Argentometric Microdetermination of Halogens Using Sodium-SensitiveGlass Electrode in Nonaqueous Medium Keiichiro Hozumi and Naoshige Akimoto Faculty of Pharmaceutical Sciences, Kyoto University, Kyoto, Japan An automatic microtitration of halides using a titrant of 0.005M or 0.0005M silver nitrate solution and a sodiumsensitive glass electrode as an end point indicator in nonaqueous medium is described. The potential curves mark sharp inflections closely at the equivalence point and the inflection points precisely reproduce over the limited range of pH 2-7. The behavior is interpreted by overlapping responses of the electrode to both silver and hydrogen ions. Reproducibilities of the end points are evaluated as standard deviation u = 0.004 w 0.006 (ml) against several milliliters of the 0.005M titrant and u = 0.007 (ml) against the 0.0005M titrant. Application of the method to microdetermination of organic halogens is achieved by oxygen-flask combustion and subsequent titration of the absorption liquid in the flask. Acceptable analytical errors calculated as a standard deviation u = 0.15 (%) are obtained. A NUMBER OF ELECTROCHEMICAL approaches for microtitration
of halogen ions have been reported mainly using the argentometric method involving the potentiometric indication by a silver metal electrode (1-4). Potential changes as a function of the delivery of dilute silver nitrate solution are conveniently recorded o n a chart by a n automatic titrator which enables the end point to be detected graphically. Although the silver metal electrode is simply made, it suffers from a few shortcomings-namely, during silver halide layer formation o r a deposition of precipitates, changes occur in the surface condition of the electrode which interfere with the reproducibility of the potential curves, thus causing inaccuracy of the endpoint detection. Acceptable reproducibility of the end-point detection with dilute solutions lower than 0.01M halide have, however, been obtained after careful conditioning of the electrode surface, involving sequential treatments of polishing by fine sand paper, soaking in nitric acid, rinsing with distilled water, and finally several test runs until the electrode gives reproducible potential curves. Recently, Mattock and Uncles (5) reported an interesting behavior of a sodium-sensitive glass electrode in that its response t o silver ion was greater than t o sodium ion. This readily suggested a possibility of argentometric titration with the sodium-sensitive glass electrode (6) which would eliminate the need to condition the electrode surface as required for the silver metal electrode. This advantage will be especially appreciated in routine analytical laboratories where many daily samples must be quickly analyzed. During the course of the basic investigation with the use of sodium-sensitive glass electrode, the present authors have observed a sharp inflection point in the potential curve very close to the equivalence point when the titration was carried out in high concentrations of (1) J. Haslam, J. B. Hamilton, and D. C. M. Squirrel], Analyst, 85, 556 (1960). (2) D. G. Newman and C. Tomlinson, Mikvochim. Acta, 1961, 73. ( 3 ) A. Nara and K. Ito, Jup. A t i d . , 11, 454 (1962). (4) R. Mcgillivray and S. C. Woodger, Aiialyst, 91,611 (1966). ( 5 ) G. Mattock and R. Uncles, ibid., 87, 977 (1962). (6) R. Geyer, K. Chojnacki, and C. Stief, 2. Atid. Cliem., 200, 326 (1964).
