Use of Empirically Derived Correction Factors Polarographic Determination of Free Cyanide in Presence of Sulfides J. H. K A R C H M E R and M A R J O R I E T. W A L K E R Humble
Oil
and Refining Co., Baytown, Tex.
.4 rapid polarographic method is presented for the determination of free cyanide in the presence of complex metallocyanides and large amounts of sulfides to show how empirically derived empirical factors can be employed to correct systematic errors. This procedure is based upon measurement of the anodic wave produced by the cyanide ion at the dropping mercury electrode. The bulk of the sodium suEde, which produces an earlier wave and would interfere with the cyanide determination, is removed by treatment with a slightly less than stoichiometrical amount of silver nitrate, The cyanide ion losses, incurred by the presence of large amounts of the silFer sulfide precipitate, can be corrected by a series of correction factors developed in this study.
types of saniplp- by distillation, it was decided to determine if the free cyanide could be determined with reasonable accuracy by employing a correction factor to compensate for the cyanide lost by occluqion on the heavy metal precipitate. I) EVE LOPllENT 0 F ANA LYI’ICA L PROCEDURE:
The 1)olarographic method, although not as sensitive as a colorimetric method, Tvas cmployed for actually determining the free cJ-anide because it offered one advantage-namely, that the cyanide ion could be determined anodically in presence of nioderat,e concentrations of sulfide ion. I n preliminary esperinients the sulfide ion concent,ration, which could be tolerated, was found to be about ten times the cysnide ion concentration, l~rovidingthe sulfide ion concentration in the polarographic cell did not exceed 5 X 10-4.1f. For this reason complete removal of the sulfide was unnecessary and the possible loss of cyanide ions by coniplexation with excess metal ion, used in precipitating the sulfide ion, was avoided. The part,ial removal of the sulfides was effected by a potentiometric titrat,ion of an aliquot sample with silver nitrate solution. The titrat.ion was stopped just short of t,he end point, so as to avoid an excess of silver ions in the solution which may complex with the cyanide ions. Although this procedure proved satisfactory for samples containing small amounts of sulfides or large amounts of cyanides, IOU results were always obtained on synthetic samples when a large precipitate of silver sulfide wm present in the treated portion and if the cyanide concentration \vas low, This dependence of the recovery of the cyanide ion on both the concentration of the cyanide and the weight, of t’he silver sulfide present suggested that the cyanide loss was the result of a Freundlich-type adsorption. Although preliminary n-ork indicated that with pure solutions Freundlich isot,herm constants could be obtained over a limited range, subsequent work on plant samples indicated that this was not the only factor responsible for the losses. For example, sodium thiosulfate and sodium bisulfite, which could form by oxidation of the sodium sulfide present, could interfere with the polarographic cyanide wave. Furthermore, other oxidation products of sodium sulfide are elemental sulfur and sodium polysulfide xhich can react with the cyanide to form a thiocyanate. Because the rate of osidation of the sodium sulfide to the thiosulfate, bisulfite, or polysulfide is related to the hydroxide ion, the [OH-] concentration of the original sample also becomes a variable that, could influence the accuracy of the method.
