Table 11. Difference in Free Energy of Solvation, Ai&, for Diastereorneric Saturated and Unsaturated Alkylcarbinyl a-Acetox ypropionates H
H
I I Apo (CH3-C-COrC-R’) I I OAc
R = R’
=
Me Et
n-Pr
Meb (Old
43
41
-CH=CH$c 0 41 46
of solvation in cal/mola
R
-C(CH3)=CHzc -40 5
Meb
R’=
n-Bu i-Pr
-CH=CHzC
-C(CH+CH2c 14 38
54
15
Condition specified in Table IV of ref. 3. Po (RS)- Po (SS). Po (SS)- Po (RS). d The data were retrieved from Table IV of ref. 3, except for the sign of the -40 value.
a
As could be seen, there is almost a constant difference between any two rows or columns, i.e., a given structural change produces a constant difference in the free energy of solvation of the isomers. It would be of great interest to see if such generalities hold also when polar substituents like: halogen, nitrogen deriva-
tives, etc, replace a n alkyl group and not only when the replacement is done between two alkyl groups.
RECEIVED for review January 21, 1971. Accepted March 10, 1971.
Spectrophotometric Determination of Cyanide, Sulfide, and Sulfite with Mercuric Chloranilate R a y E. Humphrey and Willie Hinze Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340
THEAPPLICATION of various metal chloranilates for the spectrophotometric determination of a number of anions using either the ultraviolet maximum at 330 nm or the visible peak at 525 nm is well known ( 1 ) . In some instances, the insoluble metal salt of the anion is formed releasing the chloranilate ion while other reactions involve the formation of soluble, slightly dissociated metal compounds with the chloranilate ion going into solution. Probably the most common example of the latter is the determination of chloride with mercuric chloranilate which involves the formation of soluble, undissociated mercuric chloride (2, 3). A visual comparative procedure has been used for the estimation of cyanide ion with mercuric chloranilate, in which soluble, slightly dissociated, mercuric cyanide is apparently formed, and for sulfide ion, where the very insoluble mercuric sulfide precipitates (4). The useful concentration range for the visible absorption method for these two anions was not indicated. Apparently, neither cyanide ion nor sulfide ion has been determined by the chloranilate method using the ultraviolet peak at 330 nm. The molar absorptivity of the chloranilate ion is much higher at this wavelength than in the visible region (3). We have found that sulfite ion can be determined by reaction with mercuric chloranilate, presumably to form the soluble, nondissociated (1) L. S. Bark, Itid. Chem., 40 (3), 153 (1964). (2) J. E. Barney I1 and R . J. Bertolacini, ANAL. CHEM., 29, 1187 (1957). (3) R. J. Bertolacini and J. E. Barney 11, ibid., 30, 202 (1958). (4) E. Hoffman, Z . Anal. Chem., 185,372 (1962).
1100
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
mercuric sulfite as shown in Equation 1. The sensitivity for sulfite at both the UV and visible peaks is considerably higher than that for chloride ion. HgCh
+ so32-
4
HgS03
+ Ch2-
(1)
Also, the determination of cyanide ion and sulfide ion with mercuric chloranilate employing the measurement of absorption at 525 nm has been investigated and data on the sensitivity of the visible absorption for these anions is reported. EXPERIMENTAL
Apparatus. Absorption measurements were made with a Beckman DB-G spectrophotometer and with a Beckman DK-2A spectrophotometer. Reagents. Mercuric chloranilate was an Eastman Reagent chemical. For part of this work the compound was washed with ethanol several times and dried in order to remove any chloranilic acid present and hence lower the blank (5). Chloranilic acid was also a n Eastman Reagent Chemical and was used as purchased. The compounds KCN, Nd2S03, and Na2S.9 H 2 0 were all Baker Analyzed reagent chemicals and were used as received. All other chemicals and solvents were used without purification. Procedure. A mixed solvent, prepared by using equal volumes of ethanol and water, was employed in obtaining all of the Beer’s law and recovery data. The solubility of mercuric chloranilate was low in this solvent and hence the ( 5 ) C. F. Hammer and J. H. Craig, ANAL.CHEM., 42, 1588 (1970).