1312
organic solvent over the limited range of pH. Such behavior has been interpreted as overlapping responses of sodiumsensitive glass electrode to both silver and hydrogen ions, the latter holding the potential at the same level until the excess of the former abruptly breaks the potential at the equivalence point. The sharp inflection points are precisely reproduced at the equivalence points under low concentrations of halides ranging 0.005-0.0005M. The method is simple, accurate, and practical, not only for inorganic halogens but also for organic halogens if the latter are mineralized with oxygen-flask combustion. EXPERIMENTAL
Automatic Titrator. Metrohm Potentiograph E436 (Metrohm Co., Herisau, Switzerland) performed best over the potential range of 100 mV, but any other type of automatic titrator having the potential range of about 250 mV can also be employed. Automatic delivery speed control functioning to the rate of potential change facilitates rapid titration of the samples though it is not absolutely required. Sodium-Sensitive Glass Electrode. Horiba sodium-sensitive glass electrode 1512-05T (Horiba Instruments Co., Kyoto, Japan) was used. A parallel test with Corning sodium-sensitive glass electrode NAS 11-18 (Corning Glass Works Co., Medfield, Mass.) behaved practically the same on the titration curve as the former electrode, in its response characteristics to the silver and the hydrogen ions although their glass constituents would not b e identical. The electrode is recommended to be soaked in 0.01M aqueous silver nitrate solution when not in use. Reference Electrode. Horiba saturated calomel reference electrode 1826-05T for microvolume measurement was modified to have an outer sleeve filled with 0.1M ammonium nitrate solution and plugged with a small cork at the lower end in order to avoid a diffusion of potassium chloride from the electrode to the sample solution. Ordinary calomel reference electrode can also be employed using agar-salt bridge containing 0.1M ammonium nitrate. Reagents. Standard solutions of 0.005M and 0.0005M silver nitrate were prepared by dissolving 850 mg and 85.0 mg of crystal silver nitrate, respectively, in 200 ml of water and diluting to 1 liter with isopropanol. Solutions of 0.005M halide and 0.0005M chloride were made by dissolving the calculated amounts of corresponding potassium halides iLq 1 liter of distilled water. Organic solvents such as methanol, ethanol, isopropanol, and acetone were distilled from the reagent grade materials which had been freed of aldehyde by the conventional way using alkaline silver nitrate. Procedure of Titration. An aliquot volume of aqueous sample solution less than 5 ml is accurately pipetted into a 100-ml beaker and its p H adjusted at 2-7 by ammonium hydroxide or nitric acid, if necessary. The solution is diluted with 50 ml of acetone to attain approximately 90 vol of acetone concentration. Automatic titration is then carried out by the 0.005M o r 0.0005M silver nitrate solution at the delivery speed of 0.5 ml/min using a combination of the sodium-sensitive glass electrode and the modified reference electrode as the potential indicator. Direct sunlight or other strong light sources must be screened off from the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
titration in order t o avoid photochemical reduction of colloidal silver halides produced in the sample solution. The electrode potential gradually decreases as the titration proceeds toward the end point, but it suddenly rises marking a sharp inflection a t the end point as illustrated in Figure 1. The volume of the titrant dispensed up t o the end point is read on the recorder chart. The molarity factor of the silver nitrate solution must first be determined against 5.000 ml of the corresponding standard potassium halide solution using the same procedure described above. RESULTS AND DISCUSSION
Reproducibility and Regression Analysis. Repeated titrations of 2.500 ml of 0.005M halide solutions with 0.005M silver nitrate solution were carried out to compare the reproducibilities of the observed end points. Table I represents some statistical results involving mean values and standard deviations of the volumes of the titrants. The data suggest no significant differences in the molarity factor of the silver nitrate solution against different halides. However, the present authors still advise that such a dilute silver nitrate solution be standardized at regular intervals because of a possible deterioration of the reagent. In most cases it will be practical t o standardize the titrant against the halide most frequently requested. The coefficients of variations are given as 0.16% for chloride, 0.20% for bromide, and 0.24% for iodide, but in case of 10 ml of the titrant the coefficients of variations will become of the above values, if the same standard deviations are assumed. A regression analysis on the relation between the amount of chloride and the volume of titrant was studied using stepwise changes of 1 ml in the volumes of 0.005M hydrochloric acid solution from 1.OOO ml to 15.000 ml. The hydrochloric acid was preferred t o the potassium chloride t o avoid any problem arising from varied amounts of potassium ion in the test solutions, Experiments with less than 5 ml of hydrochloric acid were run after making the sample solution up t o 5 ml with distilled water in order to equalize the condition of titration. On the other hand, experiments with more than 10 ml exhibited a small interference with the sharp inflections at the end points because of lower acetone concentration, although the inflection points were still quite discernible. From the data of the above titrations, the linear regression expressed as the following equation, Y
