Determination of Microgram Quantities of Water in Organic Solvents and Electrolytes by the Lead Tetraacetate Method C. D. Thompson, F. D. Bogar, and
R. T. Foley
Chemistry Department, The American Uniuersity, Washington, D . C.
A method is described for the determination of pg quantities of water (0-100 ppm) in nonaqueous solvents. Lead tetraacetate in benzene (3 grams per 100 ml) is reacted with the water in the solvent to produce lead dioxide and acetic acid. The resulting lead dioxide solution absorbs at 396-402 mp and as little as 5 pg of water, or 2.5 ppm in a 2.0-ml sample can be measured. The test was found to be applicable to a wide range of pure solvents (acetic acid, acetone, acetonitrile, anisole, 2-bromopropane, y-butyrolactone, chloroform, diethylcarbonate, ethylacetate, propylene carbonate) and to certain solutions commonly employed in electrochemical investigations such as solutions of LiC104, KPF6, and (n-C4H9)aNCI04in ybutyrolactone, propylene carbonate, and acetonitrile, respectively.
ONEOF THE CRITICAL PROBLEMS in experimental investigations in nonaqueous electrochemistry is the determination of water in the 0-100 ppm range. Water in this concentration reacts significantly with active anodes such as Li, and increases the solubility of inorganic cathode materials such as CuCl or AgCI. Further, in the reduction of organic species containing -NO* groups or other reducible species, the presence of protons can completely alter the reduction scheme. The well known Karl Fischer method is unreliable below about 20 ppm, and certain functional groups in organic solvents interfere with the reaction. A gas chromatographic method utilizing a Poropak Q column has been successful in analyzing for water in cyclic esters but is not particularly useful for solutions of inorganic electrolytes because the salt accumulates in the injection chamber (1-3). This necessitates frequent dismantling for cleaning and produces misleading results when the salt decomposes at elevated temperature, A spectrophotometric method utilizing the 1.9-p water band in the near infrared has been found applicable for solutions of LiC104 in propylene carbonate containing above 20 pprn water (4) and in nonaqueous plating solutions (5). However, solvents such as dimethyl sulfoxide, dimethyl formamide, tetrahydrofuran, and acetonitrile, which are now being employed in electrochemistry, absorb at 1.9 I.( and interfere in a way which is difficult to compensate. Further, there is a decrease in absorptivity with increase in LiClO4 concentration requiring different calibration curves for different LiC104 concentrations. Other methods investigated in this Laboratory and found t o lack sufficient sensitivity in the desired range include refractive index, density, electrical conductivity, and reaction with calcium hydride. The refractive index of y-butyrolactone varies only 0.004 unit for water concentrations of 0.0 ppm to 2080 ppm. The density of pure y-butyrolactone is 1.1254 grams/ml (25.0 "C) while that of a 2080-ppm water solution is 1.1243 grams/ml. The electrical conductivity of (1) 0. L. Hollis and W. V. Hayes, J. Cas Chromatogr., 4,235 (1966). (2) R. T. Foley and F. D. Bogar, Fifth Progress Report on Contract NGR 09-003-005, April 30, 1967. (3) R. J. Jasinski and S . Kirkland, ANAL.CHEM., 39, 1663 (1967). (4) R. Jasinski and S. Carroll, ibid., 40, 1908 (1968). ( 5 ) R. S . Cichorz, Contract AT (29-1)-1106, Aug. 29, 1969. 1474
y-butyrolactone and water solutions varies from 7.48 x lo-' ohm-lcm-l for the pure solvent to 8.87 X 10-7 ohm-1cm-' for solutions that are 1.O water by volume. The high solubility of hydrogen in the cyclic esters renders the calcium hydride method unsuitable. It is known lhat lead tetraacetate reacts with water to yield acetic acid and lead oxide which is brown in color according t o the reaction (CHzC00)d Pb
+ 2 H20
1
PbOn
+ 4 CHpCOOH
(1)
Pesez (6) has put this reaction on a quantitative basis by determining the amount of precipitated PbOr by reacting with iodine and back-titrating with sodium thiosulfate solution. He reported the determination of water in amounts of 1 t o 20 mg in samples of 0.5 t o 10 ml. This method has been modified and extended to the determination of microgram quantities of water in a number of solvents and electrolytic solutions of particular interest to current investigations in nonaqueous electrochemistry. EXPERIMENTAL Reagents. Acetic acid was distilled through a 25-cm Vigreux column at a rate of 5 ml/minute and the 117.0117.3 "C fraction was taken and stored over Molecular Sieve (Linde type 3A) to ensure dryness. Benzene was distilled through the same column and stored over sodium. Lead tetraacetate was synthesized by a procedure similar to that described by Pesez (6). Forty grams of red lead oxide, Pb303,was added t o 160 ml of dry acetic acid and stirred at 40-50 "C for 15 minutes. The