March 15, 1935
ANALYTICAL EDITION
METHOD O F STANDARDIZATION OF 0.01 N POTASSIUM PERMANTo each cc. of 0.01 N potassium ferrocyanide add 2 cc. of N sulfuric acid and titrate with 0.01 N potassium permanganate in the presence of 0.05 cc. of 0.1 per cent aqueous erioglaucine. Subtract an end-point correction of 0.012 cc. from the titer for each 0.05 cc. of erioglaucine. (The authors' samples of this dye showed no deterioration on long standing.) GANATE.
LITERATURE CITED (1) Haen, de, Ann., 90, 160 (1854). (2) Halverson and Bergeim, J. ISD. ENG.CHEV.,10, 119 (1918).
1.21
(3) Knop, 2. anal. Chem., 77, 111 (1929). (4) Knop and Kubelkova, Ibid.,85, 401 (1931). ( 5 ) Kolthoff, Rec. trav. chim., 41, 343 (1922). (6) Kolthoff and Pearson, IND.EKQ.CHEX, Anal. Ed., 3, 381 (1931). (7) Lang, 2. anorg. allgem. Chem., 152, 197 (1926). (8) McBride, J. Am. Chem. Soc., 34, 393 (1912). (9) Maller and Lauterbach, 2. anal. Chem., 61, 398 (1922). (IO) Shohl, J. Am. Chem. SOC.,50, 417 (1928). RBCEIVBXJ October 23, 1934.
Separation and Detection of Cyanide L. J. CURTMAN AND S. M. EDUONDS College of the City of New York, New York, 'N. Y.
H
YDROGEN cyanide is rapidly displaced when a n
inert gas is bubbled through its aqueous solution. Utilizing this fact, Chellel described a n apparatus in which the acid is separated by means of a stream of carbon dioxide-free air and caught in alkali; in the alkali solution the test for cyanide is subsequently made. This paper describes a simpler and much more convenient set-up, though employing the same principle, which is particularly useful for the detection of cyanide in highly insoluble cyanides, in solid mixtures containing carbonate, and in sodium carbonate solution. The apparatus (Figure 1) consists of two 15 X 180 mm. test tubes and a 100-cc. Erlenmeyer flask connected by rubber and glass tubing as indicated. Two-tenths gram of the substance to be analyzed for cyanide (or 3 cc. of a prepared solution, made by treating 3 grams of the substance with 50 cc. of 1.5 M sodium carbonate, boiling for 3 minutes, and filtering) is introduced into the flask. The first test tube contains 6 cc. of 3 M hydrochloric acid, and the second, 10 cc. of 6 fif sodium hydroxide. A plug of absorbent cotton in the neck of the flask prevents any of the liquid from being carried over into the alkali. By slowly turning on the compressed air, the acid in the first test tube is forced over into the flask. A slow stream of air is then allowed to bubble through the flask for 30 minutes. Any cyanide in the mixture is thus carried over into the second test tube, where it is absorbed by the alkali. It is detected in this solution by means of the Prussian blue reaction carried out as follows: To the alkaline solution are added a few drops of a freshly prepared 1 M ferrous sulfate solution, the mixture is heated almost to boiling and then thoroughly cooled. The solution is then carefully neutralized with 12 M hydrochloric acid and a few drops of 1 M ferric chloride solution are added. The formation of a blue precipitate or, with very small amounts of cyanide, of a blue or blue-green coloration indicates the presence of cyanide.
chloride. However, a satisfactory final test is obtained if the solution, after the addition of ferrous sulfate and acidification, is boiled for one minute to expel the greater part of the hydrogen sulfide and then treated with ferric chloride. I n this way a test for cyanide was obtained in a mixture of 1 mg. of cyanide as sodium cyanide and 0.2 gram of finely powdered ferrous sulfide. I n the presence of nitrite the separation and tests for cyanide fail, doubtless because of oxidation of the cyanide.