=
0.996 X
+ 0.003
(1)
where X is the volume of hydrochloric acid and Y is the volume of titrant was obtained by the least square method and the mean square of the deviations from the Equation 1 was calculated as VI.z = 0.0001 from which the standard deviation of d c z= 0.010 (ml) is derived. Since the blank value represented as 0.003 ml in Equation 1 is practically negligible at the ordinary value of X ranging 1-10 ml, one can simply estimate the slope of the regression line, CIZ.,the normality factor of the silver nitrate solution, by taking the average of a few repeated tests with 5.000 ml or other aliquot volumes of standard halide solutions. The same experiments for more dilute solution of 0.0005M chloride were carried out with the titrant of 0.0005M silver nitrate resulting in a mean volume of 2.542 ml with a reproducibility represented as the standard deviation of 0.007 (ml). The mean volume higher than the theoretical was probably due to the contamination of the solvent and/or glassware which was difficult to remove completely. The reproducibility of 0.007 ml was still acceptable for normal analy-
-’
z
O1.5
2.0 0.005M OR
2.5
3.0
3.5
0.0005M AgNO, , ml
Figure 1. Potential curves of chloride in 90 vol acetone medium I. 2.500 ml of 0.005M chloride 11. 2.500 ml of 0.0005M chloride
Table I. Sequential Titrations of 2.500 ml of 0.005M Halides with 0.005M Silver Nitrate Solution a t Delivery Speed of 0.5 ml/min Mean vol of 15 titrations Coefficient of Halides (ml) Std dev (ml) variation KCI 2.516 0.004 0.16 KBr 2.509 0.005 0.20 KI 2.502 0.006 0.24
(x)
sis needs since its coefficient of variation at the titration volume of 2.5 ml was 0.28 %. In the case of the titration volume of 10 ml, the coefficient of variation decreases t o 0.07 %. The regression analysis with varied volumes of 0.0005M hydrochloric acid and the corresponding volumes of titrant of 0.0005M silver nitrate solution was studied. The linear regression was expressed as the following Equation 2, where the blank value of 0.021 ml
Y = 1.020 X
+ 0.021
(2)
could not be neglected. Therefore it is advisable t o draw a calibration line with the standard procedure of titration changing the volume of standard chloride solution. The standard deviation from the linear regression was 0.015 (ml). Bromide and iodide are presumed to have the same behavior at this low concentration level. Response Characteristics of Sodium-Sensitive Glass Electrode, The response characteristics of the sodium-sensitive glass electrode to silver and hydrogen ions depend strongly upon the glass constituents (6-8). A basic survey was carried out with the Horiba electrode 1512-05T to draw the response curves illustrated in Figure 2. The electrode in aqueous medium responded to silver ion more than 100 times greater than sodium ion, and to hydrogen ion approximately 10 times more. The Nernstian slopes which deviated a little from the theoretical value were followed at the concentrations down to pAg 6 and pNa 4, while at the lower concentrations they were leveled by hydrogen ion potential. It appeared that the re(7) G. Eisenman, “Electrochemistry of Cation-Sensitive Glass Electrodes” in “Advances in Analytical Chemistry and Instrumentation,” C. N. Reilley, Ed., Vol. 4, Wiley (Interscience), New York, N. Y . , 1965, pp 213-369. (8) G. Eisenman, “Glass Electrodes for Hydrogen and Other Cations,” Marcel Dekker, New York, N. Y . , 1967.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1313
t
-200
-3OOL
0
0.5
1.0
1.5
2.0
2.5
3.0
:
I
0.005M AgNO, , m I Figure 5. Effect of pH on titrations of 2.500 ml of 0.005M chloride
J
l
l
l
l
l
l
l
a
7
6
5
4
3
2
1
-LOG [ C A T I O N ] Figure 2. Potential responses of sodium-sensitive glass electrode as functions of [Ag+], [H+l, and [Na+l concentrations
loot
1
0
0.5
1.0
3.0 0.005M AgNO,, rnl 1.5
2.0
2.5
3.5
Figure 3. Potential curves of 2.500 ml of 0.005M halides in aqueous medium I. Chloride II. Bromide 111. Iodide
IV
-100
1.5
2:u
2.5 3.0 0.005M AgNO, , rnl
I
3.5
Figure 4. Titrations of 2.500 ml of 0.005M chloride in 90 vol organic solvents I. Methanol 11. Acetone
111. Isopropanol IV. Ethanol 1314
ANALYTICAL CHEMISTRY,
