spectively, 0.0, 0.8, 0.7, 0.7, and 0.7 X 10-6 equivalent per gram. While an alkaline ash was obtained on the combustion of cellulose recovered from the alkaline medium in the distillation apparatus, the number of equivalents of alkali determined in the ash was far too small to account for the equivalents of carboxyl calculated from the equivalents of NH3 liberated. These results lead to the conclusion that the carboxyl group formed on the alkaline hydrolysis of the nitrile is labile under the experimental conditions used. Of added interest is the observation that the Kiliani value for cotton sliver was increased from 2- to 3.5-fold when it was spread thinly over a wire frame and exposed to irridiation from a mercury vapor lamp under conditions described earlier ( 1 3 ) . For a speciment of cotton sliver with an initial Kiliani value of 4.5 x 10-5 equivalent per gram, values of 10 X to 15 x 10-5 equivalent were determined after irridiation. The impression gained from these experiments was that the extent of alteration was dependent on the degree of spreading (13) V. L. Frarnpton, L. P. Foley, and H. H. Weber, Arch. Biochern., 18,345 (1948).
of the sliver over the frame. Moreover, the Kiliani value seemed to reach a limiting level, since larger values were not obtained on exposures up to 139 hours. A fragment (4% of the sliver) was cleaved from the irridiated cellulose when it was steeped in 1% aqueous Na2C03 solution. This cleaved fragment was recovered and observed to have a Kiliani value of 1.2 X equivalent per gram. A further fragment was cleaved (8%) when the sliver was then steeped in 10% aqueous NaOH. This cleaved fraction had a Kiliani value of 3.4 X loF4 equivalent per gram. The residual cellulose had a Kiliani value of 0.5 X equivalent per gram. N o acidic properties were detected in these three fractions or in the irridiated cellulose from which they were derived. ACKNOWLEDGMENT
The technical assistance of I. G. Peters and J. G . Malone is acknowledged. RECEIVED for review August 12,1971. Accepted November 3, 1971. This research was supported in part under Aeronautical Research Laboratory Contract No. A F 33(616)-4.
Determination of Submicrogram Quantities of Mercury by the CouIometric-l odimetric Titration of Cyanide Produced in a Ligand-Exchange Reaction Timothy J. Rohm, Henry C. Nipper,’ and William C. Purdy Department of Chemistry, University of Maryland, College Park, Md. 20742 RECENTLY PUBLIC ATTENTION has been drawn to the mercury content of certain varieties of fish as well as the mercury contamination of streams and rivers. The methods commonly used to measure the mercury concentrations of these samples involve a colorimetric determination with dithizone (1-3) or flameless atomic absorption spectrophotometry (4-6). Both of these methods are very sensitive for the determination of concentrations as low as l to 2 ppb of mercury in aqueous solution. We have developed a method for the determination of similar concentrations of mercury which does not require the use of an atomic absorption spectrophotometer. The method is more selective than the dithizone procedure. Kinetic and catalytic methods of analysis have been used to great advantage in many qualitative and quantitative deterPresent address, Clinical Laboratories, University of Maryland Hospital, Baltimore, Md. 21201. (1) C . T. Elly, presented at the 18th Anachern Conference, Detroit, Mich., Oct. 1970. (2) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” Vol. 111, Interscience Publishers, New York, N.Y., 1944. DD 72-93. (3) A. C..Rolfe, T. R. Russell, and N. T. Wilkinson, Analyst, 80, 523 (1955). (4) W. R. Hatch and W. L. Ott, ANAL. CHEM., 40, 2085 (1968). (5) C. Ling, ibid.,40,1876 (1968). (6) V. A. Thorpe, J. Ass. O f i c . Anal. Chem., 54, 206 (1971).
