Determination of Micro Quantities of Cyanide in Presence of a large Excess of Sulfide M A U R I C E 0.BAKER, R I C H A R D A. FOSTER, B E N G. POST, and T. ALDON HlETT Houston Refinery Research Laboratory, Shell O i l Co., Houston, Tex.
Benzidine-Pyridine Reagent. Dissolve 18 ml. of pyridine in 12 ml. of water and add 3 ml. of concentrated hydrochloric acid. Add 10 ml. of benzidine eolution and shake until any precipitate dissolves. This solution must be prepared daily. Sulfide Solution. Dissolve 0.75 gram of sodium sulfide (Ka& 9H20) in water and dilute to 100 ml. This solution contains 1 mg. per ml. of sulfide.
The Aldridge procedure for the determination of cyanide has been modified to minimize interference from the sulfide ion and thus to eliminate the necessity for its prior removal from the sample. Sulfide is oxidized to sulfuric acid with bromine at the same time that the cyanide is brominated to form cyanogen bromide. Cyanogen bromide then reacts with a pyridine-benzidine mixture to form a colored compound which may be measured colorimetrically. This method is sensitive to less than 0.5 y of cyanide in the presence of 2500 y of sulfide and is accurate to =t5% at a concentration of 10 yof cyanide. This procedure has been used for the analysis of refinery dry gases and separator water from the distilling units.
PROCEDURE
Calibration. Pipet into 25-ml. volumetric flasks, containing 1 ml. of 1%by weight of sodium hydroxide, appropriate volumes of standard cyanide solution to give concentrations of 0, 1, 3, 5,7, and 10 y of cyanide, and add 2.5 ml. of sulfide solution to each flask. Add a small (3 X 3 mm.) piece of Alkacid paper to each flask, acidify with glacial acetic acid, then add 0.5 ml. in excess. Immediately after acidification, add 2 ml. of saturated bromine water, and swirl the flask to mix thoroughly. Let the so!utions stand with occasional shaking for 10 minutes. If a precipitate of elemental sulfur remains or if the bromine is consumed, add bromine water in 0.2-ml. increments until an excess is present as shown by the color. When the solutions clear, add arsenious acid solution dropwise and gently swirl until the mixtures are free from bromine, then add 0.2 ml. in excess. Add 4 ml. of pyridine-benzidine reagent, and swirl to mix thoroughly. Wait 30 seconds, then add 5 ml. of ethyl alcohol, and dilute to volume with water.
T
HE determination of micro quantities of cyanide in the presence of a large excess of sulfide in refinery dry gases and in certain aqueous streams has been handicapped by the lack of a sensitive procedure that does not require sulfide removal. Since casutic solutions from scrubbing of dry gas and separator water from distillation columns may contain several thousand times as much sulfide as cyanide, there is a definite risk of losing the cyanide if prior sulfide removal is necessary. Several sensitive procedures have been developed (3, 6) for cyanide determination for similar applications, but sulfide must first be removed. The pyridine-benzidine procedure of Aldridge (1) and the pyridine-pyrazolone procedure of Epstein ( 5 ) are sufficiently sensitive, and it appeared that sulfide would not seriously interfere with either. Both methods have been successfully applied to the analysis of sewage and industrial wastes (4, 7-9), where the sulfide content was apparently very low. Because of the availability of reagents, the Aldridge procedure was chosen for this work. However, when applied to cyanide solutions containing as little as 5 y of sulfide, results were 8 to 10% low, and more serious interference was encountered a t high sulfide concentrations. This procedure was therefore modified to minimize the interference from sulfide and thus to eliminate the necessity for its removal. Aldridge ( I , 2 ) brominates cyanide and thiocyanate in neutral or acid solution to form cyanogen bromide. This reacts with a mixture of pyridine and benzidine to form a red colored compound which may be measured colorimetrically. Cyanide can be distinguished from, and determined in the presence of, thiocyanate oviing to the difference in volatility of the two acids. Hydrogen cyanide is removed by aeration of an acidified aliquot of the solution, and the difference before and after aeration gives the cyanide content.