sponse characteristics for the three cations were most similar to NAS 11-18B (7,8) which has moderate sensitivity to silver ion among the sodium-sensitive glass electrode by roughly estimating the relative selectivities in Figure 2. Titration Curves. Titrations in aqueous solution were carried out using 2.500 ml of 0.005M halide solutions. The curves in the vicinity of the equivalence point exhibited sharper inflections of the potential curves at lower solubilities of the silver halides as illustrated in Figure 3. This phenomenon readily suggests that the use of organic solvent, to decrease the solubilities of silver halides, would give sharper inflection at the end point. Figure 4 illustrates the potential curves in approximately 90 vol % concentrations of some organic solvents using 2.500 ml of 0.005M chloride solution. The sharpness of the inflections varied with different organic solvents, but the inflection points exactly coincided with theoretical end point. Acetone was much the best solvent because of its abrupt potential rise after the inflection point. Further investigation was carried out changing the concentration of acetone in the titration media. The results showed that the titration of chloride must be run at the concentration of 90 vol % or higher, but the bromide can be done at the concentration down to 70 vol % and the iodide can be titrated in aqueous medium. The present authors suggest, however, that when all types of halides are analyzed routinely, it is advisable to use about 90 vol % acetone concentration as the standard procedure. Large volume of aqueous titrant may also dilute the acetone concentration. Therefore, 80 vol isopropanol was recommended for the preparation of the 0.005M silver nitrate solution because of the favorable solubility of silver nitrate and a low vapor pressure. Titration of mixed halides exhibited one end point at the sum of the individual halide concentration that was somewhat different from the supposition in ordinary potentiometric titration using silver metal electrode. Effect of pH. It is naturally supposed that the p H of the sample solution should exert a considerable effect on the potential curves, since the electrode responds to hydrogen ion. A result of an example with chloride ion under varying p H using nitric acid is illustrated in Figure 5 where potential rises after the inflection points become insignificant at lower p H value while the inflection points reproduce well at the same position. Therefore the titration should avoid strongly acidic solutions although acidic solution down to p H 2 can still be determined accurately. Effect of Foreign Cations. Sodium and potassium are two of the foreign cations most often present in halide solutions. At the sodium ion concentration 100 times higher than chloride ion, the titration curve gave no end point, while the
VOL. 42, NO. 12, OCTOBER 1970
Table 11. Effects of Varied Delivery Speed of Titrant on 2.500 ml of 0.005M Chloride Mean vol of Delivery speed of titrant (ml/min) 15 titrations (ml) Std dev (ml) 0.25 2.507 0.004 0.5 2.516 0.004 1 2.521 0,008 Table 111. Relative Selectivity Ratios of Sodium-Sensitive Glass Electrode between [Ag+] and [H+] in Aqueous and 90 vol % Acetone Media Concn Potential Potential of cations for [Ag+] for [H+] Log K P o t -1.6 119 mV 44 mV I [Cation] = 0 . 1 -1.6 71 mV -4 mV [Cation] = 0.01 64 mV -2.6 I1 [Cation] = 0.1 183 mV -2.7 140mV 20 mV [Cation] = 0.01 I: Aqueous medium. 11: 90 vol acetone medium.
I/
I
I
-loot
2.4
2.6
2.5
0.005M AgN03 , m l
potassium ion a t the same concentration caused a dull end point. I n either case, the cation concentration must b e limited below 10 times that of the halide concentration. Monovalent cations of lithium and ammonium gave n o significant interference with the titration even at concentrations 100 times higher &han the halide. Other polyvalent cations such as calcium and cupric ions also did not influence the potential curve, whereas an addition of equal concentration of ferric ion resulted in a complicated curve at the equivalence point for some unknown reason. Delivery Speed of Titration. Since the precipitation normally involves a small delay from the stoichiometric reaction, the speed of titration affects the end point. Table I1 shows a comparison between three delivery speeds of 0.005M silver nitrate solution against 2.500 ml of 0.005M chloride solution. Higher delivery speed resulted in larger titers and a t the same time poorer reproducibility of the volume of titrant was obtained at the highest speed. The present authors, therefore suggest the delivery speed be standardized at 0.5 ml/min as this gives a sharp coincidence of the inflection points at the theoretical equivalence point. Interpretation of Titration Curve. The concentration of silver ion along the course of the precipitation titration of chloride proceeds as in the following Equation 3, (3) where KA&C~ is the solubility constant of AgCl in the test solution, Ccl and CAPare the concentrations of the initial chloride sample solution and the titrant of silver nitrate, respectively, A is the volume of initial chloride sample solution taken, B is the volume of titrant delivered, and V is the volume of the acetone solution made up before the titration (9). It should be noticed that Equation 3 is only an approximation for the system under consideration because in 90 vol % acetone, a ) the activity coefficient may be somewhat below unity, and b ) AgX2- complex will be formed much more extensively than in water. Solving Equation 3, [Ag+] is expressed as the function of the volume of titrant B. (CClA - C*,B)2
[&+I
=
- CA,B + ~ K A ~-CCciA I +
-
2
(4)
(9) J. F. Coetzee, “Equilibria in Precipitation Titration Reactions and Precipitation Lines,” in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Part I, Vol. 1, Interscience, New York, N. Y . , 1959, p 767.
