Anal. Chem. W81, 53,1711-1712
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Determination of Cyanide by Conversion to) Ammonia and Ultraviolet Absorption Spectrometry in the Gas Phase Sir: The cyanide ion lhas been determined by a variety of methods, including titration with silver, spectrophotometry, ion-selectiveelectrodes, coulometry, and others. As far as users of atomic absorption instrumentation have been concerned, cyanide was determined indirectly, for example by measuring silver, iron ( I , 2),or copper (3) associated with a cyanidemetal complex. Recently we reported a method for the determination of the ammonium ion by evolution of ammonia and ultraviolet absorption spectrometry in the gas phase (4). In that work, an atomic absorption spectrophotometer was modified to function as a convenient nonflame molecular absorption spectrophotometer, a glam reaction vessel was used to evolve ammonia, and measurements of transient absorbance were made in the gas phase, Similar apparatus and approach have also been used by one of the authors and co-workers in the determination of nitrite evolved as NOCl(5), of sulfide evolved as H2S (6),of sulfite evolved as SO2 (7,8), and of iodide and bromide evolved as 1, and Br, (9). The purpose of the present paper is to report a direct, rapid, and sensitive method for t h e determination of cyanide using the same technique. Simple sample treatment rapidly converts cyanide to the form of the ammonium ion and analysis is completed by the reported (4) ammonium method. EXPERIMENTAL SECT1 ON Apparatus. Absorbance measurements were made on the Perkin-Elmer Model 460 atomic absorption spectrophotometer and recorded on a 10-mV strip chart recorder. The burner head was removed and replaced by a holder supporting a 15 cm long quartz-windowed flow-through glass absorption cell. The deuterium arc lamp served as the source of radiation absorbed by the gaseous ammonia which was generated in the reaction vessel and carried through the absorption cell by a stream of carrier gas. Absorbance was measured at 194 nm with a slit setting of 2 nm. The reaction vessel is illustrated in Figure 1 of ref 6. The volume of the cylindrical glass vessel is approximately 60 mL. Aliquots of NaOH solution are delivered into the vessel from a buret via a side arm. Ammonium-containingsamples are injected by means of a 1-mL Hamilton syringe through an injection port covered with a rubber septum. Nitrogen enters the reaction vessel through a glass tube extending almost to the bottom of the vessel. It flows at a rate of 1.5 L/min and sweeps the evolved ammonia to the absorption cell. The carrier gas is never turned off while the apparatus is in use, causing continuous flushing of the reaction vessel and of the absorption cell. Spent reagents are removed quickly via the stopcock at tlhe bottom of the reaction vessel. The operations of injecting the sample, recording the maximum of the absorbance signal, draining the vessel, and refilling it with a fresh aliquot of base take less than 1 min. Procedure. An aliquot of CN--containing solution is pipetted into a 100-mL volumetric flask. Approximately 10 mL of 10 N NaOH and 15 mL of 0.12 M KMn04 are added. The solution is swirled and allowed to stand about 1 min. Approximately 10 mL of 16 N H2S04is then carefully added, the solution is cooled and diluted to volume with distilled water. The concentration of t h e ammonium ion generated in the resulting solution is evaluated by comparison with aqueous ammonium standards prepared by dissolving ammonium chloride in water and making appropriate dilutions. One-milliliter injections of samples and standards are made in triplicate into fresh 6.0-mL aliquots of 10 N NaOH in the reaction vessel. Signals are taken from the continuously recorded absorbance traces by measuring the height of the recorded maximum from the base line preceding the injection. The complete description of the procedure for evolution and measurement of ammonia by this technique is given in detail on page 143 of
ref 4. The general appearance of the recorded signal is illustrated in ref 5. Caution: Addition of acid to incompletely oxidized cyanide results in the evolution of the highly toxic HCN gas. Sample treatment should be done in a fume hood.