I
N C E R T A I S cases when no accurate methods are avni1:ible for carrying out a specified determination, it may be necesssary t o employ a procedure which is known to contain inherent systematic errors. In such a case, where the error can be niathematically defined and predicted from a knowledge of the sample, the use of correction factors may prove useful-for esample, the determination of free cyanide in petroleum refinery units producing or fractionating light catalytic gases. This determination is important, for the free cyanide is thought to be related t’o a type of corrosion known as “hydrogen blistering” (1). Systematically low results were observed when 61 sxnthetic samplrs, containing various combinations of concentrations of contaminants, were analyzed for free cyanide using s polarographic method. These results were used as a basis to prepare a series of correction factors. The application of statistical tecliniques to the corrected values indicated that t.he use of correction factors would be satisfactory for practical purposes. The complicating Factors found in the determination of free cyanide in these types of refinery samples are the possible presence of high concentrations of soluble sulfides and complex metallocyanides (the free cyanide must be differentiated from the complex cyanides because the complex cyanides are not thought to cause hydrogen blistering). The presence of large amounts of sulfides affects the accuracy of the determination. In many methods of determining the cyanide ion (6, 7 , 9, 11) large concentrations of sulfide interfere and if the sulfide ion is removed by precipitat,ion as a heavy metal sulfide ( d ) , appreciable amounts of free cyanide will be occluded on the precipitate and hence low results will be obtained. The presence of the complex metallocyanide prevents the use of a precision acidic dist,illation technique-i.e., those of Fisher and Brown ( 3 )and Serfass ef al. (10)-for separat,ing the free cyanide ion from a heavy metal sulfide because the metallocyanide complexes (as the ferrocyanide ion, known to be present, in some of these samples) decompose to hydrocyanic acid on heing heated in acidic solutions ( 3 , 7 ) . Recent work of Dodge and Zshban ( d ) , i n studying the volatility of hydrocj-anic acid, showed that a t pH 6.5 simple cyanides rould be volatilized as hydrocyanic acid; at this pH comples metallocyanides would not yield hydrocyanic acid. This work indicates that development of an analytical method for determining free cyanide ion may be possible. In the absence of a developed procedure foF analyzing these
CORRECTIO;\( FACTORS
To determine the magnitude of these errors, a series of 61 synthetic samples, containing varying amounts of sodium cyanide, sodium thiosulfate, sodium hydroxide, and sodium sulfide, was analyzed for sodium cyanide. (Sodium ferrocyanide was not included in the study. because it has no effect upon the polarographic determination of the cyanide ion.) This series of samples was run in a 3 X 3 X 3 X 2 completely randomized factorial design so as to yield pertinent statistical information. The statistical examination of these raw data reveals that the largest effect was apparently due to the interaction of the sulfide ion and the cyanide ion concentrations, and that the remaining interactions were small. Since the sulfide ion could be readily approximated, an attempt was made to correct for the losses of cyanide ion due to the presence of sulfide ion. Therefore, another series of synthetic samples was prepared containing various sulfide and
37
ANALYTICAL CHEMISTRY
38 cyanide ion ratios to simulate conditions in given aliquots of plant samples. From these values a series of correction curves was obtained and these correction factors were applied to the original 61 synthetic samples, In preparing the synthetic samples sodium sulfide was prepared from hydrogen sulfide and sodium hydroxide because available commercial grades of sodium sulfide were contaminated with small quantities of sodium thiosulfate. Although the samples mere blanketed with inert gas, no synthetic sample was allowed to remain longer than 48 hours before being analyzed.
and sodium cyanide, in 0.1N sodium borate solution (adjusted to. pH 10.8) as the supporting electrolyte. For concentrations of 5 X 10-4M, the half waves are about -0.63 and -0.26 us. t h e saturated calomel electrode. Therefore the oxidation waves were spaced sufficiently far apart to allow the formation of well-defined waves. However, there were certain limits to the amount of sulfide ion that could be present and to the ratio of the sulfide to. the cyanide ion. If these limits were exceeded the sulfide wave would merge into the cyanide one. For this reason it was necessary to study the effect of sulfide concentration upon the accuracy of the cyanide determination and the changes in the location of the waves produced by the concentration. The effect of t h e hydroxyl ion concentration was likewise considered. During the studies with Bynthetic sodium sulfide samples the amount of sodium thiosulfate that was present as R result of t h e sulfide oxidation was found to be sufficiently high to interfere with the accurate cyanide determination. Sodium thiosulfate has a half wave of approximately -0.10 (Figure 2, C) which falls between the cyanide (Figure 2, B ) and the hydroxide waves and ia close enough to the cyanide wave to cause trouble if it is present. in sufficient quantities.
