February, 1926
INDUSTRIAL A.VD ENGINEERIINC CHENISTRY
139 ~
Determination of Strength of Liquid Hydrocyanic Acid by Specific Gravity’ I n the mariufacture and use of liquid hydrocyanic acid on a large scale in southern California a need developed for a quick end accurate method for determination of the ouritv of t h e acid. which could be oerformed in
the Eeld.. Liehie’s eravimetric method did not lend itself to this work because of the difficulties encountered in attempting to analyze hydrocyanic acid of high purity. Inasmuch as the density of pure hydrocyanic acid is less than threefourths that of water, it was recognized that t h e determination of t h e specific gravity could be utilized in determining the percentage purity of t h e liquid. This paper deals with the methods and apparatus em-
.. ..
ployed for preparing the set of specific gravity tdbles of liquid hydrocyanic acid which are appended. From these tables, given t h e specific gravity and temperature of t h e liquid, t h e percentage of purity can be readily determined. The method and tables are only adapted to solutions of high concentration. After long and continued use of these tables t h e specific gravity determination of liquid hydrocyanic acid 01 relathely high purity can be said to be accurate to within * 0.1 per cent. With t h e limitations necessarily imposed, t h e tables can he recommended for quickiy and accurately determining the percentage composition of commercial liquid hydrocyanic acid. t.....
L
‘ID hydrocyanic acid is now manufactured and used owing to the low viscosity of the liquid and the difficulty of in southern California for the control of citrus pests keeping the stopcocks sufficiently tight. The apparatus 8s well as for the fumigat,ion of ships, grain wareshown in Figure 1 was therefore devised to obviate the houses, factories, and dwelling houses for the extermination difficultiesmentioned. of vermin. This liquid is shipped in metal drums holding APPAxATns-The essential parts of this apparatus ,are the up io 100 pounds net weight. Manufacturers are held rebulb and the absorption flask. The former 1s fitted sponsible to the State of California for the delivery of liquid sampling with two.Xwav stoncorks. the . - ~~, ~ ~ ~one ~ of . which ~ . communicates ~ ~ ~ , with . . ~ hydrocyanic acid in accordance with their specifications. air and is protected by a soda-lime guard tube, while the other I n the manufacture and use of this liquid a need developed communicates directly with the 500-cc.sample bottle and also for a quick and accurate method for the determination of the with the 200-cc. absorption flask. The connection with the is made by means of an intermediate bulb carrying a short prcentage of hydrocyanic acid. The following work was latter stem on the ~ I D D C end and a lone slender stem on the lower end. therefore undertaken. The absorptio; Aask is prote&d by a soda-lrme guard tube against any moisture which might diffuse backward from a Gravimetric Methods of Analysis The fundamental principles underlying the gravimetric analysis of hydrocyanic acid offernothing new, since the work from t h e upper consists merely in absorbing a known weight of liquid hy- I& stopcock catches t h e drocyanic acid in a caustic soda solution, thus forming a SD. o v e r f l o w . The hydium cyanide solution which is titrated with standard silver d r o m e t e r and thernitratt: solution using potassium iodide indicator, according mometer in thecylinder are not essential to this to the Nac.4rthur modification of Liebig’s method.* part of the work. Simple as this process appcars, there are several inherent ABS~RPTION OF HYdifficulties which are riot apparent to the novice. I n the first D R O C Y A N l C ACIDplace, since in many cases a product of high purity is to Le N i n e t y c u b i c centiof 25 per cent analyzed, it is necessary to weigh accurately the a m p l e taken. metexs caustic soda solution Pure hydrocyanic acid boils at 26.5’ C., and since during the are placed in the absummer months in Caliornia the temperature is normally s o r p t i o n flask. The at or above this point during the workmg hours, the loss of stopper carrying the is inserted and . the more volatile and pure portion of the sample is not only bulb the flask and contents possible but highly probable unless special precautions are are weighed on an anataken, thus very seriously affecting the accuracy of the work. lytical balance. The In gravimetric analyses of liquid hydrocyanic acid of high flask is then attached to the absorption apconcentration, it has been found that the procedure of de- paratus and the stoptermining the specific gravity of the liquid and then measur- cocksareturned so that ing the volume taken for analysis is not sdiciently accu- the hydrocyanic acid rate; therefore, i t is necessary to actually weigh the portion may flow from thesample bottle through the used in the determination. On account of the vapor pressure receiving bulb and out ol the material, an ordinary weighing bottle is not satisfactory into thecylinder. The Figure I-Apparafua foe Cravtmet=ie Determination of Hydrocyanic A d d for this purpose, and even a specially constructed weighing mo.cc,bottle tube with glass stopcocks on each end hae, proved inadequate ing the sample to be is next attached, and the liquid hydrocyanic acid is to hold the fluid without loss during the weighing operation, analyzed pumped throurh the apparatus till both are at the Same temperaI Presented before the Division of Physical and Inorganic Chemistry the 70th Meeting of the Amciicau Chemical Society, Loa Angeiee. Calif,. August 3 to 8, 1925. * Clenoeli. “The Chemistry oi Cyanide Solutiom,” 1904, p. 8.
