1183
Anal. Chem. 1980, 52, 1183-1186
Pneumatoamperometric Determination of Nanogram Amounts of Cyanide Premysl Beran' and Stanley Bruckenstein' Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214
Part-per-biiiion levels of cyanide react rapidly and quantitatively with excess iodine at pH -6.0 to form iodine cyanide. The excess iodine is selectively removed from solution by nitrogen purging. Acidification of the resultant iodine cyanide solution with perchloric acid in the presence of iodide to a final concentration of 3.0 M HCi04 and 1.0 X M I- rapidly and quantitatively regenerates iodine in an amount equivalent to the original cyanide. This iodine is transferred to the gas phase by purging with nitrogen, where it is determined at a gold gas porous electrode by electro-oxidation to iodate. The detection limit for cyanide is estimated to be 5 ng in a 4-mL sample.
T h e determination of small amounts of cyanide is very important because of its toxic properties. A variety of techniques for determining low levels of cyanide exist and details of spectrophotometric and specific ion electrode procedures recommended for water and wastewater samples are given elsewhere ( I ) . Despite the existence of these and other cyanide determinations, we felt it was worthwhile to examine the utility of pneumatoamperometry as a cyanide determination technique. In pneumatoamperometry, a fuel cell-type electrode structure is used t~ directly determine the concentration of an electroactive species in the gas phase. Thus, if dissolved cyanide could be converted to a volatile, electroactive species, this electroactive product could then be purged from the solution and determined using a gas porous electrode structure. The procedure used in this paper is based on the use of a gold gas porous electrode (Au GPE), which has been shown capable of determining nanogram amounts of volatile species (2, 3) and in particular, iodine ( 2 ) . T h e reaction of iodine with cyanide is well-known, and papers dealing with this subject date back to the end of the last century ( 4 ) and the early parts of this century (5-7). The equilibrium 12
+ HCN e H+ + I- + I C N
(1)
is p H dependent ( K = 0.73 ( 8 ) ) ,and the p H dependence is the basis of the determination developed herein. T h e reaction between iodine and cyanide is quantitative over a wide p H range, 2-7, although kinetics become detectable a t lower p H values. Thus we form ICN by reaction of cyanide with excess iodine a t p H -6. T h e presence of excess iodine guarantees that any reducing agents capable of reacting with ICN (or 12)at p H -6 are oxidized. The excess iodine is removed from solution by purging with nitrogen for a few minutes. At p H 6.0, the amount of iodine in equilibrium with ICN through Equation 1 is extremely small. Hence, the rate of removal, by purging, of iodine produced by the dissociation of ICN is negligible and only excess iodine is removed. At this stage, the solution contains the same molar quantity of ICN as the amount of cyanide originally present, and a t least an equivalent amount of iodide. From Equation 'Permanent address: Department of Analytical Chemistry, Charles University, 12840 Prague, Czechoslovakia. 0003-2700/80/0352-1183$01 .OO/O
1,we see t h a t it should be possible to regenerate an amount of iodine equivalent to the ICN by adding sufficient strong acid to t h e ICN solution. This iodine could then be determined by purging it from solution with nitrogen and using pneumatoamperometry. We report here the details of the successful development of this procedure for the determination of cyanide in the range 25 to 500 ng CN- in 4-mL sample volumes.
EXPERIMENTAL Chemicals. All chemicals were of reagent grade quality. Iodine cyanide was prepared according to Kolthoff (9)by reacting a mixture of iodine and mercuric cyanide for several days while exposed to sunlight. Iodine cyanide was sublimed from the mixture and purified by resublimation. Pure water was obtained by passing house distilled water through a Millipore system. Nitrogen was obtained from the boil-off of liquid nitrogen. Equipment and Apparatus. The gold rotating disk electrode used had an area of 0.49 cm2. It was polished using standard techniques. The three-electrode potentiostat was a conventional operational amplifier design with a ground referenced currentto-voltage converter. The Au GPE used in the pneumatoamperometric studies has been described previously, as has the cell, the reference saturated calomel electrode, salt bridge, and counter electrode ( 2 , I O ) . The cell supporting electrolyte was 1 M sulfuric acid. All potentials are reported vs. the saturated calomel electrode. The pneumatoamperometric determination of iodine was performed at 1.35 V, at which potential iodine is oxidized to iodate (2).