Figure 1. Cyanide Ion Correction Graph Z A - S O D I U M SULFIDE
The lower limit of detectability of the method using synthetic samples is about 0.00125 gram of cyanide ion per liter in the absence of appreciable quantities of sulfide. If the sulfide content is 30.0 grams per liter, the detectability of the method is reduced to 0.02 gram per liter, as the sample size has to be limited because of the excessive sulfide concentration. The series of samples used for the preparation of correction factors was analyzed in a 5 X 5 complete factorial design using 0.5, 1.0, 2.0, 3.0, and 5.0 mg. of cyanide ion and 2.5, 25.0, 50.0, 100.0, and 750.0 mg. of sulfide ion. Two to five replicates were analyzed on each sample with a total of 70 determinations. The cyanide concentration of each sample !vas obtained according to the proposed procedure. T o obtain the fraction of cyanide recovered, the average of the replicates of each sample was divided by the amount of cyanide present. The reciprocal of the fraction recovered represents the correction factor or that value by which the amount of cyanide found has to be multiplied in order to obtain the amount of cyanide originally present in the aliquot. The calibration curve was prepared by plotting the correction factors against the amount of cyanide ion found for a given amount of sulfide in the aliquot. Figure 1 shows the correction curves for 2.5, 25, 100, 300, and 750 mg. of sulfide ion. (Extrapolations may be made for intermediate amounts of sulfide present.) DISCUSSION O F POLAROGRAPHIC VARIABLES
The polarographic determination of cyanides is based upon the fact that, when an increasing positive voltage is applied to the dropping mercury electrode, an anodic wave is produced by the oxidation of the mercury. In ions forming complexes or insoluble compounds with the mercury, the anodic waves are shifted to more negative potentials depending upon the solubility product of the compounds or the dissociation constants of the complexes. The anodic current, produced by the Oxidation of the mercury, is governed by the rate of diffusion of the anion to the electrode surface and hence is proportional to the anion concentration in the body of the solution. While the half-wave potential of the cyanide ion is largely fixed by the solubility product of mercuric cyanide, a further variation in the half wave is due to the concentration effect. Kolthoff and Miller ( 6 ) have found that the following equation is applicable for predicting the shift in half wave due to concentrations for anions of this type:
Eliz = constant - o.l: log concn. of anion Figure 2, A and B are idealized polarograms of sodium sulfide
0.6
c-
0EGlNNlNG
OF
[OH].
4
WAVE--/
L
1.6
0.8
2C
- SODIUM
THIOSULFATE
0.8
-0.8
-0.6 .0.4 -0.2 APPLIED E . M F. VS. S.C.E.
0
t0.2
Figure 2. Anode Waves Given by Sulfide, Cyanide, and Thiosulfate Ions with Dropping Mercury Electrode Anion concn., 5 X 10-41M Supporting electrolyte, sodium hydroxide and boric acid, adjusted t o pH 10.8
Effect of Sulfide Ion Concentration. A well-defined sulfide wave can be obtained a t about -0.63 volt when the concentration is about 5 X 10-4M. At concentrations significantly above this value, the wave becomes irregular in shape and the apparent half wave shifts appreciably to more positive voltages. The irregular shape of the wave causes it to merge with the cyanide wave. The erratic behavior of the sulfide is presumably due t o the coating of the mercury drops with a film of mercuric sulfide. Because a cell concentration of sulfide in excess of 5 X lO-*M is undesirable, and the cyanide concentration in many samples is several hundred times smaller than that of the sulfide, some of the sulfide must be removed because the amount of cyanide in samples containing the appropriately low concentrations of the sulfide would not be detected on the polarograph. This removal
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5
39
can be facilitated by selecting an adequate size sample, so as to yield a cyanide concentration in the cell of a t least 10-6M, and potentiometrically titrating with 0.25M silver nitrate solution. The titration is stopped just short of the inflection point, leaving a small amount of sulfide in the sample which, being below 5 X lO-'M, will not interfere with the subsequent cyanide wave.
thiosulfate is shown in Figure 2, C, as being -0.10 volt us. the saturated calomel electrode. T o study the effect of thiosulfate upon the cyanide determination a series of synthetic blends TI as prepared containing known amounts of sodium cyanide and sodium thiosulfate. The results are given in Table I. When the thiosulfate ion concentration in the cell is less than or approximately equal to 5.0 X 10-4M, the accuracy is satisfactory. Higher concentrations of thiosulfate produce lower cyanide results, or entirely prevent its measurement by merging with the cyanide u ave. Hence the presence of more than 2.0 X l O - 3 N (cell concentration) thiosulfate forms one of the limitations of the method. This restriction, however, is not too serious, as this would correspond to a large amount of thiosulfate in the original sample, and usually the sample can be diluted so that the thiosulfate will be below the critical cell concentration. I n the event the thiosulfate is unusually high and merges with the cyanide wwe, the analysis is of no value because of the alteration in the position and shape of the polarographic wave.
