Determination of l o w Hydrogen Cyanide in Acrylonitrile ROBERT L. MAUTE and M. L. OWENS,
JR.
Monranto Chemical Co., Texas City, Tex.
The need for a precise but simple method for the determination of free and combined hydrogen cyanide in the range of 0 to 100 p.p.m. in acrylonitrile led to a study of four different methods. Simple potentiometric procedures give sufficient precision and accuracy for the routine determination of either free hydrogen cyanide or total (free plus that combined as lactonitrile) hydrogen cyanide. Where a higher precision is required the phenolphthalin colorimetric procedure has been found more satisfactory. The visual end point of the LiebigDenighs titration method for total cyanide is useful for concentrations down to about 20 p.p,m., but is not recommended for routine application when lower concentrations are to be encountered.
I
S T H E course of investigations of systems containing acrylonitrile, hydrogen cyanide, and lactonitrile, it became necessary to apply routinely analytical procedures for the precise and accurate determination of low concentrations of hydrogen cyanide, both as free cyanide and as total (free hydrogen cyanide plus that combined as lactonitrile) cyanide. The literature contains several excellent reviews of cyanide methods, but generally these have appeared in specialized, not readily available journals. Moreover, the procedures are usually for application to plating baths, sewage samples, and other aqueous samples. Investigations in these and Monsanto's Dayton Laboratories resulted in four different routinely applicable procedures for the rapid, accurate determination of 0 to 100 p.p.m. of cyanide in acrylonitrile. The first of these is a colorimetric procedure involving phenolphthalin and is recommended where an accuracy of zt0.5 p.p.m. (in the 0 to 10 p.p.m. range) of total cyanide present is required. The Denighs titration to a visual end point with silver ion is the most rapid procedure and is applicable where concentrations of total cyanide greater than about 20 p.p.m. are expected. For slightly better accuracy and precision, and especially below 20 p.p.m., the potentiometric titration for total cyanide is satisfactory. \Then free cyanide is desired, the fourth procedure, potentiometric titration in acidic medium, results in precise and accurate values for free hydrogen cyanide without interference from hydrogen cyanide combined as lactonitrile.
,Imnionia-pota.bium iodide indicator, 30 grains of potassium iodide dissolved in 200 ml. of concentrated ammonium hydroxide. I'qr 2 ml. for a 100-ml. sample volume. Methanol, C.P. Thymolphthalein indicator solution, 0.1 xeight 70aqueous Trisodium phosphate, saturated solution. Sodium iodide solution, standard 0.025S. Thymol blue indicator solution, O.l70 aqueous. METHODS AND PROCEDURES
Phenolphthalin Method for Total Cyanide. One of the most qensitive and Lvidely used tests for hydrogen CJ nnidtl is the phenolphthalin method ( 6 , 14). This method has been used by various workers to detect and determine concentrations of hydrogen c j anide in air and aqueous solutions ( 2 , S, 4 , 1 2 , 1 7 ) . Kolthoff ( 4 ) reports that 0.15 mg. of hydrogen cyanide prr litcr can be detected by visual comparison with known standards. -4s little as 0.02 p.p,m. of hydrogen cyanide in air can be detected by this method. Rohde and Swope (IS) reported the use of the phenolphthalin method for the determination of cyanides in u aste water by colorimetric measurement using a photometer and green filter. They also determined free and uncomplexed cyanide and total cyanide in plating wastes or similar solutions. Nicholson (11) modified the phenolphthalin method by use of o-cresolphthalin and addition of a stabilizer after color development. He found that the color produced by c-cresolphthalin was more sensitive and stable than that from phenolphthalin. These advantages, however, are offset by the fact that the phenolphthalin (the reduced form of phenolphthalein) is available commercially while the o-cresolphthalin is not. The phenolphthalin method is based on the quantitative oxidation of phenolphthalin to phenolphthalein by cupric ion in the presence of cyanide. The color developed in an alkaline medium is a measure of the total cyanide content. This method is sensitive to various oxidizing agents, and the various agents reported to interfere are: free halogens, ferricyanide, hydrogen sulfide, sodium and potassium perchlorate, and sodium hypochlorite. Ferrocyanides, chromates, nitric acid, ferric chloride, and halogen salts are reported not to interfere. Experience during this work indicated that peroxides are the most likely interferences when present in concentrations greater than 5 p.p.m. S o interferences were found in any of the commercial scrylonitrile used.
