Determination of Water by Direct Injection Enthalpimetry C. A. Reynolds’ and Sister Marie Joan Harris Department of Chemistry, Unioersity of Kansas, Lawrence, Kans. THEDETERMINATION of water in organic solvents, hydrated salts, desiccants and other salts has most frequently been accomplished with Karl Fischer reagent. The limitations of the method have been thoroughly treated by Mitchell ( I ) . Jacobs (2), in his book on food analysis, discusses several other common procedures for the determination of water in a sample. With the development of thermistors, Harris and Nash (3) proposed method of analyzing traces of water vapor in gases based on the exothermic reaction of the vapor with calcium hydride. Jordan ( 4 ) later introduced thermometric titrations which employ enthalpimetric measurements for quantitative analysis. Wasilewski (5) adapted Jordan’s method and introduced a new technique known as direct injection enthalpimetry (DIE). DIE combined with the Karl Fischer method provided additional sensitivity to a widely used method for water determination (6). Spink and Spink (7) have recently used DIE to analyze binary mixtures of water and miscible organic liquids by injection into sulfuric acid. The method proposed in this paper employs DIE, but utilizes a temperature pulse obtained from the rapid exothermic adsorption of water by molecular sieves. EXPERIMENTAL
Reagents. All the chemicals used were reagent grade. The desiccants were anhydrous calcium chloride (4mesh) and an$ydrous calcium sulfate (8 mesh). The molecular sieves, 4 A, were obtained in a powder form (The Linde Company). The Karl Fischer reagent was a stabilized, premixed, single solution (Matheson, Coleman and Bell). Apparatus. The equipment consisted of an adiabatic cell with Styrofoam insulation and a Wheatstone bridge (8). The bridge was connected to a 2.5-mV strip chart recorder. A lipless beaker was constructed from 38-mm glass tubing to contain a maximum volume of 50 ml. A close fitting lid was milled from Teflon by Dupont and glass tubing was inserted as a guide for the hypodermic needle. Four matched 100,000Q thermistors (Victory Engineering Corp., A-192), which were operated in parallel, were also inserted into the lid. The voltage across the thermistors was supplied by a n absolute dc 1
Present address, Edgewood Arsenal, Edgewood Arsenal, Md.
(1) J. Mitchell, Jr., and D. M. Smith, “Aquametry,” Interscience, New York, N. Y . , 1948. (2) M. B. Jacobs, “The Chemical Analysis of Foods and Food Products,” 3rd ed, D. Van Nostrand Co., New York, N. Y., 1958. (3) F. E. Harris and L. K. Nash, ANAL.CHEM., 23, 736 (1951). (4) J. Jordan, Rec. Chem. Progr., 19, 191 (1958). (5) J. C. Wasilewski, P. T-S Pei, and J. Jordan, ANAL.CHEM., 36, 2131 (1964). (6) J. C. Wasilewski and D. C. Miller, ibid., 38, 1750 (1966). (7) M. Y. Spink and C. H. Spink, ibid., 40, 617 (1968). (8) B. C. Tyson, Jr., W. H. McCurdy, and C. E. Bricker, ibid., 33, 1640 (1961). 348
ANALYTICAL CHEMISTRY
0 P S I CENT
W A l l R X 10’
Figure 1. A comparison of calibrations curves for several different solvents Chloroform B. Toluene C. Benzene D. Ethyl propionate A.
power supply (Kay Lab, San Diego, Calif., Model 30B-100). Magnetic stirring was used with the stirring rate controlled by a transformer. Procedure. The solvents were dried over molecular sieves for a minimum of 24 hours. Three to four standard solutions of known water content were prepared by volume with a IO-pl syringe (Hamilton Co., Whittier, Calif.) to add the water to the reagent grade solvents. If nitrobenzene were the solvent being studied, then 25 ml of nitrobenzene and approximately 0.6 gram of molecular sieves were placed in the reaction beaker. The beaker was stoppered with the lid made of Teflon (Dupont) and placed in the Styrofoam nest, The bucking circuit was used to adjust the scale on the recorder to obtain a 5-mV full-scale deflection. The sensing circuit was set so that 11.0 V were placed across the thermistors. A 1.0-ml hypodermic syringe was used to inject 0.2-ml samples of the standard water solutions of nitrobenzene and a sample of the reagent grade solvent into the reaction cell. The contents of the reaction cell were replaced when the sieves were consumed or the temperature of the contents exceeded that of the sample being injected. A calibration curve was constructed from a plot of the deflection us. the per cent water added. The per cent water originally present in the nitrobenzene was read from the calibration curve by the deflection obtained from the injection of the reagent grade solvent. The same procedure was followed for the other solvents that were studied. For the determination of water in hydrated salts, a calibration curve was constructed for standard solutions of water in N,N-dimethylformamide (DMF). Weighed samples of the hydrated salt were dissolved in D M F and diluted to a total volume of 50 ml. The deflection due to the DMF salt solu-