0.C
I
I
C 1.4
-0.2
I
I
0.0 -0.4 -0.2 A P P L I E D V O L T A G E VS. S.C.E.
I 0.0
0.5
Figure 3.
Effect of Hydroxyl Ion on Cyanide Determination
I n a L 0
0.4
s
Effect of Hydroxyl Ion Concentration. Figure, 2, B, showi the beginning of the oxidation wave of the hydroxyl ion which is present in the electrolyte From Equation I it may be observed tho t for every tenfold increase in the hydroxyl ion concentration, the half-Tvave potential of that ion shifts approximately 0.03 volt in a more negative direction; and conversely, for every tenfold decrease in the cyanide ion concentration there is a 0.03-volt shift of its half wave in a more positive direction. Thus, for sample. of low cranide concentration and high pH, there is a possibility that the two TI aves may approach each other sufficiently closely to make the measurement of the cyanide wave difficult. For this reason it was decided to reduce the hydroxyl ion concentration to as low a value as possible, so as to thrust its nave in a positive direction. Hoir ever, too low a pH is undesirable because of the ease ivith u hich sodium cyanide can be hydrolyzed to hydrocyanic acid and lost to the solution during the bubbling operation to remove dissolved oxygen. Calculation of the appropriate pH of the equivalence point of sodium hydroxide and hydrocyanic acid to yield a 0.01S solution of the salt revealed that this value was 10.6. Therefore, some value slightly above this pH was selected. I n Figure 3 the eft ect of the hydroxyl ion on the cyanide wive shows how the two TT aves tend to merge if the pH is too high. To achieve the desired pH of 10.8 f 0.2, the excess sodium hydroxide in the aliquot is neutralized with a saturated solution of boric acid beyond the equivalence point (pH 11.3 for 0.1;11 sodium orthoborate) to the desired pH. Effect of Thiosulfate. In using a synthetic sample of sodium sulfide and sodium cyanide with the pH of the solution reduced to 10.8, the hydroxyl ion oxidation m v e seemed to begin much too early and appeared on the shoulder of the cyanide wave. This t r p e of poorly defined cyanide wave was also found in some plant samples Upon investigation sodium thiosulfate was found to be formed by oxidation of the sodium sulfide in presence of water: 2Na2S
+ 202 + HzO
-P
+ 2NaOH
Xa2Sz03
The half-wave potential of a 5 X 10-41V1 solution of sodium
c
w z /AV€ 0:
a
D U E TO CYANIDE ION
0.3
z
0 3 LL
0
02
- /
0.I WbVE DUE TO REMNANT OF S U L F I D E ION
0.c -0.6
-0.4 b P P L l E D VOLTAGE
-0.2
0.0
V S . S.C.E.
Figure 4. Polarogram of Typical Plant Sample
Table I. Effect of Thiosulfate on Cyanide Determination &OS--Added, CN-, Cell Concn., M C N - ReCell Concn., M 0.0 2.60 x 10-4 5.20 1.04 1.04
x
10-4
x 10-3 x 10-3 2.08 x 10-8 5 . 2 0 x lo-* Q
Added 6.29 x 10-4 i.ofi x 10-3 1.06 1.06
5 29 5 29 5 29
x in-3 x 10-3 x 10-4 x 10-4 x 10-4
Found 5 . 3 3 x 10-4 1.04 x 10-3 1.05 X 10-3 1.00 x 10-3 4 . 8 0 X IO-' 4 . 6 0 X JO-4
...
covery.