at
.
flowsdownthrough theem~,tybuIbontheabsorbiiigilask,innbicli liquid vaporizes and then bubbles slowly under perfcct control through the caustic soda solution, where it is completely absorbed. Aftcr the recriviiip bulb has thus been gradually emptied of the liquid, gentle suction is applied to the lowei guard tube, tlie upper stopcock is opened to coiinect with the sodalime tube, which is opened to the air, and a current of air is slowly bubblcd t l m w h the liquid in the absorption flask, thus removing all traces of hydrocyanic acid from tlic receiving bull>. ?'lie absorl~eris thcir detached, thc guard tube rcirioved,and after cooling in the balaoce case the flask and contents ase reweighed. T h c gain in weight reprcserrts the hydrocyanic acid absorbed. The entire contents o i t h e absorber are then transferred to 3 liter k, made up to volimic, and wcil mixed, after which duplicate ce. imtioiis are titrated in the usual way. S ~ M Wthe
With proper precautions this nietliod fiilfil.; all requirements It is, liomvcr, a lengthy operation and it is apparent that its enip i o y m e u t rnust be ronfined to a wellcquipped laboratory, thus precludingits use i11 the field where Ereqricntly tliesc determinations liave i,o be made. In view of iliese fa& a quicker 111 e t 110 d b e c a m e necessary. This riceri was early xccognized by GriLy rind Hulbirt, wlro prcpiwed the first set of specific gmvity tabics to be used in determining t,he strength of liquid Irydrocyaiiii: ncid.8 It mas discovered by the writers, liowover, t h a t s o m e rat.her wide Figure 2 - A ~ p a r a f u a lor Determinarariations existed betion of specific Gravity of Liquid Hydrocyanic Acid tween tlie percentages of hydrocyanic acid delcrixriiird by thc gsavimetrii: inetliod and by the specific gravity iiietliod. The investigatbii described herein wati therefore shrbed iii order to obinin a closer agrecmcint. between the t x o methods mid to prqxirc a correclcd set of tables. f o v acriiriioy.
wrre obiaiiicd from the U. S. llureau of ;jtandards. 'fliese iii.i~riirncnts incasure 25.7 em. (lO'/8 inches) from tip of stem to i x h i . of bulb. Tlic scnle reads from 0.650 to 0.750 and roiit,:iins fifty divisions which extend over a stem length of 106.4 em. (4a/,nimhes), so the space betwen two graduations o r ~ q ~ i e2 si i i ~ r i . ('/,? inch) and therefore it is quite eirsy to read ncciir:~telyt,o the third decimal place. Standard Centigrade t,herrnometers paduated froin 0" to 50" in 0.1 degrees were empioyed for temperature observations. Yo1 only is accuracy of observation necessary, but it is cssent.inl for accurate work that tlic temperature of the liquid be held constant while the observations arc being taken. 1:igiire 2 slioivs the app;rr.atus used for this work. The inner cylindrical ghss coiitaiiier is surroundcd by a glass jacket i n wlricli ilic water may bo kept at airy desired temperature !,y a re(~ii1atedsupply from tile reservoir at the side. The inlier cyliiidcr is closed at its upper end by a perforated rubber stopper, and a capillary tube inserted therein maintains atmospheric pressure in the interior without evaporation of the liquid, ivliile a glass st,opcoek at tho lorser end retains tire liquid in the cylinder for observation as long as desired. Preparation of Pure Liquid Hydrocyanic Acid
Ii is evident that the liquid hydroeyaiiic acid used for this work muct be absolutely pure, so commercial hydrocyanic acid of 06 to 08 per cent purity was slightly acidified witlr sulfuric acid and was then redistilled. This distillate was allon-cd to starid over calcium cliloride for a few days, after which it was again distilled, yielding anhydrous hydrocyanic acid free from impurities. From tlik stock supply, by dilutioii and by subsequent analyses, the various percentages of liquid needed for t.he investigatiori were prepared.