1.0
-
0.6
A-
CN-
B-
CN-+2500pqS'
0 w
f
0.6
a
' 53
0.4 BECKMAN "e" INSTRUMENT: CELL LENGTH! I ccm m WAVELENGTH: WAVELENGTH I 5 3 0 m p
0.2
0 0
2
4
6 MICROGRAMS
I
I
8
IO
12
CYANIDE
Figure 1. Calibration with and without added sulfide
After 15 minutes measure the absorbance of each solution a t 530 mp. The red color formed is stable for approximately 30 minutes. Prepare a calibration curve of concentration versus absorbance. A new calibration curve should be made for each batch of reagents. Typical calibration curves with and without added sulfide are shown in Figure 1. Unknown. Determine the sulfide concentration of the unknown by an appropriate method such as electrometric titration (10). If the sulfide content is above 2500 per milliliter, take an aliquot for analysis in which the sulfide IS below this amount. If the sulfide is lees than 50 p.p.m., add sufficient sulfide solution to the volumetric flask to give 100 to 2500 y of sulfide in the reaction mixture. Failure to add this sulfide will cause a high cyanide value. Pipet 1 to 5 ml. of the aqueous solution or aliquot into a 25-ml. volumetric flask and proceed a8 described above. Measure the absorbance a t 530 mp against a reagent blank containing approximately the same sulfide concentration as the sample tested. Determine the cyanide concentration by reference to the calibration curve.
APPARATUS
Spectrophotometer, Beckman Model B or DU or equivalent, or colorimeter equipped with green filter such as Corning 401. REAGENTS
The reagents used in these experiments were the same as those described by Aldridge ( 2 ) , except for the benzidine solution and the benzidine-pyridine reagent. The benzidine was not found to be soluble in dilute hydrochloric acid to the extent indicated by Aldridge. Baker and Adamson's reagent pyridine gave a clear color without being redistilled. Benzidine Solution. .4dd 0.5 gram of benzidine to 50 ml. of 0.5.V hydrochloric acid. Heat t o boiling, cool, and filter the solution. Store the solution in a dark botfle. 448
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 Table I.
Effect of -4dded Sulfide on Absorbance 200
S-- Added,
CS-, 4
y
S - - in Blank, y 1000
2500
Absorbance
0.335 0.350 0.370
200 1000 2500
4 4
449
0,320
0.330 0.350
0.300 0.310 0.330 __
EXPERIMENTAL
Determination of Wave Length of Maximum Absorption. The color was developed in a solution containing 10 y of cyanide. The absorption spectrum was determined between 380 and io0 mp with a Beckman Model B spectrophotometer. X o sharp peaks were found, but the masimum absorbance occurred a t 530 nib. All subsequent measurements were made a t this wave length. Optimum Concentration Range. Since varying amounts of bromine water must be added, depending on the amount of sulfide present, it is necessary to dilute the reaction mixture to a definite final volume before absorbance measurement. When 30 y of cyanide in 25 ml. of final solution was exceeded, a red precipitate formed. This was prevented by the addition of 5 nil. of ethyl alcohol immediately after the addition of the benzidinepyridine reagent. K i t h a Beckman Model B spectrophotometer using a 1-cm. cell, the absorbance approaches 1 when the color is developed from 10 y of cyanide in 25 ml. of solution. -4s reasonably accurate measurements can be made up to an absorbance of 2.5 with this instrument, about 25 y of cyanide in 25 ml. of final solution is the maximum amount which can be determined without dilution. However, 10 y in 25 ml. is probably the upper limit for most colorimeters unless smaller cells are used. Application to Thiocyanates. Solutions of ammonium thiocyanate were substituted for potassium cyanide and the calibration was repeated. Calculation of the observed data into terms of equivalent cyanide concentrations gave a series of points which were on the previous calibration curve. Accordingly, the method is equally applicable to either ion if the concentration levels are properly adjusted. Effect of Sulfides. Solutions containing various concentrations of cyanide (0 to 10 y) and sulfide (0 to 2500 y ) mere analyzed by the Aldridge procedure, except that the solutions were diluted to 25 ml. before measurement of absorbance. The results are shown in Figure 2. Even 5 y of sulfide caused a significant error, while larger amounts of sulfide caused up to 40% decrease in absorbance. A t the higher sulfide concentrations much of the sulfide [vas not