PROCEDURE. .411 reagents are prepared with oxygen-free water.
APPARATUS
Beckman (H2) pH meter with a silver indicator electrode and a mercury-mercurous sulfate-potassium sulfate reference electrode. Xagnetic stirrer. Beckman Model B spectrophotometer with 50-mm. cells. Stopwatch. REAGENTS
Cupric sulfate solution, 0.02 \%-eight% aqueous. Phosphate buffer solution, 2.14 grams of anhydrous disodium hydrogen phosphate diluted to 1 liter. Sodium hydroxide solution, 0.1N. Phenolphthalin stock solution, 0.05 gram of phenolphthalin dissolved in 100 ml. of ethyl alcohol. Keep refrigerated. Phenolpht,halin reagent solution, 3 ml. of phenolphthalin stock solution diluted to 100 ml. with 0.02% cupric sulfate solution. Prepare fresh daily. Stabilizer solution, 5.0 grams of anhydrous sodium sulfite and 0.224 gram of triethanolamine hydrochloride dissolved in 100 ml. of d i d l e d water. Renew every 5 days. Silver nitrate solution, standard 0.01 or 0.025N.
;1blank, omitting acrylonitrile, is carried through the procedure.
Place approximately 25 mi. of oxygen-free distilled Ivater in a 5 0 - ~ 1 .glass stoppered volumetric flask. Add 10 of phosphate buffer, 2 ml. of 0.1N sodium hydroxide, and 5 nil. of phenolphthalin reagent. Add 2 ml. of sample (2 ml. of sample represents the amount of acrylonitrile rapidly soluble in the total volume) and shake. Start timing upon the addition of thc sample, and exactly 5 minutes after sample addition add 2 ml. of stabilizer. Time carefully. Dilute to 50 ml. with distilled water and immediately read the absorbance with a Beckman B or similar spectrophotometer a t 525 mp using 50-mm. cells. The color should be read within 5 minutes after addition of the stabilizing reagent. Convert the reading to hydrogen cyanide concentration by use of a predetermined calibration chart. The calibration curve was prepared following the given procedure using assayed, aqueous sodium cyanide solutions diluted with cyanide-free acrylonitrile. Dry sodium cyanide (Baker and Adamson) was used throughout all this work. Assay by Liebig-Denigbs macrotitration and by a gravimetric method agreed within 1% of the manufacturer's assay. Dilute solutions (1 to 2%) of cyanide n-ere prepared, and further diluted with volumetric glassxare (uncalibrated) to prepare known solutions.
1723
1724
ANALYTICAL CHEMISTRY Phenolphthalin Method for HCN Determination
Table I.
H C N Known,P.P.M.
HCN Found, P.P.M. 1, 1 1, 1, 1 4, 4, 4 5 5
1 1
4
5 6 7
.
7. .. 7
ii
10,10,10 21 38 38 51, 52 60
21 36 39 50 61
0
PO
40
HCN CONCENTRATION,
60
P.P.M.
Figure 1. Calibration Curve for Cyanide in Acrylonitrile
Hydrogen
Beckman B spectrophotometer, 50-mm . cell, 525 mp
The calibration curve for hydrogen cyanide is given in Figure 1.