70
+- 02 .. 80 -
1.0
6.0 9.3 -13.0
1 . 0 4 X 10-2 5 . 2 9 x 10-4 ... Cyanide wave could not be measured, as it merged with thiosulfate vave.
ANALYTICAL PROCEDURE
Apparatus. POLAROGRAPH, Sargent, Model X S I . H-TYPE POLAROGRAPHIC CELL, fitted with calomel half cell (8).
TITRATOR, such as the Fisher Titrimeter, equipped with stirring motor and stand. ELECTRODES FOR TITRATOR. Calomel electrode, Beckman #4970; silver wire electrode, Beckman #1281-5; and glass elec-
ANALYTICAL CHEMISTRY
40
trode, Beckman Type E, for sodium solutions in range of pH 9 to 14. Table 11. Cyanide Concentrations in Synthetic Saniples after Application of NITROGEX GAS, CYLINDER, AKD REG Empirical Correction ULATOR, oxygen-free, for bubbling samS--. 0.1G./L. S--, 3.0 G./L. S--. 30 G./L. ple to remove dissolved oxygen. NaOH, G./L. Solutions. Sodium hydroxide, 0.5.V. 1.0 25.0 1.0 25.0 1.0 25.0 Boric acid, 0.4M. Dissolve 12.36 Si03 - - , c s - , C S - Found grams of boric acid in about 250 of nearly G./L. G./L. G./L. 70 G./L. '0 G./L. 70 G./L. c'b G./L. 70 G . / L "0 boiling water. Make up volume to 500 0 . 0 0.02 0.019 95 0.02' 100 0.015 75 0.18 90 0.020 100 0.024 120 ml. with cool water. Adjust to volume 0.10 0.101 101 0.101 101 0.100 100 0 . 0 8 6 86 0.101 101 0.096 96 when solution reaches room temperature. 0.125 125 1.02 102 0.99 99 0.95 95 1.07 107 0.98 98 0.93 93 Silver nitrate, 0.2524'. 1.0 Sodium cyanide stock solution, 0.2500 0.04 0.02 0.018 90 0.020 100 0.021 105 0.020 100 0,023 115 0.021 105 gram per liter of CN-. Dissolve 0.2466 0.021 105 gram of 95.5 weight % sodium cyanide 0.10 0.099 99 0.097 97 0 106 106 0.095 95 0.100 100 0.090 90 0.101 101 in 0.5N sodium hydroxide solution, and 1.0 1.03 1 0 3 1 . 0 2 IO? 1.02 1 0 2 1 . 0 5 105 1.12 1 1 2 1 . 1 1 1 1 1 make up to 500 ml. with the alkaline 1.03 103 solution. The true cyanide content of 0.13 0.02 0.016 80 0.015 75 0.020 100 0.020 100 0.021 105 0.020 100 each batch of sodium cyanide should 0.018 90 be determined by an argentimetric titra0 . 1 0 0.094 94 0 . 0 8 8 88 0,090 90 0.099 99 0.080 80 0.09 90 tion (5). 1.101 101 0.095 95 Procedure. CALIBRATION.Prepare 1.0 1.00 100 0.96 96 1.00 100 1.02 102 1.06 106 1.01 101 sodium cyanide stock solution containing 0.2500 gram per liter of CN-. Pipet 10 ml. of this solution into a 250-ml. beaker containing about 25 ml. of 0.5-4' sodium hydroxide soluto about -0.1, which is the top of the cyanide wave. Measure tion. Add about 100 ml. of Tvater; adjust to an end point of the height of the cyanide wave (Figure 4) which appears between pH 10.8 with boric acid; make up volume to 250 ml. ait'h water. -0.4 and -0.2 volt. (The position of the half wave shifts slightly Transfer a portion of the solution to the polarographic cell and obas the cyanide concentration varies.) The earlier wave beginning tain polarogram. Use this standard to calculate concentrat,ion of a t about -0.8 is due to the sulfide which was allowed to remain sample. in sample. If too much sulfide has been left in the sample from SAMPLE h ~ A S I P U L A T I O S . d d d 25 ml. of 0.5AVsodium hydroside the silver nitrate titration, this wave will be irregular and large to a 250-ml. beaker, and pipet a portion of the sample into this enough to merge with the cyanide wave. In such a case, repeat solution. The amount of sample selected should be governed by the determination, making certain that more of the sulfide is rethe cyanide and the sulfide content-the object being to use an moved. On the other hand when analyzing sulfide-bearing samamount of sample that. contains a minimum of sulfide and yet ples, if no sulfide wave is present, it may mean that too much contains 0.25 to 10.0 mg. of cyanide. silver nitrate was added and hence the cyanide results ma.y be If the sulfide content cannot be estimated at this time, an low. In such a case, repeat the determination. arbitrary amount of sample may be selected and this step repeated after the sulfide content has been