Water Determination by Near Infrared Spectrophotometry
SIR: Determination of water in some organic systems is difficult or impossible by the classical Karl Fischer method; thus alternate methods for water are always of value. The advent of instrumentation and techniques for near infrared spectrophotometry has provided a useful analytical tool for this purpose. The 1.9-p near infrared water absorption band has been used by several investigators, including Cordes and Tait (a), who determined water in pure and impure hydrazines, and Chapman and Nacey ( I ) , who used the 1.9-p band for the determination of water in glycerols. Recently, Streim and coworkers (3) published a near infrared procedure for the determination of water in 1,l-dimethyl-hydrazine, diethylenetriamine, and mixtures of the two compounds, in which the preparation of a reference sample by drying with a Molecular Sieve 4A column was first described. In the present correspondence, the preparation of a dry reference sample by batch drying with Molecular Sieve 4A is described. The near infrared determination of water, Table 1. Drying Efficiency of Molecular Sieve 4A
Drying time, hr.
Compound Acetone 2-Propanol
Isopropyl ether
Methyl Cellosolve
n-Butylamine
0 2.5 21 0 3 20 26 0 3 21 26 0 I 2.5 18 0 24
H20,
wt. % 0.45 0.007 0.001
0.14 0.025
0.007 0.003 0.12 0.003 0.002 0,001 0,090 0.020 0.010 0.003 2.85 0.013
using the dried sample for calibration, is applicable to many classes of organic compounds. EXPERIMENTAL
A Beckman Model DK-2 recording spectrophotometer with lead sulfide detector and tungsten lamp source was used for the spectrophotometric measurements. The cells were near infrared silica, 1-cm. path length, Beckman No. 46008. Karl Fischer reagent, prepared in methyl Cellosolve (Umon Carbide Chemicals Co.), (2-methoxyethanol), with a factor of 0.1, was used to determine the water content for comparison with the near infrared results. The titrations were performed in methyl Cellosolve, pyridine, or acetic acid. I n the drying step, 150 ml. of liquid were mixed with 40 to 80 grams of Linde Molecular Sieve 4A, 1/16-inch pellets, in 250-ml. glass-stoppered flasks. For the determination of the water content with Karl Fischer reagent, a portion of the sample was withdrawn with a hypodermic syringe. For the near infrared spectrophotometric measurements, a portion of the sample, dried for 24 hours, was used as the reference solution for the spectrophotometric measurements and calibration standards were prepared from the remainder of the dried sample by adding a weighed amount of water to a weighed amount of sample. The calibration standards and samples were scanned from 2.2 p to 1.7 p in the near infrared on the Beckman DK-2. The absorbance of the water band was measured as the difference between the peak maximum near 1.9 p and a horizontal baseline, which was drawn from some reproducible point on the curve not affected by the water concentration. RESULTS A N D DISCUSSION
Molecular Sieve Drying. The investigation of the efficiency of drying by Molecular Sieve 4A was limited to those liquids in which water could be determined with Karl Fischer reagent. However, the basic aim of the study was to determine which general classes of compounds could be dried, and to
Table II. Effect of Original Water Content and Amount of Molecular Sieve 4A on the Drying of Methyl Cellosolve
Original water
content, yo
a
Weight Molecular Sieve 4A,
Drying time, hours, efficiency
%a
g. 1 3 6 96 0.094 96 40 72 97.1 91.8 0.56 40 60 95.6 40 68.8 90.1 0.93 98.3 0.98 80 89.8 97.8 93.4 88.3 1.97 40 70.6 97.1 80 86.0 94.0 2.00 The efficiency was calculated as the relative per cent of the water removed.