solution was heated to 70 O C while stirring to ensure complete dissolution and then filtered hot through a Buchner funnel. The filtrate was allowed t o cool to room temperature and filtered. The crude lead tetraacetate was then recrystallized twice from dry acetic acid. Lead tetraacetate prepared in this manner may be stored in a vacuum desiccator over anhydrous CaS04 for a period of at least several months. The lead tetraacetate reagent was prepared by dissolving about 3.0 grams of lead tetraacetate in 100 ml of dry benzene and allowing the solution to stand in a glass stoppered flask in a desiccator over P205for at least 12 hours. At the end of this time, the reagent was usually clear and colorless and could be used with no further treatment. However, if the reagent remained discolored, or as a general procedure to facilitate handling in any case, the following method rendered the reagent clear. An oven dried glass column approximately 5 by 25 cm was fitted with a stopper at the top which was vented to the air through a drying tube. The drying tube was a 3-ml plastic syringe, plunger removed, filled with Aquasorb (Mallinckrodt No. 6063) and plugged with a cork with a small hole. The syringe needle of the drying tube was forced through the stopper in the top of the column. The bottom of the column was fitted with a stopcock which in turn was attached to a syringe needle to allow addition of the reagent directly into the sample test tubes without exposure to the air. A cotton plug was placed in the bottom (6) M. Pesez, BUN. Chim.SOC.Fr., 15, 1108-9 (1948).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
of the column and about 150 ml of Aquasorb was added to the column. The reagent was poured into the column and allowed to stand for approximately 30 minutes prior to use. After that time the required amount of reagent could be metered out through the stopcock, and by flowing through the Aquasorb becomes clear and colorless. A subsequent modification to the test involved the use of more dilute solutions (1.0 gram of lead tetraacetate per 100 ml of dry benzene). This solution usually yields clearer solutions without the column treatment. A standard solution containing approximately 1.0 mg of water per 1.0 ml of methanol was used for preparation of standard curves. The titer of this solution was determined with Karl Fischer reagent, which in turn was standardized by microliter amounts of water using a dead-stop electrometric end point (7). The titer of the standard solution was found to be 1.13 mg of water per ml of solution by this method. This solution was stored in a 100-ml rubber capped syringe bottle for easy usage. Organic Solvents. The organic solvents to which the lead tetraacetate method has been applied are listed in Table I along with the source and method of purification. In a number of cases electrolyte solutions (e.g., 0.5M LiC104 in 7-butyrolactone) were dried by passing through a crushed molecular sieve column. It has been generally observed that, whereas it is easy to prepare solvents of low HzO content (lo0 ppm) HzO is brought into the solution by anhydrous salts such as perchlorate, chlorides, hexafluorophosphates, etc. T o prepare the column the commercial Molecular Sieve 3A was crushed with mortar and pestle and sieved to 20-80 mesh. This was washed thoroughly with water to remove fines and dried at 230 O C and 50 mm Hg for 24 hours. A column 30 cm long and 1.6 cm in diameter was fitted with a 1000-ml flask on top and a No. 25 syringe needle on the bottom. The column was packed with 120 ml of the crushed molecular sieve and the solution allowed to pass through the column at about 15 ml per hour into a vented syringe bottle. Butyrolactone (0.5M LiClO,) treated in this manner was found to have a water concentration of 22.2 i 1.3 ppm. Distilled acetonitrile similarly treated was found to have a water concentration of 7.0 i 1.6 ppm. Procedure. All glassware and syringes were oven dried and removed to cool only immediately before use. Ten test tubes (15 mm X 125 mm) previously etched at 7.0-ml capacity which represented a small, convenient, and standard volume, were fitted with rubber syringe caps, also oven dried. The tubes were vented to the air through 3-ml syringes filled with Aquasorb. The vents were stored in a desiccator over P205when not in use. Table I1 shows the addition and volumes employed in a typical determination. T o the first 6 test tubes from which the standard curve is constructed is added 2.0 ml of dry benzene by syringe through the vented caps. The solvent or solution to be tested is added by syringe to tubes 7-10 and enough dry benzene added to make the volume 2.0 ml. The test solutions were stored in rubber capped syringe (7) I. M. Kolthoff and P. J. Elving, "Treatise on Analytical Chemistry," Part 11, Vol. I., Interscience Publishers, New York, N. Y.,
1961, p 86.