approximated. .4dd distilled water to bring volume to about 100 ml. and prepare to titrate potentiometrically with 0.25N silver nitrate solution (or Table 111. Analysis of Variance any convenient Concentration) using a silver sulfide and a calomel electrode. Prepare silver sulfide electrode by immersing silver Degrees Sum of of Mean electrode in a solution of sodium sulfide of any convenient conSource Squares Freedom Square F centration-such as, 5 grams of sodium sulfide per liter-until a 11.14659804 2 5.57329902 4098.28' uniform black coating of silver sulfide is obtained. Insert elec0 00098193 2 0.00049096 N.S.a trodes and stirring device into solution and connect electrodes to a OH 0.00003585 1 0.00003585 N.S. 0.00568248 2 0.00284124 N.S. s203 potentiometer, such as the Fisher Titrimeter (or the Beckman 0.00211851 4 0.00052963 N.S. Model G p H meter), Add silver nitrate solution slowly to remove 0.00003448 2 0.00001724 N.S. the bulk of the sulfide present. Plot the voltage readings against C N x s203 0.00956430 4 0.00239108 N.S. 0.00244904 2 0.00122452 N.S. S X OH volume of silver nitrate added after the addition of each incre0.00416474 4 0.00104118 N.S. mental portion and stop the titration a t the beginning of the first 0.00021515 2 0.00010758 N.S. break in the curve. This will leave a small amount of sulfide ion C N x s x OH 0.00543963 4 0.00135991 5.23e 8 0.00118260 4.55c 0.00946082 C N X S X 5203 in the solution which does not interfere with the polarographic 0,00100252 4 0.00025063 N.S. cyanide wave; an excess of silver, however, would cause low re0,00079363 4 0.00019841 N.S. sults. After the technique has become familiar, an actual plot C N x s x OH x s202 o.00207970 0.00025996 5.04~ will not be necessary for the approach of the sulfide end point Error 0,00036100 , 0.00005157 11.19062082 53 will be indicated by the increasingly large changes in voltage per a Statistical significance a t 0.1% level. addition of a unit, volume of titrant, If the product of the volume a Not significant. of silver nitrate, in milliliters, times its normality exceeds 46.8, C Statistical significance a t 5 % level. it is advisable to discard that portion of the sample and to select a smaller aliquot of the sample so that the silver nitrate t'iter times its normality does not exceed that value. Disconnect the silver sulfide electrode from the instrument and remove from beaker. Calculation. From the silver nitrate potentiometric titration Replace it with the glass electrode. (If instrument is a Beckman approximate the amount of sulfide precipitate in the sample aliModel G pH meter, throw switch on instrument so that dial will quot, using the following equation: read pH.) ,Idd 0.4M boric acid solution until pH reads 10.8. ,4110~bulk of precipitate of silver sulfide to settle, and transfer in solution to a 250-ml. volumetric flask. Wash the precipitate remg. = vO1. Of normalit,y clgNOa,In'. of AgN03 X 16 (2) maining in the beaker with a t least three changes of water. The solution in t'he volunietrir flask does not have to be perfectly Calculate weight of cyanide ion found in sample aliquot: clear, for small amounts of silver sulfide have no effect on bhe results. Make up volume to 250 ml. with water and mix. Trans[ C y ]- found, mg. uncorrected = current produced by sample X fer a portion of this solution of an H-t'ype polarographic cell havvolume to rvhich sample portion diluted X wt. of [CN]- present ing a calomel half cell built in the cell and connected t'o the in standard, mg./current produced by standard X volume t o sample-containing portion by a conventional agar plug. Insert nrhich standard was diluted (3) dropping mercury electrode into the solution and then bubble the The volume to which the sample was diluted and volume to solution 5 minutes with nitrogen to remove dissolved oxygen. Set dials on polarograph, so that an anodic wave can be recorded, which standard was diluted cancel, as the procedure recommends that