Current-time curves were recorded using a Health-Schlumberger Model SR-255B recorder. Gas flow rates were controlled using a Union Carbide Linde Division flowmeter. Cyanide Determination Procedure. The apparatus used is shown in Figure 1. As required, buffer and cyanide, iodide and iodine solutions are introduced into the Reaction Vessel, RV, through inlet Port I with a pipet, or using a hypodermic syringe fitted with a needle through Inlet Port I, which is fitted with a rubber septum. Nitrogen enters RV when Stopcock S1 is in position 2 and purges the contents of RV when it escapes from a gas dispersion tube sealed to the gas inlet of RV. Solutions are drained from RV using Stopcock S5. RV is cleaned with water after opening inlet I of RV, with Stopcock S5 open. The tubing between RV and the syringe SY is cleaned by closing S5, filling RV with water, S4 in position 1,sucking water from RV into SY and emptying SY again. Finally, when RV is washed and empty (S5 is closed), water in the tube between RV and S4 is sucked out with SY and then S4 is closed. Strong acid, used in the conversion of iodine cyanide to iodine, is added to RV by filling beaker B with thoroughly nitrogen-purged 6 M perchloric acid, filling SY with Stopcock S4 in position 2, changing S4 to position 1, and injecting into RV. T is a trap required to catch any spray produced in RV during the nitrogen purging step. EV is the electrochemical cell and consists of a gold gas porous electrode, Au GPE, a reference electrode, Ref, and an auxiliary electrode, Aux. V is a vessel which is an exact duplication of RV, except that there are no provisions for adding 'strong acid. V is required because at some steps in the cyanide procedure, nitrogen must be diverted from RV. When the nitrogen is diverted into the atmosphere, the pressure at the Au GPE changes, producing a 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
AuGPE Aux ?
9
a a
Ref
- 9
V Y
50
n
Ne-
06
04
02
0
-02
-04
-66
v Figure 2. Voltametric curves of iodine and iodine cyanide-Au rotating disk electrode; w = 2500 rpm; scan rate, 0.2 V/min; scan +to- and -to+ Curve 1: 0.05 M H,SO,, 2 X M I,; curve 2: 0.05 M H,S04, E,
EV
Figure 1. Sketch of apparatus change in the background response of the Au GPE. This problem is largely alleviated by diverting the nitrogen gas stream through V which is filled with an appropriate volume of distilled water or a solution similar to that in RV. The steps in the cyanide determination and the positions of the various stopcocks are given below. A nitrogen flow rate of about 12 mL s-l is used. Step 1. Adjust the stopcocks to positions S1-2, S2-1, S3-2, S4-closed and S5-open. Next remove the inlet rubber septum to RV; then wash RV with distilled water using a washbottle through open inlet. Step 2. Adjust the stopcocks to positions S1-1, S4-1,S5-closed, S2 and S3 unchanged. Fill RV with water through I, empty RV into SY and then empty SY into RV. Step 3. Adust the stopcocks to positions S4-closed, S1-2, and S5-open. Repeat steps 1 and 2 three times. Next, after draining RV, with stopcock S4-1, suck a little gas into the tube between RV and S4 using S Y and then close S4. Step 4. Adjust the stopcocks to positions S5-closed,S1-2, S2-1, S3-1,and S4-unchanged. Close inlet I of RV with a rubber septum. The reaction vessel is now prepared for the next measurement. Step 5. Load SY with the required amount of deoxygenated 6 M perchloric acid from beaker B with Stopcock S4 in position 2 and then close S4. To prepare a calibration curve follow steps 6 and 7 . To analyze an unknown in a volume, follow steps 6’ and 7‘. Step 6. Adjust stopcocks to positions S1-2, S2-1, S3-1, S4closed, and S5-closed. Add 4.0 mL of pH -6.0 phosphate buffer (0.01 M) and 50 gL of 0.01 M iodide solution to RV through I. This step mixes the buffer and iodide and removes oxygen. Wait about 60 s. Step 7. Adjust stopcocks to positions Sl-1, S3-2, and all others unchanged. Add 100 p L of saturated iodine and up to 10 pL of 2.00 X or 2.00 X M KCN samples through I. Wait 30 s for ICN to form. Go to step 8. Step 6’. Adjust stopcocks to the same positions used in step 6. Then add the alkaline cyanide sample. Step 7’. Adjust stopcocks to the same positions used in step 7 . Then add concentrated phosphate buffer and water to bring the pH to -6 and volume to 4.0 mL. Add 50 pL of 0.01 M iodide solution. The phosphate concentration should be chosen to obtain a phosphate level close to 0.01 M at the final dilutions. Add 100 pL of saturated iodine solution. Wait 30 s for ICN to form. Step 8. Adjust stopcocks to positions S1-2, S3-2, and all others unchanged. Purge contents of RV with nitrogen for 5 to 20 min to remove excess unreacted iodine. Step 9. Adjust stopcocks to position S3-1 and all others unchanged. Wait a minute or so for the pressure change induced in the Au GPE current to subside and check that all excess iodine has been removed by noting the Au GPE current. Step 10. Adjust stopcocks to positions Sl-1, S2-1, S3-1, S4-1, and S5-closed. Inject 4.0 mL of 6 M perchloric acid from SY and close 54. Wait 5 min for ICN to react with I- and H+ to form 12, Wait 10 min for very low cyanide concentrations. Step 11. Adjust stopcocks to positions S1-2 and all others unchanged. Purge iodine from RV and then record Au GPE current-time curve.