absorbance blank was small. Usually about 10 mg of the mercuric chloranilate was added to the solution of the anion for the work involving UV measurement and the mixture stirred. For the work involving visible absorption, about 25 mg of mercuric chloranilate was used. Approximately 15-20 minutes were required for maximum reaction. The excess solid was then removed by centrifuging and the absorbance of the solution measured. Filtration of the reaction solutions was tried but was not helpful. Absorbance readings were reasonably constant for several hours. The volume of solution was usually 10 ml. Sodium sulfite and sodium sulfide solutions were made up just before use in order to avoid loss due to oxidation of these anions. RESULTS AND DISCUSSION
Stoichiometry of the Mercuric ChloranilateSulfite Reaction. A mole ratio study was made in order to determine whether mercuric sulfite or a sulfite complex was formed when sulfite ion reacted with mercuric chloranilate. Various amounts of a standard sulfite solution were added to approximately constant amounts of mercuric chloranilate and the absorbance was measured at 525 nm after allowing about 20 minutes for the reaction to occur and centrifuging to remove any remaining mercuric chloranilate. The solvent was 1:1 ethanol-water. The absorbance reached a maximum value at a mole ratio of 1 : 1 and then decreased at higher ratios of sulfite ion to chloranilate. This decrease is apparently due to reaction of sulfite ion with the chloranilate ion released to produce a product which does not absorb at this wavelength. The absorbance of a standard chloranilic acid solution was found to decrease on adding sulfite ion. This reaction would account for the decrease in absorbance beyond the 1 :1 ratio in the mole ratio study. Possibly a 1,Caddition reaction involving bisulfite ion occurs, such interactions having been reported for several substituted p-benzoquinones (6). The formation of a stable mercuric sulfite species has been used as the basis for an indirect spectrophotometric method for sulfite ion. The decrease in absorbance of the mercuric diphenylcarbazone complex on removal of the mercury by reaction with sulfite ion is measured (7). The stoichiometry of the mercuric sulfite species formed was not definitely established. A mercuric sulfite species is also used to stabilize sulfite in the absorbing solution employed in the West-Gaeke method for sulfur dioxide. The stoichiometry of the species formed has been reported to be 1 :1 (8) and 1 :2 sulfite ions per mercuric ion (9). Determination of Sulfite. Mercuric chloranilate reacts readily with sulfite ion in 1 : 1 ethanol-water solvent to release the chloranilate ion. Reaction requires about 15 to 20 minutes. The Beer’s law plots at the visible peak, 525 nm, and at the ultraviolet maximum, 330 nm, are linear and the reproducibility is good. The useful concentration range measuring at 525 nm is about 5-100 pg/ml SOZ,with the blank having an absorbance of about 0.05. The concentration range which can be determined measuring at 330 nm is approximately 0.5-8.0 pg/ml SO2. The blank at this wavelength can range from A = 0.2 to A = 0.5, depending on how well the mercuric chloranilate has been cleaned up by washing with ethanol. Recovery of sulfite from synthetic samples at both the visible (6) C. A. Bishop, R . F. Porter, and L. K . J. Tong, J . Amer. Chem. SOC.,85, 3991 (1963). (7) T. Okutani and S . Utsumi, BuII. Chem. SOC.Jup., 40, 1386 ( 1967). (8) N. V. Nauman, P. W. West, F. Tron, and G. C. Gaeke, ANAL. CHEM., 32,1307 (1960). (9) P. W. West and G. C. Gaeke, ibid.,28, 1816 (1956).