in both cases the volume be made up to 250 ml. with the initial voltage being set a t about -0.8 volt and the apKnowing the amount of the sulfide precipitate and the amount plied voltage across the cell increasing in a positive direction. (For a Sargent Model X X I polarograph, flip the switches SO t.hat of the cyanide ion found, use the correction graph, Figure 1, to they are on opposed, positive, and 3-volt span, and manipulate find correction factor. the dials, so that voltmeters read 2.0 volts on bridge, and 0.8-volt Calculate concentration of [CN]- in sample: initial voltage.) Obtain polarogram over the range of -0.8 volt
8"
g: E 5H
:z E2i:01
%: "0%
s
V O L U M E 2 7 , NO. 1, J A N U A R Y 1 9 5 5 Table IV.
41
Summary of Accuracy and Reproducibility
(Concentration of principal components, grams per liter) Cyanide Ion _________ 0.02 0.10 AI.. C S - found, &/I. St. dev. (single det.), 4~ St. dev. from true value.
*
% error
1.00
0.10 0.018 0.0022
3.00 0,019 0.0022
30.0 0,021 0.0020
0.10 0.097 0.0047
3.0 80.0 0.10 0.090 0.097 1.01 0 . 0 0 6 4 0 . 0 1 4 1 0.0250
3.00 30.0 1.01 1.04 0 . 0 4 0 7 0.0760
0.0025 12.5
0.0026 13.0
0.0023 11.5
0.0067
0,0073 7.3
0.0430 4.3
Concn. of [CN] - in original sample, g./l. = uncorrected [ C S ]found, mg. X correction factor/sample aliquot, ml. (4) EVALUATION OF RESULTS
Statistical Data on Synthetic Samples. The results of each of the 61 samples (which contained all possible combinations of three levels each of cyanide concentrations, thiosulfate concentration, and sulfide concentration, and two levels of sodium hydroxide concentration) were multiplied by the appropriate correction factor based upon the amount of cyanide and sulfide found in the aliquot. The corrected values are reported in Table 11. The analysis of variance (I%’), given in Table 111, and the corrected data in Table I1 show that the [CS]-variation is now the only significant one. The 5% level of significance of [CN] X [SI X [OH], the [CY] X [SI X [S203], and the [CX] X [SI X [OH] X [S203]interactions, indicate that these interactions are atatistically, but probably riot chemically, significant. A summary of the results showing the accuracy and reproducibility of the over-all method, using the correction factors, is presented in Table IV. Plant Samples. In the absence of any plant sariiples of known cyanide ion content, the method was evaluated by adding known amounts of sodium cyanide t o plant samples. The results of such a series obtained on 12 different samples from sis sample points are shown in Table V. Reasonable recoveries of the computed amounts of cyanide ion were obtained. Some results, however, are low by as much as 17%. This procedure is believed to be actually better than this method of evaluation indicates, becauw certain plant samples contain some contaminant 1%hich reacts with the cyanide ion. Because this reaction is not instantaneous but is partially dependent upon the concentration of the cyanide ion, any increase in the cyanide ion concentration would result in an equilibrium shift h Sample of this type, in which some reactive sulfur types may be present, is continually changing. T o illustrate this instability a typical sample of this type was selectcd and allom-ed to stand in the laboratory in a screw-top glass bottle for 25 days. Free cyanide determinations were carried out periodically. The data presented in Table VI show that the free cyanide content had
5.7
0.0148 14.3
0.0261 2.61
0.0840 8.4
fallen to about one third of the original concentration in 25 days. Also, the sulfide ion concentration decreases The sulfide was thought to be slowly osidated by air to a polysulfide which, in turn, reacted with the cyanide to form a thiocyanate. Qualitative tests for both the polysulfide and the thiocyanate have been obtained on these samples which have been allowed to oxidize. This emphasizes the importance of protecting samples from oxidation and analyzing them as soon as possible.