If the concentration of hydrogen cyanide in acrylonitrile is greater than 60 p.p.m., a diluted sample may be used. The curve deviates slightly from Beer’s law. The procedure was carried out a t room temperature and the diurnal change in temperature had little effect on the results. Working samples of standard cyanide solutions were prepared by using cyanide-free acrylonitrile plus lactonitrile or liquid hydrogen cyanide. Results of known cyanide solutions are given in Table I. Several hundred samples of known and unknown cyanide content were analyzed by the phenolphthalin method in order to determine the precision of the method. The method was found to be reproducible to *0.5 p.p.m. from 0 to 10 p.p.m. of hydrogen cyanide and A 2 p.p.m. in the range 15 to 50 p . p m of hydrogen cyanide. Titration Method for Total Cyanide. Alternate methods for the determination of cyanide were also desired. The LiebigDenigh titration appeared to have good possibilities. Denigbs in 1893 introduced the use of potassium iodide as an indicator in cyanide titration, and the method has been widely used since that time. However, the importance of the proper concentration of ammonia has not been properly stressed in the literature (9). With too little ammonia the end point appears too soon, and with a large excess of ammonia the end point appears too late. DenigPs used 1 to 2 ml. of 6 N ammonia and 0.2 gram of potassium iodide for 100 ml. of solution. Kolthoff (9) obtained better results by use of 5 to 6 ml. of 6N ammonia. Thompson and Wick (16, 18) investigated and concluded that the Liebig-DenigPs titration was accurate to better than 0.2%
relative. However, they were working with a high cyanide concentration. Saredo (15) studied all the classic methods for cyanide determination and concluded that the DenigPs method was preferred. Therefore, the method was investigated and extended into the 0 to 100 p.p.m. range. PROCEDURE. To 25 ml. of distilled mater, 2 ml. of ammoniapotassium iodide indicator, 10 ml. of 0.1.4- sodium hydroxide, and 40 ml. of methanol, add 25 ml. of acrylonitrile containing cyanide. Titrate to a faint opalesence against a dull black background using 0.01 or 0.025-Vsilver nitrate. Run a blank omitting the sample.
9 silver nitrate solution as dilute as O.OO5N was used for this titration; however, a t this concentration the end point is not sharp and is somewhat difficult to see. The omission of sodium hydroxide did not have any noticeable effect in the range studied. The end point for samples containing 0 to 20 p.p.m. of cyanide is difficult to see; however, with extreme care, good results can be obtained. Various dilutions of weighed sodium cyanide (96% assay) were analyzed by the phenolphthalin and Denigks titration, and the precision and accuracy of the two methods are compared in Table 11. The methods agree with each other and with the known values within 10% relative at 0 to 70 p.p.m. Potentiometric Method for Total Cyanide. A third method investigated was the potentiometric titration of cyanides. Muller and Lauterbach (IO),in a study of the potentiometric titration of cyanide, concluded that the first break in potential occurs 0.5 to 1 % too early or the second break too late by the same percentage. S o discussion of this error was given. Silver and calomel electrodes were used. Runge et al. ( 1 4 ) of Monsanto’s Dayton Research Laboratories determined total cyanide by the potentiometric titration of cyanide with silver nitrate in a trisodium phosphate buffer solution. A silver indicator electrode and a mercury-mercurous sulfate-potassium sulfate reference electrode were employed. The method was used for higher concentrations of cyanides than considered here. However, using dilute reagents and a larger sample, the procedure gave good precision and accuracy. PROCEDURE. Into a 400-ml. beaker containing 75 ml. of distilled water, 10 ml. of saturated trisodium phosphate, 50 ml. of methanol and 5 drops of thymolphthalein indicator, put 50 ml. of acrylonitrile. Titrate potentiometrically, while stirring, with 0.01 or 0.0252v silver nitrate using a Beckman (H2) pH meter with a silver indicator electrode and a mercury-mercurous sulfateTable 11.
Comparison of Phenolphthalin and DenigZs Methods for HCN Determination
H C X Known, P.P.11,
n 1
3
HCK Found, P.P.M. DenigBs method Phenolphthalin metho2 n 1 i. 1 i, 1 2 3
A
I
1
6
6, .i 10,lP 27
3. 7 12,lO 26 31 36 72 230
11
27 32 36 73 220
32 36 74 220
Table 111. Comparison of Colorimetric, Potentiometric, and Titration Methods for HCN Determination HCiX Found, P . P . M . H C N Known, P.P.M. 1
1 5
Potentiometric method 2 , 1, 1 4 . 4, 4. 4 2
Phenolphthalin method 1, 1, 1
... ..