minations (7-9). A group of ligand-exchange reactions, that are sensitive to traces of metal ions, has been reported (7,8). In the present study the sensitivity of one of these reactions has been coupled with the accuracy and precision of coulometric titrations to yield a method capable of determining submicrogram quantities of mercury. In the reaction Fe(CN)6-4
+ 3 o-phen
4
+ 6 CN-
Fe(o-phen)3+z
(1)
the iron-bound cyanide is released under the catalytic influence of certain heavy-metal ions which form acid-resisting cyanides. The free iron(I1) is complexed by 1,lO-phenanthroline (0-phen) forming the redox indicator, ferroin. The metals which have been reported to catalyze this replacement are silver and gold (8), mercury(I1) ( I O ) , and palladium (7). Karas and Pinter (10) made colorimetric measurements at 510 nm of the ferroin produced when various amounts of mercury(I1) were used to catalyze the reaction. Their calibration curve of the absorbance us. the logarithm of the mercury(I1) concentration was linear over the range of 1.33 X IO-Bto 17.28 X 10-6M. In the present study, the cyanide ion released by the replacement reaction is titrated with electrically generated (7) F.Feigl and A. Caldas, Anal. Chim. Acta, 13,526 (1955). (8) K. B. Yatsimirskii, “Kinetic Methods of Analysis,” Pergamon Press, New York, N.Y., 1966, p 143. (9) P. W. West, ANAL.CHEM., 23, l(1951). (10) V. Karas and T. Pinter, Croat. Chem. Acta, 30,141 (1958). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
869
e *
LOG MERCURY ADDED
1 RI
51K 12 w
0 .!io 0
" AI 4
w
a 0.30 :
TIMER
m
CELL
a
0
0.20
m
0.1 0
o.ooo
20
IO
MERCURY
3c
ADDED,
Figure 2. Kinetic-colorimetric determination of mercury
1
RECORDER
1
Figure 1. Circuit diagram of the constantcurrent coulometer iodine. A procedure is described for the determination of submicrogram quantities of mercury(I1) and the interference of the other heavy-metal ions is evaluated.
EXPERIMENTAL Apparatus. Absorbance was measured with a Beckman Model DB spectrophotometer using 1-cm silica cells. Coulometric titrations were performed with a constant-current apparatus constructed in this laboratory (see Figure 1). Further details of this apparatus and its operation are described elsewhere (11). A 5-pl Eppendorf pipet was used to deliver the metal-ion solutions. Reagents. All chemicals were reagent grade and were used without further purification. Mercury(I1) stock solutions were prepared by dissolving 0.1345 gram of mercuric chloride in 100 ml of 0.1N sulfuric acid. These stock solutions were diluted by a factor of fifty to yield working solutions which contained 0.1 pg/5 pl. Gold stock solutions were prepared by dissolving 19.25 mg of ammonium chloraurate (NH4AuC14.5Hz0) in 10 ml of water. Palladium stock solutions were prepared by dissolving 100 mg of palladium metal in 4 ml of aqua regia and diluting to 100 ml with water. These stock solutions were further diluted by a factor of ten to yield working solutions containing 0.5 pg of metal ion per 5 pl. Mercury cyclohexanebutyrate solution was prepared by dissolving 0.1462 gram of the dried reagent in 100 ml of cyclohexane. This solution was diluted by a factor of five with cyclohexane to obtain a solution that contained 0.5 pg of mercury(I1) per 5 pl. Silver stock solution, O.O998N, for use in the Liebig-Deniges titration of cyanide was prepared by dissolving 16.963 grams of silver nitrate in water and adjusting the volume to 1 liter. This stock solution was diluted by a factor of one hundred to yield a solution which contained 0.54 pg of silver per 5 111; this diluted solution was employed in the studies of the kinetic reaction. A 0.001M ferrocyanide solution was prepared by dissolving 0.4224 gram of potassium ferrocyanide trihydrate [K4Fe(CN)6.3Hz0]in 1 liter of water. (11) J. H. Ladenson and W. C. Purdy, Clin. Chern., 17, 908 (1971). 870
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
A Log scale
0 Linear scale