Table VI.
Loss of Free Cyanide on Storage
( I n plant sample containing sulfide ion) Cyanide Ion, G./L. Sulfide Ion, G./I.. Day9 of Storage“ 0 1.17 10.6 3 1.16 9.8 7 0.94 8.4 25 0.36 4.4
*
Stored in screw-top glass bottle.
Although the use of these correction factors does not yield results as precise as may be desired, the over-all method is sufficiently accurate and precise to be usefully employed in plant control for many practical p u r p o w . ACKZiOW LEDGBIENT
The authors thank C. T . Shewell for his valuable assiytance in formulating the statistical design and for preparing the analysis of variance table and data, and the Humble Oil and Refining Co. for permission to publish this paper. LITERATURE CITED
(1)IBonner. K A., Bornham, H. O., Conradi, J. J., and Skei, T.. ‘‘Prevention of Hydrogen Attack on Steel in Refinery Equipment,” American Petroleum Institute, Division of Refining, New York. N. Y., RIay 12, 1953. (2) Dodge, B. F., and Zabbon, ‘Ar., PZating. 39, 1133-9, 1235-41 (1952). . 24, 1440 (1952) (3) Fisher, F. B., and Brown, J. S., A N ~ LCHEM., Ibid., 20, 911 (1948). (4) Karchmer, J. H , and Dunahoe, J. W., (5)
Kolthoff, I. AI., and Furman, 11. H., “Potentiometric Titrations.” 2nd ed.. p. 159, Wiley, S’ew York, 1931. (6) Kolthoff, I. AI., and lliller. C. S..J . Am. Chem. Soc.. 63, 1405, 2732 (1941).
,
Table V. Analyses of Plant Samples Grams per Liter
Sample
CN-
Sulfide. found
Found
Added
Tot.CN-, found
Wt. 3’%
2.32 2.68 8.2 7.5 10.4 10.4 2.3 4.2 4.6
0.036 0.046 0.15 0.17 0.81 0.81 0.0 0.0 0.0
0.037 0.187 0.46 0.28 0.38 0.77 0.18 0.037 0.037
0.070 0.230 0.63 0.46 1.07 1.45 0.17 0.035 0.040
104.0 98.6 97.0 102.1 83.0 91.8 94.5 94.6 108.0
10.0 9.0
0.41 0.37
0.93 0.37
1.15 0.65
86.0 87.9
Depropanizer overhead D-3-2 11 10.1 0.82 0.38 accumulator water D-3-2 12 10.9 0.91 0.46 No. 5’ is the same as 5 excelit a different amount of S a C N was added.
1.17 1.22
97.5 89.1
Description
Point
No.
Cat. cracker condensate accumulator water
D-3-1 D-3-1 D-22-1 D-22-1 D-2-2 D-2-2 B.6. B.S. B.S.
1 2 3 4 5 5”’
Depentanizer overhead accumulator water
D-23-1 D-23-1
6
6
9 10
Rec,
( 7 ) Kruse, J. AI., and llellon, 31. G., -%VAL. CHEM., 25, 446 (1953). (8) Lingane, J. J.,and Laitinen, H. A., IND. EXG. C H E V , A N ~ L ED., . 11, 504 (1939). (9) Scott, W.IT., “Standard Methods of
Chemical Analysis,” 5th ed., p . bv H. Furman. Van Nostrand,-Nefi: York, 1949. (10) Serfass, E. J., Freeman, R. B., Dodge, B. F., and Zabbon, W., Plating, 39, 661. ed.
267-73 (1952). (11) Smith, R. G . , J. A m . Chem. SOC.,51, 1171-4 (1929). (12) Youden, W. J., “Statistical Nethods
for Chemists,” Wiley, Kew York, 1951.
RBCEIVEDfor review November 30, 1953. Accepted September 7, 1954.