Principles Involved
Tire specific gravity tables are appiicuble to iioiiiriit! liquid hydrocyanic acid made arid distributed in southcrii ('nlifomia, because this product contains noiliing but water ill addition to pure hydrocyanic acid. It beeoirics apparent upon st,udying tire relation between the specific gmvity and tlic percentage of liydrocyrinic acid that the method is best adapted to solutions of high coiicent,ration and is not at a11 suited to solutions of very high dilution. Fortunately, liowever, the dilute sohtions arc easily analy~edby simple volumetric methods and asti of niinor importance, since commercial liquid hydrocyiinie acid usually contains 96 to 08 per cent hydrocya,nic acid. It is evident here, as in all andyscs of high-purity products, that small errors in tlie observed data will seriously a Fieure 3 iiie caiciilated perccnlage. Therefore ordinary coinme Iiydromet,ers by reason of their short a i d e o m p r e s d scalc, Gravimetric Analysis of Samples were found to be Quite unsuited for this work. Accordinrrlv. I.. a riumber of specially made, accurate hydrometers were scTlie gravimetric method already described was used for cured and cert,ifir,ateias to their accuracy nt 15.6" C. (60" P.) t.lris work. The buret was carefully checked and bhe correetions so obtained were applied to the work. Titratioiis were Univeisilg 01 Criiloriiin, Buli. SO8 (June, 1919).
142
INDUSTRIAL ARD ENGINEERING CHEMISTRY
so arranged that each one used between 40 and 50 cc. of silver nitrate, in which region the buret corrections were almost negligible. The standard silver nitrate (approx. 0.1N) was very carefully standardized by precipitation with a known slight excess of 0.1 N hydrochloric acid followed by filtration on a Gooch crucible and subsequent weighing in the usual way. These determinations were made in triplicate and the average was taken. Since the maximum divergence between the weights of silver chloride found was only a milligram and since the samples of hydrocyanic acid used for subsequent analysis weighed 10 grams or more, the error in the analyses of hydrocyanic acid, due to standardization of the silver nitrate solution would not exceed 0.00127, when e x p r e s s e d i n percentage of hydroc y a n i c a c i d in the sample analyzed. Determination of Specific Gravity
The various samples p r e p a r e d a n d analyzed as indicated were cooled to 0' C. The jacket was filled w i t h w a t e r at this temperature and the sample of liquid hydrocyanic acid was p u m p e d f r o m the sample bottle into the inner cylinder. When Figure 4-Portable Apparatus for De- temperature equilibtermining Specific Gravity of Liquid Hyrium was attained, drocyanic Acid i n Drums duplicate or triplicate readings of the sDecific gravity and the temperature were taken a n d recordeh. T h e temperature of t h e jacket water was then changed 1.5 degrees and after equilibrium between the two liquids was once more attained a second set of readings was taken. This procedure was repeated again and again until a temperature of 25" C. was reached. This method of operation was used for each of the ten different strengths of hydrocyanic acid, The points thus determined are plotted on the accompanying graph (Figure 3) and show for each percentage of hydrocyanic acid the corresponding specific gravity and temperature. From these curves a second set, also shown on the graph, were plotted, and from this second set of curves, plotted in practice for every degree Centigrade, the specific gravity table was constructed. Figure 4 shows an apparatus designed for use in sampling liquid hydrocyanic acid in shipping drums either in the field or the factory. By its use the operator is spared much of the annoyance and hazard ordinarily attendant on such an operation, and can obtain percentages which are correct t o less than 0.2 per cent. I n using this apparatus the hydrocyanic acid is sucked up from the drum into the glass cylinder by use of the suction bulb shown at the top. If desired, this bulb may be replaced by a bicycle pump, the discharge hose from which is attached t o the right-angled copper tube which passes through the rubber stopper at the right of the vertical tube. If a sample of the liquid is desired for any purpose it can be drawn from the glass cylinder through the small copper tube emerging from the bottom of the cylinder at the left.