tion was measured and the per cent water present in each solution was read from the calibration curve. From this, the per cent water of hydration was calculated. The value obtained was verified with a Karl Fischer titration on a methanol solution of the same salt. To measure adsorbed water, C a C 0 3 and AgCl were placed on a platform in a dessicator containing a saturated solution of NaCl for two days. Weighed amounts of the salts were placed in 50 ml of D M F and 50 ml of methanol, respectively. The per cent water was determined by DIE and Karl Fischer reagent as above. The desiccants were exposed to the air, divided between two stoppered flasks and weighed, Fifty milliliters of methanol were added to one flask and 50 ml of D M F to the other, then readings were taken. The per cent water was again determined by DIE and Karl Fischer reagent as above. All Karl Fischer titrations were performed on 10.0-ml samples with a 10.0-ml titrating buret and a dead-stop amperometric method for detecting the end point (9). RESULTS AND DISCUSSION The solvents studied, the slope of the calibration curves, and the standard deviations for each determination are listed in Table I. Each point on a calibration curve represents an average of three or four measurements of the same water standard. Figure 1 compares the calibration curves obtained for several different solvents. Each measurement took approximately 5 minutes to complete. A minimum of four samples could be injected into a solvent before it and the sieves had to be replaced. However, if the sample should contain a large percentage of water, the number of samples that could be injected would be limited by the amount of molecular sieves present and by the change in temperature between the solvent and the water standard being injected. Three solvents were tested which could not be studied under the conditions stated. Acetonitrile and methanol, because of their size, were adsorbed by the 4 A molecular sieves upon contact. It would seem probable that the use of 3 A molecular sieves would solve this problem because such size would still be sensitive to water but not to these solvents. Glycerol could not be tested because the increase in viscosity upon addition of the sieves prevented effective stirring. In addition, hydrocarbons were not included in the solvents studied because of the difficulty involved in preparing water standards in solvents of such low water solubility. The sample size was varied from 1.0 ml to 0.1 ml. Although a larger deflection was obtained with the larger sample, differences in temperature between the sample and the solvent became more important. For this reason, 0.2-ml samples were chosen as those giving the most suitable results. The activity of the molecular sieves could limit the sensitivity of the method. Originally, the molecular sieves were stored in a jar in a dry box. Under the same experimental conditions except that the sieves were stored in an air-tight can in a dry box, the slope of the calibration curves increased by a factor of 1.7. As a result, it is essential that sieves of the same activity be used for all measurements on a given solvent. Avoidance of heat loss or gain during a series of measurements is also essential to the sensitivity of the method. After an hour, the Styrofoam surroundings and the glass beaker began to retain the heat given off in the reaction. This heat retention may be corrected by surrounding the reaction beaker with a constant temperature water jacket.
(9) W. Grand and F. J. Hopkinson, IND.ENG.CHEM.,ANAL.ED., 15, 212 (1943).
Table I. Organic Solvents Slope, mVjO.1
Solvents Methylene chloride Benzene Nitromethane Ethyl ether Toluene Butyl acetate Dioxane i-Propyl alcohol Acetone Carbon tetrachloride N,N-Dimeth ylformamide Chloroform Ethyl propionate n-Butanol N,N-Dirnethyllauramide n-Amyl alcohol
x
x
Std dev, =!=0.004 0.002 0.003 0.003
Hz0 1.38 0.92 1.18 0.58 1.36 0.95 0.71 0.69 0.80 1.97 0.71 1.57
0.001
0.004 0.003 0.005 0.004 0.001 0.006 0.002
0.003 0.003 0.003 0.002
0.60
0.64 0.56 0.73
Table 11. Comparison of Results Water of Hydration
z
Salt CuS04.5 Hz0 cO(N03)~.6 Hz0 Cr(N03)319 H20
H20-DIE 34.7 i 0.8 35.510.2 40.5 i 0 . 2
H20-
Calcd
K. Fischer 35.0 i0 . 8 36.3k0.5 40.8 i 0 . 6
36.07 36.12 40.50
z
Adsorbed Water on Solids Salt CaS04 CaClz AgCl CaC03
z
H20-DIE 2.31 1 0.14 5.61 =t0.15 0.78 i 0.25 8.57i0.16
Z Hz0K. Fischer 2.23 1 0.03 5.78 i 0.17 0.99 i 0.14 8.53 i 0.03
Although a constant dc voltage source was used to apply a potential across the thermistors, two 12-V batteries in parallel will most probably be as efficient and as stable. Table I1 compares the results obtained for the determination of water by DIE and the Karl Fischer reagent. According to Mitchell (IO), a Karl Fischer titration of C u S 0 4 . 5Hz0 results in a value of 4.5 waters of hydration (30.8z) because of the reaction of H I with copper (11). In agreement with this, the value obtained was 31.3 i 0.8x. Correcting for the copper (11) reaction, the value shown in Table I1 was obtained. The data from both methods agree within experimental error. The use of DIE for the determination of water seems especially suitable for small quantities of moisture, but is not limited to this range of analysis. The ease and rapidity with which a sample may be analyzed once a calibration curve has been obtained would make it suitable for quality control studies in industry. The economic nature of the equipment together with its size would allow its use both in the laboratory and in the field. From all appearances, DIE produces results comparable to those obtained from the Karl Fischer method without the limitations imposed by easily oxidizable or reducible materials and troublesome end point detection.
RECEIVED for review October 7, 1968. Accepted November 18, 1968. (10) J. Mitchel, Jr., D. M. Smith, E. C. Ashby, and W. M. D. Bryant, J . Amer. Chem. SOC.,63, 2927 (1941). VOL. 41, NO. 2, FEBRUARY 1969
349
