 1j ; a few drops of methyl red indicator are added, and the flask is attached to the apparatus. The sintered-glass tip of the condenser tube is immersed in 25 ml. of 1 M sodium carbonate contained in a 100-nil. graduate. A low flame is applied to the diptillation flask, and when boiling has begun, the steam from the generator is allowed to pass through the system. When reflux has begun and back pressure has caused the carbonate Polution to rise slightly in the condenser tube, 2 A4 sulfuric acid is added alowly from the dropping funnel until the sample is distinctlJacidic to methyl red. If the sample contains a large amount of carl)onate or bicarbonate, large quantities of carbon dioxide will eacape from the system during the acidification procedure. The carbon dioxide evolution will cease when the methyl red changes color and again the rapid condensation of the steam will cause a back pressure in the system, causing the carbonate solution in the graduate t o rise slightly in the condenser tube. This hack pressure should be maintained throughout the remainder of the distillation. Little if any hydrogen cyanide is lost with the evolved carbon dioxide. When the distillate volume has reached about 75 ml., the receiving graduate is lowered and the distillation flask flame is increased to provide a thorough washdown, by the condensate, of the inside of the condenser tube. The outside of the tube is washed with distilled water and the distillation is stopped. The distillate is transferred to a 100-ml. volumetric flask and diluted to volume with distilled water. A 10-ml. aliquot is pi-
Figure 2. Optical Density cs. Equivalent Hydrogen Cyanide Concentration
Both sulfites and sulfides are volatile reducing agents and will distill from an acidic solution along with the hydrogen cyanide. If either is present, a modified procedure is required, owing to their deleterious effect on the color development. Sulfites may be oxidized to sulfates by the addition of a large excess of potassium chromate to the alkaline sample in the flask just before distillation. The presence of the chromate does not affect the distillation of hydrogen cyanide. When a qualitative test indicates sulfides to be present in the sample, the following procedure is used: The sample containing from 1 to .5 mg. of cyanide is transferred to the distillation flask and immersed in an ice bath. The solution is titrated t o a pH of 2 (PHydrion paper) with 4 S hydrochloric acid, and enough of the saturated lead acetate solution is added t o precipitate all of the sulfide present and to ensurr a 2-nil. excess, which buffers the solution a t a pH of 3 to 4. The flask is then stoppered and shaken to ensure the removal of anv hydrogen sulfide above the solution. The flask is attached to the distillation assembly and stram is passed into the cold solution. A flame is placed under the distillation flask and the distillation procedure is carried out as before, eliminating, of course, the addition of the 2 M sulfuric acid. .4 portion of the distillate should be tested for sulfide with a drop of the lead acetate solution. If the test is positive, either incomplete sulfide precipitation or too low a pH in the distilling flask solution is indicated, and the procedure should be repeated with a fresh sample, making appropriate corrections. Waste Waters. This method is applicable only to the determination of those cyanides in waste waters which are readily decomposed by strong acids. The procedure is the same as that for stack gas condensate waters, except t h a t larger samples are normally required to obtain the desired quantity of cyanide in the distillate. When 500 ml. of sample contains less than 1.0 mg. of equivalent hydrogen cyanide, the sample must be concen-
1442
ANALYTICAL CHEMISTRY
trated. A predistillation of the acidified sample into 25 ml. of 5% potassium hydroxide, collecting 50 ml. of distillate for each 500 ml. of sample, suffices. Alkaline cyanide solutions cannot be concentrated by boiling, because an appreciable amount of cyanide will be lost through alkaline hydrolysis. EXPERIMENTAL
Analytical. Known amounts of cyanide were added to numerous refinery effluent samples to check the accuracy of the method. Typical results are given in Table I.
Table I. Determination of Cyanide
Sample
Interfering Substances Present
Stack gas absorber Solution I Stack gas absorber Solution I1 Stack gas condensate I Stack gaa condensate I1 Waste water effluent I
Sulfites and COz Sulfites and COz Sulfites Sulfites Sulfides
Waste water effluent I1 T a p water
Sulfide None
a So
Equivalent H C S , P.P.M. Recovered Found in after sample .4dded addition 4.7 4.7
25.0 5.0
29.5 9.5
25.3 25.3
25.0
49.9 29.9
10.0 5.0 1.0
19.9 10.0 5.0 0.96 20.0
1.1
