Amperometric Titration of Cyanide with Silver Nitrate, Using the Rotating Platinum Electrode H. A. LAITINEN, W. P. IENNINGS,
AND
T. D. PARKS, Noyes Chemical Laboratory, University of Illinois, Urbana, 111.
exactly 0.5 AT (weight basis). The two end points are comparable in precision and yield the same answer to about 0.1% in the titration of 0.1 M cyanide.
The amperometric titration of cyanide with silver is equal in accuracy and precision to the visual Denigas method, and isapplicableat much higher dilution. Chloride, bromide, hydroxide, and high concentrations of potassium nitrate or sulfate d o not interfere.
To compare the two end points for more dilute solutions of cyanide, accurate dilutions of the two stock solutions were made by diluting known weights of stock solution to known kolumes. All dilutions were carried out with 0.1 N sodium hydroxide, to prevent loss of hydrogen cyanide by hydrolysis. The dilute solutions were then compared by the usuaI volumetric technique, using as a basis of calculation the average normality of stock cyanide solution determined by the visual end point (Table I). The results of the comparison are given in Table 11. The two end points were comparable in precision and accuracy a t cyanide concentrations as low as 0.002 LV. The visual end point became indistinct a t 2 X 10-4 N cyanide and failed a t higher dilutions, whereas a distinct amperometric end point was still observed in titrating 4 X 10-6 A' cyanide' (8 X 10-8 M ) with 5 X 10-4 N silver nitrate. I n general, it is recommended that for the amperometric titration a fivefold more concentrated silver solution than cyanide be used to avoid the necessity of correcting the current for dilution effect. For the titration of very dilute cyanide, the lowest practical limit for silver nitrate concentration is of the order of 5 X 10-4 N to obtain a distinct end point.
I
T HAS been shown by Thompson ( I G ) , who used very pure potassium cyanide as a primary standard substance, that the silver nitrate titration of cyanide to Ag(CIi)2- is more accurate than'the mercuric chloride titration Wick (21) showed, however, that the potentiometric mercuric chloride titration is accurate. The original method of Liebig ( l a ) ,based on the formation of a turbidity due to silver cyanide, is subject to error in alkaline (3, 18) and ammoniacal (8, 21) solution. The Denighs (4) titration, based on a turbidity due to silver iodide in the presence of ammonia, gives high results in the presence of a large excess of ammonia (8, 9, 20, 21), but yields accurate results if the concentration of ammonia is carefully regulated (8,9,19,21). The potentiometric titration of cyanide with silver has been described by Treadwell (20), Muller and Lauterbach (16), and Clark ( 2 ) . Rick (21) concluded that the potentiometric titration gives accurate results. Read and Read (17) suggested a bimetallic electrode titration, and Gregory and Hughan (6), using the null-point equivalence potential method of Cavanagh (1), report that the potentiometric method is superior to the Liebig method (5) for the determination of cyanide in plating solutions. The potentiometric method has also been used for the indirect determination of nickel ( I d ) , cobalt (15), and zinc ( 7 , I S ) . The successful amperometric titrations of halides with silver nitrate using the rotating platinum electrode ( I O , 11) suggested a similar method for cyanide. The present paper describes an accurate comparison of the amperometric end point with the Deniges titration, which has been proved to be accurate to within 0.2% and probably 0.1% by reference to pure potassium cyanide (19) and to the potentiometric end point (I9,11).
Table
I. Comparison of Visual and Amperometric End Points in Titration of 0.1 M Cyanide V
KCN Solution Grams 19.737 19.806 19.142 20.470 20.185
=
visual, A = amperometric end point Normalit of Deviation from Stock K e N , Mean Calcd.
AgN03 Solution Grams 20.068 20.168 19.479 20.828 20.532
A 0.500 N solution of reagent quality silver nitrate and a 1.0 M solution of C.P. potassium cyanide in 0.1 X sodium hydroxide were compared accurately, using weight burets, finishin each titration with 0.01 S silver nitrate prepared by diluting a fnown weight of stock solution to a known volume, and using a volume buret for the final titration. The end point was observed by both the visual (Denighs) and amperometric methods. No blank correction was applied to the visual end point. For the amperometric titration, the apparatus and technique pkeviously described ( I O ) were used, with the mercury-mercuric iodide-potassium iodide reference electrode and a galvanometer sensitivity of about 0.02 microampere per mm. The residual current before the end point was extremely small, and very sharp end points were observed. Between titrations, the silver was removed from the platinum electrode by anodic polarization or by using nitric acid to prevent an anodic residual current due to dissolution of silver in the cyanide solution. If nitric acid was used to remove the silver, the electrode was allowed to stand in ammonium hydroxide solution for a fely minutes before rinsing with water and using it again. In Table I, experimental data are presented for the titration of 20-gram samples of 1.0 JI potassium cyanide, diluted to about 100 ml. nith 0.1 S sodium hydroxide to give a final cyanide concentration of 0.2 J4 (0.1 -Y with respect to silver). For purposes of calculation, the silver nitrate solution was assumed to be 1
I'reirni address Shell Developnierit Co
%
hv.