1,lo-Phenanthroline solution was prepared by dissolving 0.615 gram of the reagent in 1 liter of water. Cyanide solution was prepared by dissolving 6.15 grams of potassium cyanide in 20 ml of ammonium hydroxide solution, sp gr 0.90, and adjusting the volume to 1 liter with water. The concentration of the cyanide solution was found to be 0.099N by titration with silver nitrate solution using the Liebig-Deniges method. For use in the coulometric titrations, the cyanide stock solution was diluted by a factor of one thousand with 0.01N sodium hydroxide to obtain a working solution that was 9.9 X 10-5Nin cyanide. EDTA solution, 0.05M, was prepared by dissolving 18.61 grams of disodium (ethylenedinitri1o)tetraacetate dihydrate in water and adjusting the volume to 1 liter. Clark and Lubs buffer solutions of pH 3.0 and 3.5 were prepared by dissolving 10.212grams of potassium acid phthalate in 223 ml and 82 ml of 0.1N hydrochloric acid, respectively, and diluting both solutions to 1 liter with water. Borate buffer containing 0.1M potassium iodide was prepared by dissolving 3.8 grams of sodium borate decahydrate (Na2B4O7.10H20)and 16.6 grams of potassium iodide in water and diluting to 1 liter. Procedures. KARASAND PINTER(10) COLORIMETRIC DETERMINATION OF MERCURY.Three milliliters of o-phen solution, 1.5 ml of ferrocyanide solution, 0.5 ml of 0.1N hydrochloric acid, and 10 ml of water were added to a 25-ml Erlenmeyer flask, and the flask was placed in a constant temperature bath at 60 "C for 5 min. At the end of the 5min period, mercury (as HgC12) was added to the reaction flask, and the flask was placed in the bath for an additional 5 min. The reaction was then stopped by placing the reaction flask in an ice bath. The absorbance for each test solution and blank was measured at 490 nm. There was some problem with condensation on the cuvettes since it was necessary to keep the solutions cold or the reaction would resume. COULOMETRIC-IODIMETRIC PROCEDURE.The kinetic procedure of Karas and Pinter (10) was altered slightly so that the cyanide produced could be determined by constant-current coulometry, To 10 ml of the pH 3.0 buffer in a 25-1111 glassstoppered Erlenmeyer flask was added 0.1 to 2.0 pg of mercury. Buffer blanks were run in parallel with each determination. One and a half milliliters of o-phen solution was added to the reaction mixture and the open flask was placed in a 60 O C water bath for 10 min. After this heating period, 3.0 ml of 0.001M ferrocyanide was added, the flask was stoppered and replaced in the bath for 5 min. To stop the reaction, the pH was raised to 12 by the addition of 0.9 ml of 1.ON
Table I. Coulometric Titration of Cyanide CNCNNumber Av dev, added, peq found, peq of runs Error, peq 5 -0.2 0.003 0.495 0.494 0.001 0.396 3 0.0 0.396 0.003 0.297 3 0.0 0.297 0.003 0.198 3 0.0 0.198 0.005 0.101 3 2.0 0.099 Table 11. Reagent Interference in Coulometric Titration of Cyanide CN- added, 1%generated, Number req Interference peq of runs 0.396 None 0.395 2 0.027 3 None 3 ml0. 00lM Fe(CN)s40.430 2 0.396 3 ml0.001M Fe(CN)e41 . 5 ml o-phen 0.394 3 0.396 reagent
Table 111. Mercury Determination by the CoulometricIodimetric Method WII) Ip generated, added, pg Number of runs neq Av dev, neq 0.1 3 63.9 1 .o 0.2 4 120.7 7.4 0.3 4 171.9 12.8 0.4 4 229.8 14.5 297.4 17.1 0.5 10 13.3 0.6 4 332.9 0.7 4 385.9 10.4 0.8 6 412.1 22.8 0.9 5 459.3 11.6 1.o 6 503.3 16.7 1.1 2 548.3 15.3 1.2 2 606.0 2.6 1.3 2 666.2 0.4 26.1 1.5 3 737.4 2.0 3 900.6 19.4
1,000
sodium hydroxide. When this reaction solution was added to 20 ml of the pretitrated borate buffer in the titration cell, the pH fell to 9.2. The determination of cyanide ion was carried out by adding approximately 20 ml of borate buffer containing 0.1M potassium iodide to the cell and titrating until the voltage drop across Rs (see Figure 1) reached some arbitrary value. The coulometer was switched off at this point, the timer was reset to zero, and the sample was added to the cell. The titration was resumed and iodine was generated until the voltage drop reached the same value as before.