Table I. Organic Solvents to Which the Lead Tetraacetate Method Has Been Applied Solvent Source Treatment y-Butyrolactone General Aniline and Fractionated at 71-72 "C and 2.5 mm of Film Corp. Acetonitrile
Matheson, Coleman and Bell No. 2726 Fisher C-574 SpectroChloroform analyzed Acetic acid Fisher No. A-38 C Reagent-ACS Acetone Fisher A-18 ACS 2-Bromopropane Eastman No. 213
Hg
Fractionated 81 . 5 82.0" None None
None Stored 24 hours over molecular sieve Baker No. 9280 Stored 24 hours over molecular sieve Reagent-analyzed Fisher No. A-834 Stored 24 hours over molecular sieve "Certified" M, C and B No. 1296 Stored 24 hours over molecular sieve Jefferson Chem. Co., Fractionated at 90.0 M, C and B No. 7613 "C and 2.5 mm of Hg
Ethyl acetate Anisole Diethyl carbonate Propylene carbonate
bottles and the samples withdrawn by syringe and added directly to the test tubes without exposure to air. The amount of test solution necessary depends on its water concentration. Generally 1.0 ml or 2.0 ml was used which is in the range of the test for solutions of 0-50 ppm water. Additions of 1.0 ml and 2.0 ml are shown in Table I1 and, consequently, twice as much water should be indicated for tubes 9 and 10 as for 7 and 8. The water-methanol standards were added to tubes 2-6 with a 50-111 syringe in the indicated amounts. Following the addition of the dry benzene, watermethanol standard, and the solvent to be tested, 5.0 ml of lead tetraacetate reagent was added (to the 7.0-ml mark) from the reagent column or by syringe from the reagent bottle at 30-second intervals. The timing was important because the solutions darken slightly upon standing as discussed below. A Beckman DB spectrophotometer was set at 396 mp and slit width of 0.25 mm. The absorbance was adjusted to 0.00 by measuring solution No. 1 us. solution No. 1 in dry capped cells of IO-mm path length. The wavelength of maximum absorption of solution No. 2 was then determined (generally 396-402 mp varying somewhat from day to day and different preparations of reagent) and the absorbance of tubes 2 through 10 was measured at 30-second intervals. RESULTS AND DISCUSSION
The absorption spectrum for the PbO? solution is given in Figure 1. The water determination measurements were made at wavelengths 396-402 mp. Figure 2 is a standard curve recorded from the absorption data of tubes 1-6 in Table 11. The concentrations and standard deviation for 11 determinations on undistilled acetonitrile was 47.6 i 2.8 ppm; the 95% confidence limit of a single value ~ t 6 . 2 ;of the average value 11.9. Known amounts of water were
Table 11. Volumes Employed in a Typical Determination Tube number Dry benzene, ml Solvent, ml H20-methanol, p1 Water, pg Reagent, ml
1
2
2.0
2.0 ...
0.0 0
9.0 10 5.0
...
5.0
3 2.0
4
5
2.0
2.0
...
...
...
18.0
27.0
20 5.0
30
36.0 40
5.0
5.0
6
2.0 ... 45.0 50 5.0
7 1.0 1.0
... X
5.0
8
9
1.0 1.0
0.0
... X 5.0
2.0 . I .