RESULTS AND DISCUSSION Converigion of Cyanide to Ammonium Ion. Cyanide can be converted to the form of ammonium ion by a simple two-step procedure. First, cyanide is oxidized with an excess of KMn04 under conditions of high pH resulting in the formation of cyanate (IO)
CN-
+ ?,Mn04-+ 20H-
-
CNO-
+ H20 + 2Mn04"
Second, the cyanate solution is treated with an excess of acid yielding ammonium ion (11)
CNO-
+ 2H+ + 2H20
-
NH4+f H2C03
The analysis step is represented by the reaction
NH4+ + OH-
+
NH,(g)
+ HzO
The effects of several experimental conditions on the completeness and speed of these reactions were checked experimentally. The results follow. (a) Oxidation of CN- to CNO-. To a series of 10.0-mL aliquots of 1444 pg/mL CN- were added 0-12-mL aliquots of 10 N NaOH, followed by 15 mL of 0.12 M KMn04. After 5 min, 15 mL of 14.5 N HzS04was added, all solutions were diluted in 100 mL, and the NH4+concentration was evaluated. Whereas the sample which received no base permitted only 70.9% complete conversion to NH4+,all samples in which the pH was raked above about 12.5 yielded close to 100 pg/mL NH4+,indicating an average 99.4% complete conversion. A &fold molar excess of KMn04 over CN- was adequate for quantitative and rapid oxidation and no adverse effects were deteded at a 10-fold excess. Obviously, if an insufficient quantity of permanganate is added to the sample, free cyanide will be left in solution and subsequent addition of acid would cause evolution of the highly toxic and volatile hydrogen cyanide whose loss would result in low results for the analysis. The deep purple color indicative of the presence of excess permanganate should be evident when the acid is added. The oxidtition of cyanide is complete in less than 1min after mixing the reagents. When the time elapsed between the addition of KMnO, and the addition of H#04 was varied over the range of 45-720 s, complete conversion of CN- t o NH4+ was obtained throughout. (b) Conversion of CNO- to NH4+. Treatment of both standard cyanate solutions (NaCNO) and of oxidized CNsamples confirmed the expectation ( 2 1 ) of rapid and quantitative conversion to NH4+. The amount of excess H2S04 added made no difference in the results and the reaction was complete before the experimenter could finish the volumetric dilution and inject sample aliquota into the analysis apparatus (less than 2 min). Precision, Accuracy, and Sensitivity. Ten 10.0-mL aliquots of 1.44 X lo3pg/mL CN- solution were each treated with 8 mL of 10 N NaOH, 15 mL of 0.12 M KMn04, followed (after 5 minj by 8 mL of 14.5 N HzS04 and dilution to 100.0 mL with distilled water. Ten 10.0-mL aliquots of 148 p g / d CN- solution were also treated similarly. For analysis, each solution was injected four or five times and the signals were averaged. The higher concentration solutions were evaluated by direct comparison to a 100.0 pg/mL NH4+standard. The
0003-2700/81/0353-1711$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53,
NO. 11, SEPTEMBER 1981 -
-
Table I. Reproducibility and Accuracy of the Proposed Method repeated runs using the 1.44 X l o 3 pg/mL CN- solution __ CN- concn detected, pg/mL (144 pg/mL present) error, pg/mL CN_ I _ _ I _ _ _ _ _ _
____
146 150 147 149 148 136 134 142 135 148 av = 143.5 st dev = 6.3 pg/mL CNre1 st dev = 4.4%
+2 .t6 +3 +5 +4 -8
-10 -2 -9 +4 av = k5.3 re1 av error = 3.7%
___ Table 11. Effect of Concomitant Anions absorbance, peak height test anion effect of test plus 145 conanion pg/mL comitant, alone CN%
none 1001 pg/mL SO,*1005 pg/mL Br1002 pg/mL c11000 pg/mL CO,Z1000 pg/mL SCN1207 pg/mL NO; 1001 pg/mL I980 pg/mL NO;
0.000 0.000 0.000 0.000 0.300 0.000 0.000 0.000
_ _ I _
15.0 14.6 14.5 14.8 14.6 15.0 15.0 14.8 14.8 14.5 av = 14.76 st dev = 0 . 2 pg/mL CNre1 st dev = 1.4%
-
__
concn of added anion (as Na or K salt)
repeated runs using the 148 pg/mL CN- solution __ CN- concn detected, &/mL ( 1 4 . 8 p.g/mL present) error, pg/mL CN-
0.161 0.158 0.000 0.136 0.153 0.355 0.151 0.154 0.155
-1.9 -100 -15.5 -5.0 +120 -6.2 -4.3 -3.7
lower concentration solutions were evaluated from a calibration curve prepared with NH4+standards ranging from 2.72 to 21.76 pg/mL NH4+. The results are given in Table I. Whereas the true concentrations of CN- in the pretreated sets of samples were 144 and 14.8pg/mL, respectively, the averages of the two sets of ten experimental values were 143.5and 14.76 pg/mL. The average error for -nindividual determination was 3.7% at the 144 pg/mL CN level and 1.1%at the 14.8 pg/mL level. The precision of 10 repeated analyses is reflected in relative standard deviations from the mean of 4.4% and of 1.4%at the respective concentration levels. The precision of making repeated injections of the same sample was 1.4% and 2.2%,respectively. The blank gave very low, reproducible absorbance signals of