Table I. Recovery Data for Anions at 525 nm
CN-
SOP
Presentb Found 10.0 20.0 80.0
11.0 21.0 78.0 91.5
Sa-
Present
Found
Present
Found
25.4 50.8 127 253
25.9 53.7 129 250
23.2 34.7 104 139
23.0 36.0 102 130
100 Present as sulfite ion.
0
pg/ml.
Table 11. Recovery Data for Anions at 330 nm sop CNS 2Presentb Found Present Found Present Found 0.75 2.25 4.50 7.50 4
0.70 2.15 4.40 7.55
0.52 1.04 1.56 2.08
0.60 1.07 1.46 1.95
0.36 0.80 1.00 1.38
0.35 0.75 1.03 1.40
Present as sulfite ion. rg/ml.
peak, Table I, and UV maximum, Table 11, is reasonably good. These results, representative of multiple analyses, are probably as good as can be expected considering the nature of the method and the rather high absorbance of the blank. Determination of Cyanide Ion. Mercuric chloranilate has been used for the visible estimation of cyanide ion but the sensitivity is somewhat uncertain (4). This reaction presumably forms soluble, undissociated mercuric cyanide. Reaction also occurs readily with this anion but the sensitivity, in terms of molar absorptivity calculated from the cyanide concentration, is lower than that for sulfite ion. This is probably due primarily to the fact that two cyanide ions are required to release one chloranilate ion. At 330 nm, cyanide ion can be determined over the range of 0.4-4.0 pg/ml. In the visible region, using the 1 : 1 ethanol-water solvent and measuring the absorbance at 525 nm, the Beer’s law plot is linear over the range of 20-200 pg/ml cyanide. Recovery of cyanide from synthetic samples is satisfactory, as shown in Tables I and 11. Determination of Sulfide Ion. Sulfide ion displaces chloranilate ion from mercuric chloranilate and forms the very insoluble mercuric sulfide. Visible estimation of sulfide using this reaction has been reported (4,but there appear to be no reports of the application of the use of the UV absorption for this determination. At the visible peak, 525 nm, the Beer’s law plot is linear over the range of 5-200 pg/ml sulfide. Sensitivity measuring the absorption at 330 nm is considerably higher than would be expected from the molar absorptivity of the chloranilate ion at this wavelength. The useful concentration range at this wavelength is 0.1-1.5 pg/ml sulfide. Recovery data for various amounts of sullide are shown in Table I and Table 11. Effect of Solvent and Buffers. The mercuric chloranilatesulfite ion reaction was tried in an acetic acid-sodium acetate buffer of pH 4.8, a phosphate buffer of pH 6.8, and a borate buffer of pH 9.0. Reaction occurred in the first two of these but the sensitivity was not as high as in the ethanol-water solvent. The blank was very high at pH 9.0. The sensitivity of the sulfite reaction was also higher in ethanol-water than in 1 :1 methyl cellosolve-water mixture or in distilled water alone. The cyanide and sulfide reactions were studied only in the ethanol-water solvent. A 1 :1 methyl cellosolvewater solvent is recommended for the determination of chloANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1101
Table IIL Apparent Molar Absorptivities Anionn CN-
S$-
sopCeC1*04*-
e,
525 nmb 115 250c 150d
1 ,m 950
6,
330 nmb 11,500
32,000
16,800 22,800
Solvent was 1 :1 ethanol-water. Values are based on the anion concentration. c Solutions seemed to have a brown tint, possibly due to HgS which was not separated. d Sodium nitrate present at 8 x concentration. 4
b
ride ion with mercuric chloranilate (2, 3 ) . The organic solvent presumably promotes the reaction by lowering the dissociation of mercuric chloride and also lowers the blank by decreasing the solubility of mercuric chloranilate. These effects were not significant in the determination of sullite ion. The magnitude of the blank is an important factor in the choice of solvent or buffer. The blank can be decreased significantly by cleaning up the mercuric chloranilate as has been reported (5). It is likely that the extent of reaction could be improved in some buffers or solvent systems but the blank would probably increase in a similar manner. Sensitivity of the Reactions. Apparent molar absorptivities for cyanide, sulfide, and sulfite in both the UV and visible regions are presented in Table 111, Comparing the molar absorptivity values at 330 nm for these substances with that for chloranilic acid under the same conditions indicates that reaction between mercuric chloranilate and these anions is extensive at the low concentrations. The sulfite reaction is possibly slightly less than complete while the cyanide reaction would appear to be essentially complete, since two cyanide ions are required to release one chloranilate ion. The molar absorptivity for sulfide at the UV peak is considerably higher than would be expected from the stoichiometry of the reaction and the molar absorptivity of the chlor-