2 5x 0
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
10 0.0 2.0
... 2x 5.0
1475
70
9 A t \ 1 0.6
6o
t-
2ol
3
r l l l 1 . 1 1 1 1 1 360 380 400 420 440
IO 340
0.OLl 0
Wavelength in mp
Figure 1. Absorption spectra of PbO, produced in a water-methanol standard upon addition of reagent
I
I
I
I
I
1
I
I
IO 20 30 40 pg W a t e r Cppm per 1.0ml sample)
I
1
50
Figure 2. Standard curve of a typical determination Absorbance at 396 rnp observed for tubes 1 through 6 in Table I1
Table 111. Recovery of HzO by Lead Tetraacetate Method Acetonitrile 20 Fg 10 pg water added water added Amount of water found, Amount of water found, Amount 29.7 =k 2.0pg 40.3 f 1.7pg found,
No water added
52.0 =k 1.7 Pg
added to acetonitrile and y-butyrolactone; Table I11 illustrates the ability of the method to recover water in acetonitrile. From the viewpoint of its usefulness in investigations in nonaqueous electrochemistry, it is significant that the test is applicable to electrolyte solutions as well as the solvents. No interferences have been observed in solutions of 0.5M LiCIOa in y-butyrolactone or 0.5M (n-C4H&NC104in acetonitrile. It was found that the addition of the reagent (benzene solution) to 0.5M solutions of KPFGin propylene carbonate causes the salt to precipitate. This was also observed with the addition of pure benzene containing no lead tetraacetate. However, it was possible to avoid this complication by preparing the reagent solution in a 1 to 1 mixture of acetonitrile and benzene in which KPF6 is sufficiently soluble to prevent precipitation. It was necessary to use fractionated acetonitrile which had been stored over molecular sieve for a week to ensure dryness. In this case, 5.0 grams of lead tetraacetate per 100 ml of solution was used which scavenged ‘any excess water from the benzene-acetonitrile mixture. This reagent was generally clear after standing for 12 hours, or could be made clear by passage through the glass wool column described below. In quarternary ammonium halide solutions, e.g., tetrabutyl ammonium bromide in acetonitrile, a precipitate of the corresponding lead halide was observed when the reagent was added and consequently the test was not applicable to such electrolytes. The absorbance of the solutions and standards was found to be time dependent. The absorbance increases until about 1476
8 minutes after the addition of the reagent, when a plateau is reached. After about 12 minutes the absorbance slowly decreases until 17 minutes, at which time the solutions slowly begin to darken again. We do not fully understand this behavior which is observed in solutions of all water concentrations, the effect being more pronounced in solutions of greater concentration. For a standard containing 30 pg of water, the change in the absorbance us. time curve between 7 and 10 minutes, the region of the plateau, was 0.015 unit per minute, equal to 0.75 ppm per minute. For this reason, the absorbance of all the tubes was measured at the same time after the addition of the reagent. The only pertinent interferences with the test that have been observed are with some reducing agents, particularly aldehydes. For example, the addition of reagent to butyraldehyde yielded an immediate cloudy white precipitate which increased upon standing. It was concluded that the precipitate was lead diacetate resulting from the reduction of lead tetraacetate and the concomitant oxidation of the aldehyde to the acid. The equivalent reaction was observed when hydroquinone, which is easily oxidized to quinone, was added to acetonitrile. Upon addition of the reagent, the white precipitate formed immediately. That the product was lead carbonate was ruled out because the precipitate was water soluble and could not be induced in acetonitrile which had been bubbled with carbon dioxide. Some lots of commercial propylene carbonate yielded the cloudy precipitate along with the characteristic color indicating the presence of water. Formaldehyde and propionaldehyde have been tentatively identified as impurities in propylene carbonate (3), and it was assumed that oxidation of these aldehyde impurities by the lead tetraacetate resulted in ,the cloudy precipitate. Fractionation of the propylene carbonate usually removed the impurities and the water test was then straightforward. One modification of the technique has involved using a Bausch and Lomb Spectronic 20 spectrophotometer which has
ANALYTtCAL CHEMISTRY. VOL. 42, NO. 13, NOVEMBER 1970
a fixed internal reference and accepts circular cuvets of about 12-ml capacity. Matched cuvets were etched at 7.0 ml and the test performed directly in the cuvettes. This avoids the transfer of the solutions from the test tubes to capped Beckman cells. This modification also has the advantage that the internal reference eliminates any error caused by the darkening of the reference solution during the time that the other solutions are measured. One further modification of the test involved the column used to prepare the reagent. Instead of using a column filled with Aquasorb to clear the reagent, the same column was tightly packed with ovendried glass wool. The sensitivity of the test is at its greatest
with this column to the extent that 2 pg of water is easily determined, but 35 kg is the maximum detectable amount because of the high absorbance (0.85) of solutions at this concentration. It is though that the sensitivity of the reagent treated with the Aquasorb column is lessened by small amounts of P20~from the Aquasorb in the reagent solution which compete with the lead tetraacetate for water. RECEIVED for review May 22, 1970. Accepted August 6, 1970. Work supported by the U S . Army Mobility Equipment Research and Development Center, Fort Belvoir, Va., under Contract No. DA 44-009-AMC-1386 (T).