--+
i
FIGURE1
INTERFERING ACIDS. The Prussian blue test cannot be directly applied to mixtures containing ferrocyanide, ferricyanide, or thiocyanate. These acids, however, cause no interference in the proposed method, since excellent controls were obtained when 0.2 gram of potassium ferrocyanide, ferricyanide, and thiocyanate were separately analyzed. TABLE I. TESTFOR CYANIDE
EXPERIMENTAL Following the proposed method, 0.2 mg. of cyanide as sodium cyanide in 3 cc. of 1.5 M sodium carbonate yields a decided positive test. With large amounts of cyanide (10 to 100 mg.) the separation of the acid, although not quite complete in 30 minutes, is sufficient to yield a very large Prussian blue precipitate, proportional to the amount of cyanide present. EFFECT OF OTHERVOLATILE ACIDS. The presence of large amounts of carbonate as sodium carbonate (3 cc. of 1.5 M sodium carbonate containing 0.5 gram of the anhydrous salt) offers no interference. Similarly, sulfite and thiosulfate have no disturbing effect, for, with a mixture of 1 mg. of cyanide as sodium cyanide and 0.2 gram of ?;aaSO~ or Na2S20,.5Hz0, an excellent test for cyanide was obtained. In mixtures containing sulfide, the test is complicated by the precipitation of ferrous sulfide when ferrous sulfate is added and by the separation of sulfur upon the addition of ferric 1
Chelle, J . pharm. chim., 20, 166 (1919).
A I R DISPLACEYBNTPRUSSIAX BLUETEST SUBSTAXCE 0.2 gram of solid 3 cc. of ureuared soln. 3 eo. of ureuared soln. DOUbtfUlQ Faint b Doubtfulc 7 -
+ ++ ++
a
+ ++++
f -
Faint -d
Large bluish white precipitate, probably Zn(CN)z or ZnzFe(CK)s
b Indicating less than 1 mg. of cyanide.
Greenish white Drecioitate. urobablv NinFeiCNh. d Heavy brown precipitate, probablvmer&ry-formed by reduction Fith FeSOd. Mercuric cyanide also yields the same confusing result by direct test. Like the oxyoyanide, it yields a satisfactory cyanide test by the airdisplacement method. C
INSOLUBLE CYANIDES.The common insoluble cyanides (all commercial products) and their prepared solutions were tested for cyanide by the air-displacement method, with the results indicated in Table I. For comparison, the Prussian blue test was applied directly to the prepared solution of each of the substances, in accordance with the normal procedure if ferrocyanide, ferricyanide, and thiocyanate were known to be absent.
INDUSTRIAL AND ENGINEERING
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With the exception of nickel cyanide all the common insoluble cyanides tested respond to the cyanide test when subjected to the authors' procedure. Only in the case of silver cyanide does the method fail when applied to the sodium carbonate prepared solution, for the reason that the salt is not transposed by the carbonate treatment. It is particularly noteworthy that only lead cyanide responds to the direct Prussian blue test. It is thus evident that the separation must be carried out even when the interfering acids are definitely known to be absent.
CHEMISTRY
Vol. 7, No. 2
SUMMARY
A method for the detection of cyanide is proposed that depends upon the separation of the acid by air displacement in a simple apparatus, followed by the application of the Prussian blue test. The method is sensitive to 0.2 mg. of cyanide and is applicable in the presence of ferrocyanide, ferricyanide, thiocyanate, carbonate, sulfite, thiosulfate, or sulfide. RECEIVED January 17, 1935.
A Simple Photoelectric Thermoregulator WILLIAML. WALSHAND NICHOLAS A. MILAS,Massachusetts Institute of Technology, Cambridge, Mass.