DenigBs method
...
...
...
1725
V O L U M E 26, NO. 11, N O V E M B E R 1 9 5 4 Table IV.
Comparison of Potentiometric and n e n i g e s M e t h o d f o r HCN Determination H C S Found, I’ P.M.
H C N Known, P.P.M. 20 18
Potentiometric 1; 18
DenigPs 23, 2 2 18, 17
27
24 16 65 8
2b3’26 19, 16 69,66 9,9 35,34 8, 7 35,34
7 7
7
18
68
8 34 7 34
33 6 31
Table V.
Free HCN Determination H C N Found, P.P.hI.
H C N Known, P.P.hI. 3 7 13 20 145 189 356 740
3 6 13 19 139 190 351 746
potassium sulfate reference electrode. If the indicator changes to green add more trisodium phosphate solution. The initial potential may be as high as 900 mv. and the break occurs a t about 450 to 500 mv. Standard cyanide solutions were independently analyzed by several analysts, as well as the authors, in an effort to establish the reproducibility of the three methods. Typical results are given in Tables I11 and IV. The accuracy and precision of the three methods compare favorably. Potentiometric Titration of Free Hydrocyanic Acid. A procedure was also required which would distinguish cyanide present as free cyanide in acrylonitrile from the cyanohydrin of acetaldehyde (lactonitrile). Yates and Heider (19) studied the reversible reaction between acetaldehyde and hydrogen cyanide to give lactonitrile. As the equilibrium rapidly shifts to H f
CHBCHO
+ H C S J_ CH3-CHOH-CN OH -
form ionized cyanide in a basic medium, both lactonitrile and free hydrogen cyanide would be determined by the three methods discussed. However, on the acid side (pH < 4) lactonitrile does not dissociate. Therefore, a mixture of free hydrogen cyanide and lactonitrile can be analyzed by titrations both in acidic and basic media. Titration in acidic medium determines the free hydrogen cyanide, while the total cyanide value (free hydrogen cyanide plus lactonitrile) is obtained by a titration in a basic solution. Yates and Heider ( 1 9 ) determined free hydrogen cyanide in the presence of lactonitrile by addition of excess silver nitrate solution and back titration of the excess silver with thiocyanate. Although the precision and accuracy were good, the method did not lend itself for use in low hydrogen cyanide concentrations. Kolthoff ( 5 , 8) noted that the titration of sodium iodide nith silver nitrate gave better accuracy and a more distinct break with sulfuric acid present than in a neutral medium. He also found ( 7 ) that as little as 10 p.p.m. of iodide could be determined with an accuracy of a few per cent. In this work the titration was reversed-i.e., excess silver nitrate was titrated with sodium iodide; however, the conditions affecting accuracy were assumed to be the same. Anduze ( I ) determined free hydrogen cyanide in the presence of lactonitrile by addition of acrylonitrile t o excess acid silver nitrate. The excess silver nitrate was titrated potentiometrically with sodium iodide solution using a silver electrode with B mercury-mercurous sulfate-potassium sulfate reference electrode. The method was modified by the use of more dilute reagents and a larger sample size. PROCEDURE. Into a 400-ml. beaker containing 30 ml. of nater