Least Squares Curve-Fitting Method for End-Point Detection of Chelatometric Titration with Metal Indicator Hisakuni Sat0 and Kozo Momoki Laboratory for Industrial Analytical Chemistry, Faculty of Engineering, Yokohama National Unicersity, Ooka-machi, Minami-ku, Yokohama-shi, Japan For obtaining an accurate end point for a chelatometric titration with indicator, where 1:l chelates (MI and MY) are formed, a numerical calculation method by the least squares method using a digital computer i s presented. Because the conditional formation constants (KJiI and KAIY)for the chelates formed in a titration are also obtained as the results, the present method is more useful for the titration systems in which these values are not known beforehand. To ascertain its utility, the method is applied to the titration of MgZi in the absence and in the presence of 1M KCI or NaCI. In the latter case, KMI and KIIY are considerably reduced and the end point detection has been difficult. Precise and accurate results are obtained rapidly. Some problems accompanying the present method are also discussed.
PHOTOMETRIC STEP-INDICATION ( I ) employed widely in chelatometric titration has been discussed theoretically or practically on the optimal conditions for a sharp indication (2-6) or on the methods to determine the end point in the curved titration plot once obtained (7-10). The end point is often located by linear extrapolation. Musha, Munemori, and Ogawa ( 7 ) extended the linear extrapolation method in acid-base titration by Higuchi, Rehm, and Barnstein ( 1 1 ) to chelatometry. This method seems to be (1) H. Flaschka and P. Sawyer, Tulanta, 9, 249 (1962). (2) G. Schwarzenbach, “Die komplexometrische Titration,” Ferdinand Enke Verlag, Stuttgart, 1955. (3) J. M. H. Fortuin, P. Karsten, and H. L. Kies, Anal. Chim. Acta, 10, 356 (1954). (4) H. Flaschka and S. Khalafallah,Z . Anal. Chem., 156,401 (1957). (5) C. N. Reilley and R. W. Schmid, ANAL.CHEM., 31, 887 (1959). (6) M. Tanaka and G. Nakagawa, Anal. Cliim. Acta, 32, 123 (1965). (7) S. Musha, M. Munemori, and K. Ogawa, Bull. Chem. SOC. Jup., 32, 132 (1959). (8) A. Ringbom, “Complexation in Analytical Chemistry,” Interscience Publishers, New York, N. Y., 1963. (9) E. Still and A. Ringbom, Anal. Chim.Acta, 33, 50 (1965). (IO) E. Still, Suom. Kemistilehti E , 41,33 (1968). (11) T. Higuchi, C. Rehrn, and C. Barnstein, ANAL.CHEM.,28, 1506 (1956).
very ingenious giving the correct end point easily without knowing about the conditional formation constants (KaIIand KJIY). However, the linearity of this plot was often found to be questionable ascribed to the dilution effect during the titration as well as to the incomplete treatment of the concentration of indicator. In other graphical methods, by Ringbom (8) and by Still (IO) for example, KJIIand K3ly must be known beforehand for the accompanying calculation. The present paper deals with a rigorous least squares method (curve fitting method) for obtaining an accurate end point rapidly using a digital computer. The conditional constants are also obtained as the results of this method. Therefore, these values need not be known beforehand. To make the calculation program as general as possible, the theoretical equation (3, 4) to fit a set of titration data is modified to take the dilution effect into account. The present method is applied to the typical Mg-Calmagite-EDTA system and to the more difficult Mg-Calmagite-EDTA-NaC1 (or KCl) system. Although computer calculation of similar titration procedures has been described in the recent book by Dyrssen, Jagner, and Wengelin (IZ), their treatments are essentially based on the tactics by Ringbom (8) as quite different from ours, as will be seen. THEORETICAL
Fundamental Equations. Fortuin, Karsten, and Kies ( 3 ) derived the theoretical equation for the photometric titration curve, in which only 1 :1 complex formations were assumed between metal ion (M) and indicator (I) as well as metal ion and titrant (Y). Although they neglected the dilution effect during a titration, many authors have treated the photometric titration based upon the same equation. If the dilution ~~
~
~~
(12) D. Dyrssen, D. Jagner, and F. Wengelin, “Computer Calcula-
tion of Ionic Equilibria and Titration Procedures with Specific Reference to Analytical Chemistry,” Almqvist & Wiksell, Stockholm, 1968.
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