Figure 6. Details of theoretical potential curves at equivalence point Assuming an electrode responding exclusively to the silver ion, the electrode potential simply moves as a sigmoid curve in the vicinity of the equivalence point where CclA = CA,B in Equation 4. However, because the sodium-sensitive glass electrode is responsive also to hydrogen ion, it follows that the overlapping potential E should be expressed as the well known equation (6, 7), RT E = Const. - In ([Ag+l KPot[H+l) F where KPotis the selectivity ratio of [H+]t o [As+]. Combining Equation 4 and 5, one gets, RT E = Const. - In F
+
+
+
(6)
Hence the shape of the overlapping potential curve is functional t o three factors; solubility constant selectivity constant KPot, and concentration of hydrogen ion [H+]. Lower solubility of silver halide gives a higher potential break, thus contributing t o an easier end-point detection. The solubility constant of silver chloride in aqueous medium is 1.2 x 10-10 at 20 “C which showed a dull inflection of potential curve at the end point whereas silver iodide with solubility constant of 1.2 x 10-16 at 20 “C gave a sharp inflection point. The present authors therefore presumed that the solubility constant of silver chloride in 90 vol % acetone medium would be close t o 1 x 10-l6 as quoted in the literature (10) and the experiments shown in Figure 3 and 4. Relative selectivity ratio of the given electrode between silver and hydrogen ions was investigated by measuring the potentials in either of 0.1M silver nitrate and 0.1M nitric acid solutions. Calculation was done by the following Equation 7, - K I n K P O ~= E,*,+] - E , ~ + . ]
(7)
F The log KPot is given in Table 111where the relative selectivity ratio exhibited much higher value in acetone than in
(10) D. C. Luehrs, R. T. Iwamoto, and J. Kleinberg, h ~ g Chem., . 5, 201 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1315
Table IV. Representative Analyses of Organic Halogens Using Oxygen-Flask Combustions and Subsequent Argentometric Titrations with Sodium-Sensitive Glass Electrode as Indicator
X(%) Sample p-Chlorobenzoic acid C1 = 22.65% 2,4,5-Trichloroaniline CI = 54.13z 1,2,3,4,5,6-Hexachlorocyclohexane c1 = 73.14% 2,4-Dinitrochlorobenzene C1 = 17.50%
S-Benzylthiuronium chloride CI = 17.49% p-Bromoacetanilide Br = 37.33% 1,2,3,4,5,6-Hexabromocyclohexane Br = 85.99%
Iodobenzoic acid I = 51.17%
Sample wt (mg) 3.572 3.268 3.459 3.300 3.370 3.567 3.551 3.598 3.532 3.605 3.281 3.532 3.471 3.595 3.488 3.959 3.889 4.058 3.492 3.508 3.780 6.301 5.989 5.210
0.005M AgN03 (ml) 4.575 4.415 4.175 10.090 10.245 10.895 14.585 14.830 14.560 3.575 3.240 3.490 3.418 3.553 3.463 3.711 3.601 3.796 7.532 7.532 8.721 5.100 4.825 4.990
Found 22.71 22.65 22.63 54.21 53.90 54.15 72.82 73.08 73,09 17.58 17.51 17.52 17.46 17.53 17.61 37.45 37.00 37.37 86.17 85.78 85 83 51.36 51.12 50.98
Dev +0.06 10 -0.02 +0.08
I
Std Dev u
aqueous medium. This interesting phenomenon has been explained by a considerable decrease of hydrogen ion activity in high concentration of organic solvent while other metal cations retained the initial activities (7,8). By inserting K A =~ 1 ~X 10-lG, ~ KPot = 2.7 X in Equation 6, the potential curves in the vicinity of the equivalence point were computed under varied acidities and drawn as illustrated in Figure 6. The curves well explain the results shown in Figure 5 . Details of the potential curves in Figure 6 show that the inflection points shift slightly under different pH, although this shift is practically negligible for ordinary analyses. The inflection points obtained at p H 2-7 appeared t o be located at approximately 0.01 ml before the theoretical equivalence point, but they actually reproduced at the points indicated in Table 11, because of the delay from the stoichiometric reaction depending upon the delivery speed of the silver nitrate solution. It was also concluded that a sharper inflection point was obtained at lower p H value, but the lower p H value caused a smaller potential rise after the end point. The optimum p H existed between p H 3-5 at the solubility constant of 1 x 10-16. DETERMINATION OF ORGANIC HALOGENS