I
tentiometer. I n so doing, it INE cuts off the light fallin on the on the catalytic oxidaphotoelectric cell whic!, by a D RNACE t i o n of o r g a n i c subsuitable amplification, operates stances at temperatures bea sensitive relay, thereby bypassin a part of the current tween 100" and 500" C., it t,hroug% additional resistance. was found necessary to deAs the temperature of t h e velop a simple and inexpenfurnace decreases, the opposite sive device for the automatic effect is produced-. g., t h e galvanometer needle moves t o c o n t r o l of temperature, in - 115 V., D.C. N +I15 V., D.C. the right, thereby exposin the order to obtain reproducible FIGURE1. DIRECTCURRENT 23o-vOLT THERMIONIC RELAY photoelectric cell to the Bight results with the minimum exsource which is most convenA . G. E. 110-volt 60-cycle relay type C. R. 2810-1265-G1. A thin penditure of time. sheet of mica should be inderposed between the iron core of the iently rovided by a 40-watt solenoid and the control arm of this relay. This seems to be 110-voi desk l a m p p I a c e d Various temperature-coneffective in preventing sticking of the contacts. about 20 cm. (8 inches) above E . 48-Type vacuum tube trolling devices (1-12) utilizC. 22-Type vacuum tube the slit. ing either photoelectric cells D . PJ 22-Photoelectric cell E. 0.002-Mfd. condenser foil t y e or thyratron tubes have apThe photoelectric cell with L. LlO-Watt, 115-volt t u b t e n &ament G. E. Mazda lamp used as a convenient reslatant peared in the literature durthe system of amplification R i and Ra. 500-Ohm resistors i n g t h e p a s t few y e a r s . and sensitive relay is shown Rn. 400-Ohm resistor. Although, it might be best to have R I and Rz variable, this specification 1s not absolutely necessary. However, all these e m p l o y diagrammatically in Figure 1. R4. Grid leak, 1 megohm R5. Grid leak, 50 megohms very elaborate or expensive T h e entire apparatus-is asa u x i l i a r y apparatus such s e m b l e d in a v e n t i l a t e d as mirror galvanometers and other instruments of high cabinet in which the dry cell for the potentiometer may precision, the use of which is impractical in an ordinary also be placed. chemical laboratory where fumes and other corrosive subIn operation the pointed tip of the paper-covered gvaalstances are always to be found. Furthermore, these devices nometer needle is adjusted so as to balance on the zero point frequently require complex optical systems, often necessi- on the scale immediately to the right of the slit. The furnace tating a large amount of permanent wall or desk space. temperature is adjusted by variable resistance 10" to 15" The present device has been found highly suitable in higher than that desired. Too high a temperature must be certain catalytic reactions where greater accuracy of tem- avoided, since the lag created drives the galvanometer needle perature control than *0.25" is unnecessary. It is to be completely across the slit, thereby again turning on the recommended for its compactness, efficiency, and com- current. The present device has been tested using a single-junction paratively simple and inexpensive construction. Instead of a mirror galvanometer, it employs a Leeds & Northrup chromel-alumel thermocouple and a well-insulated furnace potentiometer indicator. This type of instrument, it is over a range from 100" to 500" C., and it has been found to true, represents some outlay of money, but is found in most control the temperature to within k 0 . 5 " C. Under the chemical laboratories for the measurement of temperature. same conditions a tri-junction thermocouple increased the Since its use as an auxiliary apparatus does not in any way accuracy to within *0.25" C., which is about the limit of impair its customary utility, its initial cost may be disre- accuracy of the potentiometer used. The accuracy obtained garded, and the cost of the thermoregulator will not be more in any case would also depend upon the lagging of the furnace employed. than $15 to $20. Although the present thermoregulator has been found In the construction of the authors' device, a vertical slit 2 mm. highly suitable in catalytic reactions, it could be used to wide and 8 mm. long is cut through the bottom of the galva- control the temperature of any chemical reaction. Furthernometer of a Leeds & Northrup double-range potentiometer in- more, its range should not be confined to temperatures bedicator, No. 8657C, just to the left of the zero point on the scale. This slit admits light to the compartment ordinarily occu ied tween 100" and 500" C. by a dry cell which is removed to an outside position a n a its ACKNOWLEDGMENT place taken by a photoelectric ceII. A strip of black paper slightly larger than the dimensions of the slit and pointed a t its It is a pleasure to acknowledge the valuable suggestions forward end is pasted on to the galvanometer needle which, with increasin temperature, moves to the left of the zero mark as and assistance rendered by Ralph D. Bennett of the Dethe e. m.?. of the thermocouple rises above that set on the po- partment of Electrical Engineering.
N T H E course of work