RESULTS AND DISCUSSION Reaction of 12 and CN-. Iodine is reduced (E112= 0.55) more easily than ICN ( E l I 2= -0.2) a t a rotating gold disk
2 X
M ICN
Figure 3. Formation and stability of iodine cyanide in phosphate buffer (pH -6.5). Au rotating disk electrode; w = 2500 rpm; scan rate 1 V/min; scan +to-; curve 1: 500 mL 0.1 M phosphate buffer, pH -6.5, 5X M I,; curve 2: as curve 1 + 0.3 mL 0.5 M KCN; curves 3 to 7 recorded after blowing N, through the solution for: curve 3, 15 min; curve 4, 30 min; curve 5, 60 min; curve 6, 90 min; curve 7, 210 min. Voltage origin is +0.86 V electrode as seen in Figure 2. Thus, the current-potential, i-E, curves of mixtures of iodine and cyanide provide a simple measure of their reaction to form ICN. Figure 3 illustrates the i-E curves found by adding sucessive amounts of potassium cyanide to a solution of iodine at p H 6.5. T h e reaction forming ICN and iodide ion according to Equation 1 is evident = -0.2 from the appearance of a cathodic wave having V and the appearance of an anodic current at potentials more positive than 0.50 V. The disappearance of iodine is seen by the decreased current in the range 0.6 V < E < 0.0 V. Studies of the sort shown in Figure 3 indicate ICN can be formed quantitatively over the p H range 2 to 7. We chose a p H range of 5.5 to 6.8 as the optimum for ICN formation because (1)the rate of reaction to form ICN becomes measurably slow a t low p H values, and (2) we were concerned that a t high p H values the disproportion reaction
I2 + OH-
-
IO-
+ H+ + I-
(2)
would complicate our analytical scheme. If appreciable amounts of hypoiodite were to form, it would provide a source of iodine on acidification of the mixture, and lead t o high results in a cyanide determination. Volatility of ICN. The limiting current for 5.0 X ICN a t a gold rotating disk electrode changed less than 2% when 400 mL of solution were purged with nitrogen at a rate of 160 mL/min for 2 h. Under the same conditions, iodine was completely volatilized. Thus, i t is possible to remove excess iodine from a solution of iodine cyanide by purging with nitrogen. Effect of Nitrogen Purging Rate on Au GPE Response. This experiment was performed by injecting 100 pL of saturated, aqueous iodine solution into 4 mL of water in the reaction vessel, and purging the solution with nitrogen over a range of gas flow rates. T h e Au GPE current-time curves
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
1185
I6
I 4
=L4
c2 3 w E
5 2 u
up. t
s
TIME
Figure 4. Iodine i-t curves at different nitrogen flow rates. 4 mL 1 M HCIO,, lO-’M I,; EAuGpE = 1.35 V vs. SCE; N, flow rate in mL s-’; curve 1, 1.5; curve 2, 3.3; curve 3, 5.3; curve 4, 7.5;curve 5, 10.0; curve 6,12.0, curve 7, 14.2; vertical bars on abscissa indicate times of 1, injection
,---__ -
5
10
TIME, minutes
Figure 6. Decomposition of ICN after addition of 6 M HCIO, E,, GpE = +1.35 V vs. SCE; N, flow rate, 12 mL s-’; 4 mL 0.01 M phosphate M ICN; after adding 4 mL buffer 50 pL lo-* M I- 10 pL 2 X 6 M HCIO,, nitrogen flow is stopped for variable time; curve 1, 10 min; curve 2, 7 min; curve 3, 4 min; curve 4, 2 min; curve 5, 1 min; curve 6.30 s; curve 7. 15 s.; curve 8. 5 s
+
+
TIME, minutes
Figure 5. Iodine i-tcurves at different iodide concentrations. E,, = +1.35 V vs. SCE; N, flow rate, 12 mL s-’; 4 mL 1 M HCIO4 with variable concentrations of iodide 30 pL 1.3 X 10-3 M I,; curve 1, 5 X io-, M I-; curve 2, M I-; curve 3, 2 X M I-; curve 4,
+
4 x 10-~ M I-