The method described above was applied t o microdetermination of organic halogens using the oxygen-flask combustion (11, 12) for rapid decomposition. This modification was prefered to the conventional visible titration using mercuric nitrate and diphenylcarbazone as an indicator (13,14) because serious interferences from most of the metal cations had been encountered. (11) W. Schoniger, Mikrochitn. Acta, 1956, 869. (12) A. Steyermark, “Quantitative Organic Microanalysis,” 2nd ed., Academic Press, New York, N. Y.,1961,p 291. 1316
=
-0.23 +o I02 -0.32 -0.06 -0.05 +0.08 +0.01 $0.02 -0.03 +0.04 +0.22 $0.12 -0.33 +O. 04 +o. 18 -0.21 -0.16 +o. 19 -0.07 -0.19 0.15
(z)
Procedure. Accurately weighed organic halogen samples ranging 3-6 mg are placed on cut filter papers and wrapped in the conventional manner. The filter papers are subsequently burned in oxygen-flasks containing 5 ml of water and 5 drops of 5% hydrazine hydrate solution. The flasks are vigorously shaken and stand for 30 minutes for absorption of the combustion gas. The absorption liquids are neutralized during the standing time by absorbing excess carbon dioxide from the combusted filter papers. One of the absorption liquids is transferred to a 100-ml beaker using 50 ml of acetone. The solution is immediately titrated with 0.005M silver nitrate solution using the sodiumsensitive glass electrode as the end-point indicator and with protection from direct sunlight or other strong light sources. Results. Hydrogen peroxide has been used conventionally in absorption liquid for reducing chlorine and bromine in the combustion gas to chloride and bromide, while iodine and iodate have been reduced by hydrazine hydrate (3, 15). The use of hydrogen peroxide has been very advantageous for the mercurimetric titration using diphenylcarbazone when the halogen compounds contained sulfur. In the absence of hydrogen peroxide, the sulfur dioxide produced in the combustion gas forms sulfite in the absorption liquid and this strongly interferes with the end-point detection. The hydrogen peroxide therefore acts as a strong oxidizing agent for the conversion of sulfite t o sulfate. Neither sulfite nor hydrazine gave any interference with the end-point detection using the argentometric titration presented here. The use of hydrazine hydrate is preferred because of its convenience and applicability to all three halogens. I t was of interest that the 0.005M silver nitrate solution standardized against known concentration of halide solutions (13) F.W. Cheng, Microchem. J . , 3, 537 (1959). (14) D.C.White, Mikrochim. Acta, 1961,449. (15) S.Kinoshita and K. Hozumi, Jup. A d . , 14,352 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
exhibited statistically somewhat low analytical results. The reason was probably attributed t o incomplete conversion of organic halogens to halogen ions and t o losses occuring during the transfer of the absorption liquid t o the beaker, although the conversion rate and loss are quite reproducible. Analysts are strongly advised t o standardize the concentration of silver nitrate solution against the standard organic halogen com-
pounds such as p-chlorobenzoic acid. A list of representative analyses involving different types of organic halogen compounds is shown in Table IV where the over-all distribution of the analytical errors has been evaluated as CT = 0.15 (%) which is quite acceptable for microanalysis. RECEIVED for review May 4, 1970. Accepted June 24, 1970.