A
I. O F
-
1
I O p p CN-
W V
oxidized beyond the elemental state when the bromine color disappeared. Therefore, a series of experiments was undertaken in which an excess amount of bromine required t o oxidize the sulfide to sulfuric acid was added in addition to the 0.2 ml. required for the bromination of the cyanide present. These tests were made on solutions containing 4 */ of cyanide and from 200 to 2500 y of sulfide in a final volume of 25 ml. After standing xith occasional shaking until all of the sulfur was dissolved, the color u as developed as described above. When measured against reagent blanks containing the same concentration of sulfide, a nearly constant absorbance was obtained for a given cyanide concentration. Therefore, if the same amount of sulfide is added to the reagent blank as is present in the sample to be tested, a single calibration curve may be used. The results of these experiments are shown in Table I. A similar experiment on a cyanide solution containing 5000 y of sulfide was unsuccessful, as no definite red color developed. It appears, therefore, that the upper tolerance for sulfide under these conditions is between 2500 and 5000 y. Applications. Synthetic samples containing varying amounts of +fide and cyanide were analyzed by this modified procedure. These results are presented in Table 11. Cyanide has been determined in waste water which is separated from various refinery streams. Several examples of these determinations are presented in Table 111.
Table 11. Cyanide Determination in Synthetic Samples C S - - Added,
y
S - - Added,
2.0 3.0 2.0 10.0
Table 111.
C N - - Found,
y
y
0.3 0.6 0 . 9 8 , 1.04 2.00, 2 . 0 8 2.90,2.99 5.00.5.11 10.3, 0 . 9 5
1000 1000 1000 1000 2500 2500 2.500
0.4 0.6 1 .o
Cyanide Determination in Water Separated from Various Distillates
Source of Water Cat. cracker fractionator accumulator Propane, propylene absorber Straight-run naphtha accumulator Pressure distillate tops
Sulfide, P . P. hl.
Cyanide, P.P.hI.
2500
5.4 23.0
400 0.0
270
0.7 1.2
CONCLUSIOlVS
The presence of sulfide inhibits the color development in t h e original procedure. At high sulfide concentration this may be due in part to competition of the sulfide for the available bromine. However, as shown in Figure 2, very small amounts of sulfide cause a significant drop in absorbance, although an excem of bromine may still be present. The modified procedure minimizes this interference and has been successfully applied to the analysis of synthetic and contaminated water samples. Less than 0.5 y of cyanide can be detected in the presence of 1000 y of sulfide. At the 10-y level, this method is repeatable to f 5% of the cyanide present. LITERATURE CITED
0.6
a
-=
0.2.-
0 0
Figure 2.
50
7
c
100 MICROGRAMS
I
150
SULFIDE
Effect of sulfide on absorbance with fixed bromine addition
(1) Aldridge, W.N., A n a l y s t , 69,262-5 (1944). (2) I b i d . , 70, 474-5 (1945). (3) Brooke, ll.,A I ~ A LCHEY., . 24, 583-4 (1952). (4) Eden, G. E., Hampson, B. L., and Wheatland, A. B., J. SOC. Ckem. I n d . ( L o n d o n ) , 69, 244-9 (1950). (5) Epstein, J., ANAL. CHEY.,19, 272-4 (1947). (6) Fisher, F. B., and Brown, J. S., I b i d . , 24, 1440;4 (1952). (7) Kruse, J. 11.;and llellon, 11.G., Ibid., 25, 446-50 (1953). (8) Kusbaum, I., and Skupeko, P., M e t a l F i n i s h i n g , 49, KO,10, 61-3 (1951). (9) Kusbaum, I., and Skupeko, P., Sewaoe a n d I n d . Wastes, 23, 875-9 (1951). (10) Tamele, 11. W.,Ryland, L. B., and Irvine, V. C., IND. ENQ. CHEM.,ANAL.ED.,13, 618-22 (1941).
RECEIVED for review Decembe?
31, 1953.
Accepted October 22, 1954.