0 0)
c
The titration of cyanide in ammoniacal medium with generated silver was briefly investigated. Potentiometric measurements with a silver indicator electrode yielded curves the inflection point of which was difficult to determine. With electronic differentiation of the potentiometric signal, the inflection point could be determined to 1 2 sec, but the results were irreproducible. When a sulfide-ion selective electrode was employed to follow the decrease in cyanide-ion concentration, irreproducible results were again obtained. The potentiometric measurement of cyanide ions was influenced, to a great extent, by the concentration of ammonia and control was difficult. According to Kolthoff and Furman (12):
800
a-
; 600 4
K W
z w
400
W
z
2 RESULTS AND DISCUSSION
++++-+-I
200
2 I J I
0 ;
I I I .o 1.5 MERCURY ADDED, yg
0.5
2.0
Figure 3. Calibration curve for the coulometric determination of mercury
The reaction of iodine with cyanide ion will proceed quantitatively only in alkaline medium. It is also important to keep the concentration of iodide low. The titration efficiency for the reaction was determined to be 100% at pH 9.2 by the
method of Lingane (13). Results of the coulometric titration of cyanide ion are shown in Table I. Also included in this table are the average deviations for the various titrations. The large error (2%) for the 0.099-peq cyanide sample is probably due to imprecise delivery of the sample. The possible interference of ferrocyanide ion and o-phenanthroline on the iodimetric titration of cyanide was investigated (see Table 11). Ferrocyanide ion caused a slight interference in the titration but this interference could be removed by running a blank in parallel with each sample. o-Phenanthroline did not interfere with the cyanideiodine titration. The kinetic-colorimetric determination of mercury was performed in this laboratory at 490 nm, which is somewhat below the absorbance maximum of ferroin in slightly acid solution. Our results (see Figure 2) were in good agreement with those of Karas and Pinter (IO). The blank absorbance was approximately one-fourth that of a sample containing 10 pg of mercury. Although the blank values are reproducible, the large blank-to-signal ratio is of some concern. The coulometric-iodimetric titration method was employed for the determination of mercury samples and the results of these titrations are shown in Figure 3. The portion of the curve between 0.1 and 1.0 pg of added mercury is
(12) I. M. Kolthotf and N. 13. Furman, “Volumetric Analysis,” Vol. 11, John Wiley and Sons, New York, N.Y., 1929,pp 401-402.
(1 3) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed, Interscience Publishers, New York, N.Y., 1958, p 485.
The dissociation of hydrocyanic acid is given by (3)
Combining Equations 2 and 3 yields (4)
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
871
~~
~
Table IV. Runs with Organic Mercurials and Other Metal Ions Metal ion added,
I, generated,
0.5 Pg
wq
HgC12 Hg(I1) EDTA
0.297 0.314 0.275 0.658 0 034
WdW Rza Ag
Au
a
R
Pd cyclohexanebutyrate.
0009
Number of runs 10 4
2 3
4 2
=
linear and is suitable as a calibration curve. The mercury solutions used in these studies were prepared from different stock solutions of mercuric chloride ; fresh dilutions were prepared daily. For some of the tests multiple additions were employed to achieve the desired mercury concentration and this fact is reflected in the rather high average deviations for those tests (see Table 111). Earlier studies using a pH 3.5 buffer produced results similar to those reported here. This
indicates that the pH may vary slightly without causing significant error. Inorganic and organic mercury compounds, other than mercuric chloride, exert the same catalytic effect on the kinetic reaction. Silver exerts a stronger catalytic effect (twice that of mercury) and must be absent from the test solutions. Palladium does not interfere and the interference of gold is negligible. These data are summarized in Table IV. We are presently studying methods for the digestion of urine and serum samples prior to the determination of mercury by the method described above. A digestion procedure employing nitric acid and potassium permanganate has been investigated and found suitable for these samples. As soon as conclusive data have been gathered for biological fluids, the results will be reported. RECEIVED for review September 23, 1971. Accepted November 19, 1971. From a Dissertation to be submitted to the Graduate School, University of Maryland, by Timothy J. Rohm, in partial fulfillment of the requirements for the Ph.D. degree in Chemistry.