Preparation, Analysis, and Comparative Study of Silver Perchlorate Complexes with Methylbenzenes Pinhas Avinur and Isaac Eliezer Department of Chemistry, Tel-Aviv University, Tel-Aviv, Israel
A convenient method for the preparation of crystals of silver perchlorate complexes with 0-, p-, and m-xylene, and with mesitylene i s described. A spectrophotometric method for the analysis of the crystals and determination of the ratios of the components of the molecular complexes is presented. p-Xylene and mesitylene appear to form molecular complexes of one composition only (2 p-xylene: 1 silver perchlorate, and 1 mesitylene :1 silver perchlorate), while 0-xylene and m-xylene form both the 1:l and 2:l aromatic: silver complexes. THEEXISTENCE of molecular complexes between hydrocarbons with donor properties and inorganic acceptor type compounds is well documented by now ( I , 2). These complexes can sometimes be isolated as solids of definite composition (usually of simple molecular ratios). I n other instances the interaction products are sufficiently unstable so that their formation can be recognized only through changes in the physical properties of solutions of their components. The donor properties responsible for the formation of molecular aromatic complexes are usually due to the aromatic r-electron systems. Acceptor ability of inorganic compounds in such complexes usually necessitates the availability of unfilled (mostly d) orbitals. The complexes thus formed are of considerable theoretical and experimental interest (3, 4 ) and include, as a particular case, the so-called charge transfer complexes. The ability of silver (either as nitrate or as perchlorate) t o form such complexes was noted long ago, and has been the subject of several studies (e.g,,refs. 3-9). Because of the extreme instability of the crystals of such complexes and the lack of preparative details in the literature, it was thought useful to work out a convenient laboratory (1) L. J. Andrews, Chem. Rev., 54,713 (1954). (2) H. A. Bent, ibid., 68, 587 (1968). (3) R. E. Rundle and J. H. Goring, J. Amer. Chem. Sac., 72, 5337 (1950); H. G. Smith and R. E. Rundle, ibid., 80, 5075 (1958). (4) R. S. Mulliken and W. B. Person, Ann. Reu. Phys. Chem., 13, 107 (1962). (5) G.Peyronel, G. Belrnondi, and J. M. Vezzosi, J. Znorg. Nucl. Chem., 8, 577 (1958). (6) G. Peyronel, I. M. Vezzosi, and S. Buffagni, Gazz. Chim. Ztal., 93, 1462 (1963). (7) B. G. Torre-Mori, D. Janjic, and B. P. Susz, Helu. Chim. Acta, 47, 1172 (1964). (8) D. W. A. Sharp and A. G. Sharpe, J. Chem. Soc. 1956, 1855. (9) R. J. Prosen and K. N. Trueblood, Acta Crystallogr., 9, 741 (1956).
technique for preparing crystals of silver perchlorate-aromatic complexes, as well as reliable methods of analysis of the crystals, and then use these for comparative study of the composition of crystalline molecular complexes of silver perchlorate with a series of methylbenzenes. Daasch (IO) described a method for preparing such crystals by gradual removal of the excess organic solvent in a vacuum desiccator. I n several cases, this is not practical owing to the long period required to evaporate solvents with relatively low vapor pressure (e.g., mesitylene). Peyronel et al. (5) prepared the solid benzene-silver perchlorate complex by adding ligroin to a saturated solution of silver perchlorate in benzene. This method has two main disadvantages: It is not possible to grow well-defined and large crystals necessary for X-ray work and it is very difficult to dry the fine crystals that contain ligroin. The analysis of such crystals is usually based on silver determination. EXPERIMENTAL
Spectrophotometric measurements were carried out at 22 O C using a Cary 14 instrument with matched I-cm quartz cells. The solvents used were p-xylene (B.D.H., A.R.), m-xylene (Fluka, c.P.), o-xylene (Fluka, A.R.), toluene (Merck, A . R . ) and mesitylene (B.D.H., c.P.). All reagents were used without further purification. AgC104 (B.D.H., L.R.) was dried by heating for 24 hr a t 200 O C and was kept in a desiccator over P 2 0 5[see also ref. (ZI)]. The details for the preparation of the crystals are given in Figure 1 and described in the caption. To summarize, the anhydrous silver perchlorate is dissolved in the appropriate aromatic hydrocarbon and the concentrated (nearly saturated) solutions are evaporated by passing dry nitrogen over them. With a flow rate of nitrogen of about 500 to 700 ml per second, the time of crystallization varies from 3 to 20 days, at a temperature of 25 "C. Because of the extreme sensitivity of the crystals to atmospheric moisture, their removal from the mother liquor is performed by placing vessel A (see Figure 1) in a dry box flushed with dry nitrogen. Tube Z is then detached, the mother liquor decanted, and the crystals are gently removed by tapping the upper end of Z o n black hard filter paper. (10) L. W. Daasch, Spectrochim. Acta, 9, 726 (1959). (11) L. El-Sayed and R. 0. Ragsdale, J . Znorg. Nucl. Chem., 30, 651 (1968).
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