EXPERIMENTAL 1E 20 920 19 967 19 911
End Point
2 2 20 225
0.50839 0.50914 0.50880 0.50874 0 50864 0,50873
0 50791
.I\,. 0 50816
-0.07
f0.08
+0.01 0.00 -0.03
-
V V V V V
e0.04
*o
05
II. Comparison of Visual and Amperometric Titration of 100 MI. of Dilute Potassium Cyanide in 0.1 N Sodium Hydroxide Table
with Silver Nitrate
Approximate Normality of Cy ani d e 0.02
End Poilit Visual Amperometric
x
10-3
2
x
10-4
2
x
10-5
Visual Amperometric
4
x
10-1
Visual hniperometric
Visual Amperometric Visual Amperometric
, Emer? ville, Calif.
57'4
Error +0.3,+0.2.+0.3, +0.5, +o.1 -0.3, -0.3, -0.4, -0.4, - 0 . 4 +1.0,+0.8,+0.8, +0.7, + 0 . 7 -0.4. -0.1, -0.1, -0.1, -0.1 -0.3,-3.1, -3.3, -3.3, -3.6 -2.7, -2.7, -2.6, -2.4 No end point
Average Error, 5% f0.3
0.4 +0.8
-0.2
-2.7 -2.6
+ 6+. 0 5 ,. 7-,1t. 4 6 ,. 9+1.1,
+2.3
N o end point +9.2, + 2 . 3+, 1-22..83, f 2 . 3 ,
+4.9
September, 1946
ANALYTICAL EDITION
575 LITERATURE CITED
Table
Ill. Effect of Salts and Hydroxide in Titration of 0.002 N Cyanide ( I n 0.1 N sodium hydroxide unless otherwise stated)
Salt 0.001 N I i C l 0.004 N KC1 0 . 1 N KC1 0.001 N K R r 0.004 N KBr 0.1 N KBr 1. O N KNOs l.ONKNOa+ 0 . 5 M KzS04 Satd. K ~ S O P
70 Error
Added
+ 01..0O2NNNNa aOOHH N NaOH ++ 10.02 .0 N NaOH
+ 0 . 6 , +0.6, +0.4 i - 0 . 4 , +0.3,+ 0 . 1 4-0.4, -0.2, +0.2 -0.1, i - O . 6 , + 0 . 5 +0.3, o.o,+o.7 +0.4, 4-0.4, + 0 . 4 -0.2, f O . 1 , -0.4
-0.8, -0.8 -0 6 - 0 . 2 , t0.1:-0:1 -0.8, - 0 . 6 , - 0 . 2
Average Error, 70 +0.5
+0.3 +0.1 +0.3 +0.3 +0.4
-0.2 -0.7 -0.1 -0.5
EFFECT O F CHLORIDE, BROMIDE, H Y D R O X I D E , AND HIGH C O N C E N T R A T I O N S O F SALTS
-1series of titrations of 0.002 N cyanide in the presence of various concentrations of chloride, bromide, hydroxide, nitrate, and sulfate was carried out (Table 111). The errors again were computed with reference t o the visual end point titration of 0.1 N cyanide. I n general the results are accurate t o within 0.57’in the presence of 50-fold excess of chloride or bromide or in 1 N salt solutions. High concentrations of alkali tend to give low results. INTERFERENCES
Iodide, sulfide, sulfhydryl compounds, etc., which form extremely insoluble silver salts, or materials which form more stable complexes with silver than cyanidts does, will, in general, interfere.
(1) Cavanagh, B., J . Chem. Soc., 1927, 2207. (2) Clark, W., Ibid., 1926, 749. (3) Clennell, J . E., “Chemistry of Cyanide Solutions”, New York, McGraw-Hill Book Co., 1910. (4) Denighs, G., A n n . chim. phys., 6, 7, 381 (1895). (5) Gregory, J. N., J . Council Sci. Ind. Research, 16, 185 (1943). (6) Gregory, J. N., and Hughan, R. R., ISD.ENG,CHEM.,h x a ~ . ED., 17, 109 (1945). (7) . . Kolthoff, I. M.. and Furman, N. H., “Potentiometric Titrations”, p. 168, New York, John Wley & Sons, 1931.