Determination of Absorptivities and Ligand Association Numbers of Carbanion Salts Ludvik Ambroz,’ Kam-Han Wong, and Johannes Smidz Chemistry Department, State University of N e w York College of Forestry, Syracuse, N . Y. I3210
A GREAT DEAL of information on the properties of electrolyte solutions in low polarity media and on the structure of ion pairs and their solvation complexes has been obtained from studies involving carbanion and radical anion salts (1, 2). These investigations encompass a wide area of research, such as the conductance behavior of these salts (3-5), their complexation to cation binding agents (6, 7), and the mechanism of reactions involving carbanion o r radical anion salts as intermediates--e.g., anionic polymerization (8) and electron transfer processes (1). Many of the salts have characteristic optical absorption maxima above 300 nm, and optical spectrometry has therefore been a favorite tool for determining the concentration of the ionic species. This of course requires a reliable method to obtain the molar absorptivities, which is complicated by the extreme sensitivity of these systems to air and moisture. 1 Present address, Research Institute of Macromolecular Chernistry, Brno, Czechoslovakia. 2 To whom inquiries should be addressed.
(1) M. Szwarc, “Carbanions, Living Polymers and Electron Transfer Processes,” Interscience, New York, N.Y., 1968. (2) J. Smid, Angew. Chem., 1972, in press. (3) T. E. Hogen Esch and J. Smid, J . Amer. Chem. SOC.,88, 318 (1966). (4) T. Ellingsen and J. Smid, J . Phys. Chem., 73,2712 (1969). (5) R. V. Slates and M. Szwarc, ibid.,69, 4124 (1965). (6) L. L. Chan, K. H. Wong, and J. Srnid, J. Amer. Chem. SOC.,92, 1955 (1970). (7) R. V. Slates and M. Szwarc, ibid.,89, 6043 (1967). (8) D. N. Bhattacharyya, C . L. Lee, J. Srnid, and M. Szwarc, J. Phys. Chem., 69,624 (1965). 872
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
In the past, concentrations have often been determined by terminating the carbanion salt with water or an alkylhalide, followed by the usual titration of the alkali hydroxide or halide (1). These methods are only reliable for carbanion concentration above 10-2M. Moreover, the hydroxide titration often leads to considerable errors as any hydroxide formed by termination during the preparation or storage of the carbanion salt is also counted. Also, in most of these measurements no effort is made to establish whether Beer’s law is obeyed. This is important when dealing with ionic species in low polarity media, as concentration variations can change the aggregation state of the ionic species, often leading to considerable shifts in the optical spectra. The need for accurate absorptivities and for a check of Beer’s law prompted us, therefore, to devise a rather simple titration procedure, using a completely glass enclosed apparatus which makes it feasible to obtain reliable results at carbanion concentrations as low as lO-SM. EXPERIMENTAL
Apparatus. The apparatus is shown in Figure 1. The top section containing the titrant is separated from the lower portion by a shutoff valve E’. This valve consists of a small magnet enclosed in a piece of glass which is carefully ground at the lower end so as to fit into a small female ball type joint that is part of the outside wall joining the buret and flask A . The glass enclosed magnet, placed inside the lower part of the buret, is held in place or can be lifted up by two magnets attached to the outside of the buret. When closed, no leakage of titrant to flask A is observed. Procedure. The determination of the molar absorptivity, E, of the dianion of 1,1,4,44etraphenylbutane (Ph2-CCH2-