Vol. 18, No. 2
Determination of Hydrocyanic Acid in Gaseous Mixtures' By Gilbert E. Sei1 THE KOPPERS COMPANY LABORATORIES, MELLONINSTITUTE, PIT.ISBURGH, PA.
T
HE determination of hydrocyanic acid in gaseous mix-
tures has always been a rather long and tedious operation requiring careful manipulation and unusually good technic. The method which is here described reduces the time necessary for a determination to a few minutes, requires very little apparatus, and after a little practice can be followed by an unskilled worker with satisfactory results. This method has been checked in the laboratory of The Koppers Company a t the Seaboard By-product Coke Plant against the method generally used. Synthetic mixtures of known amounts of hydrocyanic acid were analyzed with unusually accurate results. Theory
This method depends upon the following facts: I-The reaction between hydrocyanic acid and iodine in a sodium bicarbonate solution yields cyanogen iodide and hydriodic acid. The function of the excess of sodium bicarbonate is t o neutralize the hydriodic acid as i t is formed. 2-A mixture of cyanogen iodide and hydriodic acid yields hydrocyanic acid and iodine in acid solution. 3-Hydrogen sulfide and iodine yield hydriodic acid and sulfur. This reaction is irreversible under the conditions of this method. 4-The writer has found no other constituent of any gaseous mixture, which he has analyzed, which absorbs iodine in sodium bicarbonate solution and then liberates iodine upon acidification of the products of absorption.
The following reactions explain these facts: HCN CNI HzS
+ 12 + NaHC08 = N a I + C N I + COZ+ HzO + HI + HCl = HC1+ HCN + I z
+ Iz = 2HI + S
(1) (2) (3)
Usual Method
Pass the gas a t the rate of about 85 liters (3 cu. ft.) per hour through four bottle washers, each containing about 100 cc. of 15 per cent pure sodium hydroxide solution. The volume of the gas to be taken for analysis depends on the hydrocyanic acid content. At least 570 liters (20 cu. ft.) should be passed where the hydrocyanic acid content is only 0.0458 mg. per liter (2 grains per 100 cu. ft.). Wash the solution from the bottles, make up to 500 cc. and take a 50-cc. sample. Add the precipitated lead carbonate, filter, and wash. Titrate the filtrate, which should be free from sulfur, with 0.1 N silver nitrate, using a few drops of 10 per cent potassium iodide as an aid to the determination of the end point. '
Cc 0.1 N AgNOa X 10 X 0 0054 X 15.43 X 100 or
Cu. f t . of gas (cor.) C c . 0.1 N AgNOs X 10 X 0 0054
Grains HCN per ~
cu. ft.
Mg. HCN per ,iter
Liters of gas
The method is a little simpler on gas containing only a s m d amount of hydrogen sulfide, such as outlet gases from the wet purification process. On gases high in hydrogen sulfide the latter might be removed first, provided it is done without any loss of hydrocyanic acid. PREPARATION O F LEAD CARBONATE-hhke a SOlUtiOn of lead nitrate and add excess sodium carbonate solution. Allow the lead carbonate precipitate to settle and test the supernatant clear liquid for complete precipitation by adding a small quantity of sodium carbonate solution. Wash the precipitate three or four times by decantation with cold water 1
Received August 18, 1925.