obtained a t E = 1.35 V are shown in Figure 4. As can be seen the current peak, i, is constant in the gas flow range -5 to 12 cm3 The i-t curve depends on two flow rate dependent factors: (1)the rate of equilibration of iodine in solutions with the gas and (2) the response of the Au GPE electrode. The latter has been discussed elsewhere (IO) and the former is unimportant since equilibrium is reached under the specified flow conditions. The main observation is that close regulation of the gas flow rate is not required to obtain reproducible current peaks. Effect of Iodide Concentration on Au GPE Peak Currents. As mentioned in the introduction, it is possible t o have variable amounts of iodide formed in the step t h a t produces ICN if reductants are present in the unknown sample. Also, as shown below, regeneration of iodine from ICN by adding strong acid is made more quantitative by adding a suitable excess of iodide to the unknown solutions. Thus, it was necessary to study the effect of iodide ion on the pneumatoamperometric response for a known amount of iodine. Figure 5 presents the Au GPE i-t response obtained for 10 pg of 12/4.0mL sample as a function of iodide ion concentration, CI-. I t is apparent that as CI- increases, i, decreases and the i-t curve broadens. Clearly, this phenomena is related to the formation of Is-. An iodide concentration of 1.0 x M was chosen as the optimum CI-, consistent with obtaining a maximum value of i, and quantitative reconversion of ICN back to iodine. Experiments of the sort shown in Figure 5 yielded the information necessary to determine the purging time required to remove the excess iodine added in the step that converts cyanide to ICN. For samples containing 100 to 500 ng CN-, purging for 4 to 5 min proved satisfactory, while for the lowest
TIME, minutes
Figure 7. Influence of perchloric acid concentration of Au GPE current response. E,, c9E = +1.35 V vs. SCE; N, flow rate, 12 mL s-’; 4 mL phosphate buffer and 50 pL lo-’ M I- 10 pL 2 X lo3 M ICN. After
+
adding variable amount 6 M HCIO,, N2 flow through the vessel is stopped for 30 s. Curve 1, 1 mL 6 M HClO,; curve 2, 2 mL 6 M HCIO,; curve 3, 4 mL 6 M HCIO,
levels of CN-, as many as 10 to 20 min were found necessary. Effect of H+ on Conversion of ICN to 12. Experiments were performed on ICN solutions containing various amounts of I- using differing amounts of phosphoric, sulfuric, and perchloric acids. Phosphoric acid proved too weak an acid to quantitatively regenerate iodine. Sulfuric acid in large amounts (4mL concentrated acid to 4 mL water) regenerated iodine, but in the absence of cyanide produced high blanks, presumably by oxidation of iodide present in the buffer. Perchloric acid was selected as the reagent for the regeneration process since it yielded the lowest blank response a t a concentration suitable for Droducing iodine rapidly from ICN. Figure 6 shows the Au GPE current-time response found 5 s-10 min after adding the specified volume of 6 M HClO., to 4 mL of solution containing 5.0 X lo4 M ICN, 1.4 X M I- and 0.01 M phosphate buffer (pH -6). Figure 7 shows the effect of CH+on the conversion of 5.0 X lo4 M ICN to 12.The procedure used involved adding 1.0, 2.0, and 4.0 mL of 6.0 M perchloric acid to 4.0 mL of sample (pH 6.0) containing iodide and purging 30 s afterwards. The optimum amount of acid is 4 mL. Rate of Regeneration of Iodine from ICN. T h e preceding experiments indicate that the regeneration of iodine from iodine cyanide according to the reverse of Equation 1
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8,JULY 1980 11
I
I
10
ii 3
U
i
6
I
4
TIME
Flgwe 8. Au WE, current-time response using recommended analytical conditions. EA"GpE = +1.35 V vs. SCE; N, flow rate, 12 mL s-'; 6 mL phosphate buffer and 50 pL lo-' M I100 mL saturated aqueous I, solution and variable amount of CN- (ng): curve 1, blank; curve 2, 50; curve 3, 100; curve 4, 150; curve 5, 200; curve 6, 250; curve 7, 300; curve 8, 350; curve 9,400, curve 10,450; curve 11, 500. Vertical
+
bars on abscissa indicate times of is 9.0 pA
I,
injection. Curve 1 1 peak current
-