and approximately 80 ml. of methanol, put several drops of thymol blue solution and adjust to a pH of 1 to 2 (pink) with 3% sulfuric acid. Introduce 20 ml. of standard 0.025N silver nitrate solution to the mixture and add a 50-ml. sample of acrylonitrile. Stir, allow the precipitate to coagulate, and filter through a fine fritted-glass crucible. Rinse the beaker and precipitate several times with small portions of distilled water. Titrate the excess silver nitrate potentiometrically, while stirring, with freshly standardized 0.025N sodium iodide using a Beckman (H2) pH meter with a silver indicator electrode and a mercury-mercurous sulfate-potassium sulfate reference electrode. S t the beginning of the titration the potential will be low and may even be off scale. Upon addition of sodium iodide the potential rises, and the break occurs between 250 to 300 mv. Standard cyanide solutions in acrylonitrile were prepared and analyzed for free hydrogen cyanide by the above method. The data given in Table V show that results accurate to fl p.p.m. are obtainable in the range of 5 to 10 p.p.m. Acrylonitrile solutions containing sodium cyanide and redistllled lactonitrile (99.5% or better) were analyzed for free hydrogen cyanide to determine the influence of lactonitrile. No lnterference was found; thus none of the hydrogen cyanide combined as lactonitrile is titrated under these conditions. SUMMARY AND CONCLUSIONS
Four simple procedures have been developed for the determination of low concentrations (0 to 100 p.p.m.) of either free hydrogen cyanide or total hydrogen cyanide in acrvlonitrile samples. Routine application for several years has established that the modification of the phenolphthalin colorimetic procedures described here can be used b the average laboratory analyst with good results; it is especiaty satisfactory for the determination of total cyanide in concentrations below 10 p.p.m. where an accuracy and precision of 4 ~ 0 . 5p.p.m. are needed. The titrimetric procedures are useful for more rapid determinations in the range 20 to 100 p.p.m. of free or total cyanide; the resultant precision and accuracy are &lo% relative. Although designed for use with acrylonitrile samples, these methods have found application in numerous aqueous as well as organic samples. ACKNOWLEDGMENT
Some of the material in this paper mas based on the methods developed by Monsanto’s Central Research Laboratories a t Dayton, and the authors acknowledge the use of unpublished work of Paul W. Runge, Richard A. Anduze, E. M. Hubbard, and Frank R. Short. LITERATURE CITED
(1) Anduze, R. A., private commurication. (2) Childs, A. E., and Ball, W. C., A n a l y s t , 60, 294 (1936). (3) Katz, 9. H., and Longfellow, E. S.,J . I n d . Hyg.,5, 97-104 (1923). (4) Kolthoff, I. M., P h a r m . Weekblad, 54, 1157-71 (1917). (6) Kolthoff, I. M., Rec. trav. chim., 41, 176 (1922). (6) Kolthoff, I. M., Z . anal. Chem., 57, 1-15 (1918). (7) Kolthoff, I. M., and Furman, N. H., “Potentiometric Titrations,” pp. 151-2, New York, John Wiley & Sons, 1931. (8) Ibid., p. 194. (9) Kolthoff, I. M., and Stenger, V. A. “Volumetric Analysis,” 2nd ed., Vol. 11, pp. 282-3, New York, Interscience Publications, Inc., 1947. (10) Muller, E., and Lauterbach, H., Z . anorg. u. allgem. Chem., 121, 178-92 (1922). (11) Nicholson, R. I., A n a l y s t , 66,189 (1941). (12) Robbie, W. A., and Leinfelder, P. J., J . Ind. Hug. Toricol., 27, 13&9 (1945). (13) Rohde, K., and Swope, H. G., Abstracts of 118th Meeting of AM. CHEM.SOC., Division of Water, Sewage, and Sanitation, p. 39, 1950. (14) Runge, P. W., private communication. (15) Ssredo, J. F., Anules asoc. q u t m . y f a r m . U r u g u a y , 42, 59-71 (1939). (16) Thompson, M. R., Bur. 8tundards J . Research, 6 , 1051 (1931). (17) Weehuizen, F., Pharm. Weekblad, 42, 271 (1905). (18) Wick, R. M., Bur. Stallda~dsJ . Research, 7, 913 (1931). (19) Yates, W. F., and Heider, R. L., J. Am. Chem. Soc., 74, 4153 (1952). RECEIVED f o r review April 23, 1954. Accepted August 9, 1954. Presented i n part before the Ninth Annual Southeastern and Southwestern Regional Conclave, Division of Analytical Chemistry, New Orleans, La., Deoember 10 to 12. 1953.