(8) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative
Inorganic Analysis”, pp. 479, 574, Kew York, Macmillan Co.,
1943. (9) Kolthoff, I. M.,and Stenger, V. A , “Volumetric Analysis”, Yol. 11, Kew York, Interscience Publishers (in press).
(10) Laitinen, H. A., Jennings, W.P., and Parks, T. D., ISD.Exo. CHEY.,ANAL.ED.,18,355, 358 (1946). (11) Laitinen, H. .4.,and Kolthoff, I. M., J . Phys. Chem., 45, 1079 (1941). (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
Liebig, J. von, Ann., 77, 102 (1851); J . Chem. Soc., 4, 219 (1852). Muller, E., and Adam, A., 2. Elektrochem., 29, 49 (1923). Muller. E., and Lauterbach, H., 2. anal. Chem., 61, 457 (1922). Ibid., 62, 23 (1923).
Muller, E., and Lauterbach, H., 2. anorg. Chem., 121, 178 (1922). Read, H. J., and Read, C. P., Metal Finishing, 39, 612 (1941). Sharwood, W. J., J . Am. Chem. SOC.,19, 400 (1897). Thompson, M.R., Bur. Standards J. Research, 6 , 1051 (1931). Treadwell, W. D., 2. anorg. Chem., 71, 223 (1911). Wick, R. M.,Bur. Standards J . Research, 7, 913 (1931).
IXVEBTIGATIOS carried out under sponsorship of the Office of R u b b e r Roserve, Reconstruction Finance Corporation, in connection with the Government Synthetic Rubber Program.
Vapor-Liquid Equilibrium Still for Miscible Liquids DONALD T. C. GILLESPIE. Australian Scientific Research Liaison Office, Australia House, Strand, London, W.C. 2, England A new apparatus is described for the determination of vapor-liquid equilibrium data, it consists of an electrically heated still fitted with a Cottrell pump, a vapor-liquid disengagement chamber, and a condensate trap. Both the boiling liquid and the vapor circulate within the apparatus, and boiling points may b e determined with accuracy, as the system ensures complete equilibrium between the two phases. A simple test for the entrainment of liquid in the vapor is described, and it is shown that in the new apparatus less than 0.05% entrainment occurs with negligible effect upon equilibrium data,
V
APOR-liquid equilibrium determinations reported by different investigators frequently show aide inconsistencies, and it is only rarely that a new examination of a system confirms previous results. Very few systems are known to form ideal solutions, whereby the vapor-liquid relationships may be calculated by Raoult’s lam from vapor pressures, and the engineer must usually depend upon experimental data for the design of distillation and other contacting equipment. It is, therefore, not surprising to find that such equipment frequently does not operate t o the predicted specifications, and the need for greater reliability in equilibrium determinations is very evident. There is also a need for the accurate measurement of other physical properties of systems, such as boiling points and vapor pressuretemperature relationships, which enable the calculation of activity coefficients for the mathematical correlation and extension of results. Many different forms of apparatus have been proposed for the examination of the vapor-liquid relationships of miscible liquids. Some of the more recent are those of Jones, Schoenborn, and Colburn (c), Langdon and Keyes (6),Othmer (7), and the modifications by York and Holmes (10). From a careful analysis of these and other forms, it would appear that no one type of
apparatus so far described is entirely free from possible sources of error, and a still operating on a new principle is described in this paper in the hope that it may assist in progress towards the elimination of these faults, and a t the same time make it possible to determine accurately the true boiling points of the mixtures being investigated. Various methods have been adopted to prevent partial condensation and refluxing of the vapors. These methods fall generally into one or other of two main classes, in which either the vapor line is jacketed with the same vapors, or some form of external heating is applied to the exposed vapor-conveying sections, respectively. Combinations of the two methods are also described. T‘apor-jacketing when used alone does not appear to be a fully effective means of reflux-prevention. This is particularly evident with liquids of high boiling point, with which refluxing from the jacketed vapor line may become visibly quite pronounced. Further, in considerations of apparatus of this type (Y),the enrichment of the vapors with respect t o the more volatile component before they reach the vapor line does not appear always to have been recognized. I t is obvious that this enrichment can occur by partial condensation on the outer body of the still at all points above the level of the boiling liquid. In several forms of apparatus, external heating is adopted to rompensate for heat losses from the upper parts of the still ( I , 5, 6, 8, 10) and so prevent refluxing of the vapors, although the danger of evaporation of thin films and spray droplets on such heated surfaces has been pointed out ( 7 ) . Despite this possibility of error, the method appears to havr been used successfully. Carey and Lewis ( I ) have surrounded the entire vaporization chamber with an accurately heated jacket, and the careful technique of these investigators has led Jones et al. (6)to concede that the possible sources of error in this type of apparatus may not be important when special precautions are taken. Severtheless, it