should be carried out in a solution containing 1.00 x M iodide and 3.0 M perchloric acid. The procedure given in the Experimental section uses these conditions. In this procedure, an equal volume of 6.0 M perchloric acid is added to the iodine cyanide solution containing iodide. Experiments in which the purging of this mixture was delayed for various amounts of time after adding the perchloric acid solution demonstrated that, after 5 min, the peak Au GPE currents were identical. A 5-min delay was chosen for the analytical procedure a t higher levels of cyanide. A 10-min delay is recommended a t the lowest levels. Calibration C u r v e a n d Recovery of Cyanide. Using the procedure given in the Experimental section, the Au GPE current peak was determined for a range of cyanide levels. Typical current-time responses are shown in Figure 8. The Au GPE peak current response is linear in the range 10 to 500 ng CN-, the range investigated. The slope of the plot of peak current vs. amount of cyanide is 15.4 nA/ng. There was a positive intercept of 500 nA, which would correspond t o a blank of 32 ng of cyanide. Trace oxidizing agents capable of reacting with the excess iodide present in solution could give rise to this blank on adding the perchloric acid. Ten replicate experiments yielded a standard deviation of 8.7 ng a t the
300-ng level (3.1%) and 5.5 ng a t the 40-ng level (14%). Within the experimental error, the recovery of cyanide was 100%. Using pure ICN, the Au GPE peak current response, after conversion of ICN to iodine by acidification, was found to be identical to that of an equivalent amount of cyanide (100 ng) carried through our procedure. Also the peak current for iodine produced from iodine cyanide corresponded to that calculated for an equivalent amount of pure iodine. Interferences. Interferences from a number of species were checked for concentration ratios, R = CCN-/CX where X is the possible interferent and CcN-= 2.5 X M (26 ng/4 mL). For R = 1.5, nitrate, chloride, and silver did not interfere. If R < 1.5, silver cyanide precipitated and interfered. There was a slight positive interference in the presence of Ni2+,Pb2+, and Zn2+a t R = 2. Cu2+does not interfere a t R = 2. CNSproduces a strong positive interference as does NOz-. Pd2+ is a strong negative interferent. S2- and S03z- does not interfere, provided sufficient excess iodine is used. I t was not possible to determine cyanide in ferrocyanide, ferricyanide, and cobalticyanide. If a complex cyanide is to be determined, one of the existing digestion and distillation procedures should be employed prior to the pneumatoamperometric procedure ( I ) . The recommended lower level for determination of cyanide with reasonable accuracy using the pneumatoamperometric procedures is -25 ng (6 ppb). If lower levels are t o be determined, concentration by distillation would be required. LITERATURE CITED (1) A m e r l l n Public Health Association, America; Water Works Association, and Water Pollutlon Control Federatlon. Standard Methods for the Examination of Water and Wastewater", 14th ed. American Public Health Assoc.: Washington, D.C. 1976; pp 361-386. (2) Gifford, P. R.; Bruckenstein, S. Anal. Chem. 1980, in press. (3) Glfford, P. R.; Bruckenstein, S . Anal. Chem. 1980, in press. (4) Savbert, K.; Pollard, W. Ber. 1980, 23, 1062-1065. (5) Chattaway. F. C.; Wadmore, J. M. J. Chem. Soc. (London) 1902, 8 1 , 191-203. (6) Lewis, G. N.; Keves, D. B. J. Am. Chem. SOC. 1918, 40, 472-478. (7) Yost, D. M.; Stone, W. E. J. Am. Chem. Soc. 1933, 55, 1889-1895. (8) Kolthoff, I.M.; Belcher, R. "Volumetric Analysis", Volume 11; Interscience Publishers, Inc.: New York, 1957; p 302. (9) Kolthoff, I. M. Fresenlus' 2.Anal. Chem. 1920, 59, 401-415. (IO) Kosek, J.; Bruckenstein, S . Final Report, U.S. Department of Interior, Bureau of Mines. Conbact No. GO155007, Electrochemical Gas Sensors for Mine Atmospheres, February 1979.
RECEIVED for review March 3,1980. Accepted April 21,1980. This research was supported by Contract No. AFOSR 783621A from the Air Force Office of Scientific Research.