anilate ion. The reason for this is not readily apparent. Beer’s law is obeyed by this anion, however. At the higher concentrations of these anions, measuring the absorbance at 525 nm, the extent of reaction for cyanide and sulfide is rather low judging from the molar absorptivity values and comparing these to the value for chloranilic acid under the same conditions. The presence of finely-divided mercuric sulfide is evident in the sulfide reaction. In some instances sodium nitrate was added in an attempt to coagulate the sulfide. The sensitivity for cyanide and sulfide ions is somewhat less than for chloride ion, when these ions are determined using 1 :1 ethanol-water and the chloride determined in methyl cellosolve-water. On the other hand, the sulfite reaction seems to proceed to about the same extent at the higher concentrations as at the lower levels. The apparent molar absorptivity for this anion at 525 nm is very close to the value for chloranilic acid. The wavelength maxima and molar absorptivity values for a solution containing chloranilic acid are strongly pH dependent as it is possible to have the chloranilate ion, the acid chloranilate ion, or the free acid (3). In this work, reproducible results were obtained using an ethanol-water solvent without a buffer. Some work using various buffers showed a high blank while in other instances the extent of reaction with the mercuric chloranilate was less than without the buffer. Possibly some improvement might result if a buffer mixture were added after centrifuging. However, the estimate of extent of reaction of the anions with the mercuric chloranilate based on comparing molar absorptivity values of the solution with those for a chloranilic acid solution should be of some validity even without buffer since solvent and pH conditions are essentially the same.
RECEIVED for review January 25, 1971. Accepted March 19, 1971. The authors would like to express their appreciation to the Robert A. Welch Foundation of Houston, Texas, for support of this research.
Example of Flame Photometric Analysis for Methyl Parathion in Rat Whole Blood and Brain Tissue Joe Gabica, Joe Wyllie, Michael Watson, and W. W. Benson Idaho Community Studies on Pesticides, Idaho Department of Health, Statehouse, Boise, Idaho 83707 MONITORING OF ENVIRONMENTAL pesticide residues has become one of the more important practical applications of gas chromatographic analysis. Typically, gas chromatography is used to detect the presence of an increasingly great variety of chemically diverse pesticides by means of a relatively nonspecific electron capture detector, flame ionization detector, microcoulometer, or similar device. Unfortunately, the current wave of multiplicity in pesticide application presents the analyst with a need for greater specificity in his methodologies. It is possible for several chemically unrelated pesticide residues to be present in a given sample. If some of these compounds happen to behave similarly when subjected to a relatively diffuse analytical procedure such as electron capture detection, their presence could be 1102
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
easily overlooked or misinterpreted. (For example : the simultaneous presence of p,p’-DDE and ethyl parathion would elicit only a single peak on an OV-1, 3x, gas chromatographic column in conjunction with an electron capture detector.) The recently increased use of organophosphate pesticides requires that a more specific means of detection be employed. As noted by other workers ( I , 2), the extreme toxicity of some of the organophosphates requires that detection methods be highly sensitive as well as highly ( I ) D. L. Pettijean and C . D. Lantz, J . Gas Chroniarogr., 1, 23 (1963).
(2) R. A. Vukovich, A. J . Triolo, and J. M. Coon, J . Agr. Food Chem., 17, 1190 (1969).