15 10
ANALYTiCAL CHEMISTRY
24 96 97.9 98.2 99.1 96.8 99.1
use this information to predict whether a given system could be analyzed for water. To test the drying efficiency of Molecular Sieve 4A4, 150 ml. each of methyl Cellosolve, acetone, 2-propanol, isopropyl ether, n-butvlamine, and acetic acid were dried with 40 grams of Molecular Sieve 4A. Samples were withdrawn a t intervals and the water content was determined with Karl Fischer reagent. The results, shown in Table I, indicate that under the conditions used, the water content can be reduced to 1 to 4% of its original value in 12 to 24 hours, with the exception of acetic acid. No decrease in the water content of acetic acid was noted even with 80 grams of Molecular Sieve 4A. This work did not define the effect of the amount of water to be removed becauee all the samples, except n-butvlamine, contained onlv small amounts of water. Therefore, known amounts of water up to 2% by weight were added to 150 ml. of methyl Cellosolve and 40 or 80 grams of Molecular Sieve 4A were added. Samples were withdrawn a t intervals and the water was determined. As can be seen from the results shown in Table IT, the ratio of the original amount of water to the amount of Molecular Sieve is important. For example, with 2.85 grams of HzO and 40 grams of the Molecular Sieve, 3.2% of the original amount was still in solution after 24 hours, while with 2.90 grams of HzOand 80 grams of the Molecular Sieve only 0.901, remained after 24 hours. The technique of batch drying with Molecular Sieve 4A is extremely simple and requires no attention during the drying period and no special apparatus. The effectiveness of the technique mas shown by its use in the near infrsred determination of water described in the following section. Near Infrared Analysis. Calibration standards mere prepared from dried samples to contain 0.1, 0.3, 0.5, and 0.7% water, and scans in the near infrared were then obtained from 2.2 to 1.7 p with the dried material 3s the reference solution. The absorbance of the water peak near 1.9 p was measured, and plotted against weight yo water. The resulting calibration curves were straight lines in every case. Samples with an unknown water content mere also scanned with the dried material as the reference, and the water content was obtained from the calibration curve. For evaluation of the near infrared method, the water content was then determined independently by the Karl Fischer method.
Results obtained on 13 different organic liquids are compared in Table 111. Functional groups such as alcohols, amines, esters, ketones, and aldehydes are represented. In general, the agreement bctween the near infrared and Karl Fischer results is good, especially considering that only single determinations were made. I n the case of morpholine, the low results were caused by insufficient drying; the dried material contained 0.043% water by the Karl Fischer method. However, morpholine can be more efficiently dried by using a larger amount of Molecular Sieve 4%.The relatively poor results with n-butylamine are probably caused by the broadness of the water band and interference from the primary amine peak, making accurate measurement of the absorbance difficult. Excluding the few extreme errors, the accuracy is about *0.02~o, absolute, in the range 0.02 to 1% water. The absorptivity of the water band ranged from 0.52 ml. per gram-em. in triethylamine to 1.81 ml. gram-cm. in dimethylphthalate. No special precautions were taken to exclude air from the samples and standards, except that all were kept in closed containers whenever possible and the usual precautions for handling hygroscopic materials were observed. 'The various data indicate that the near infrared determination of water with batch drying by Molecular Sieve 4.i is applicable t o many organic systems containing alcohols, esters, ketones, aldehydes, and amines. The sensitivity and accuracy of the method are a t least partly dependent upon the hydrogen-bonding property of the solvent, with the poorer hydrogen-
Table 111.
Comparison of Near Infrared and Karl Fischer Methods
HzO,
Compound Acetone Acetonitrile n-Butylamine Diethylamine Dimethylphthalate Ethyl acetate Ethylene glycol 2-Propanol Methyl Cellosolve 2-Methylpentaldehyde Morpholine
wt. %
by KF 0.50 0.56 0.49 0.17 0.68 0.27 0.79 0.18 0.81 0.17 0.46 0.25 0.62 0.20 0.51 0.13 0.52 0.17 0.65 0.22 0.58 0.40
Phenyl Cellosolve
0.55 0.13 0.49
H20, wt. % by NIR 0.52 0.59 0.48 0.16 0.65 0.31 0.66 0.17 0.75 0.17 0.48 0.23 0.61 0.21 0.50 0.13 0.53 0.15 0.66 0.24 0.59 0.36 0.51 0.13 0.50
bonding solvents having better sensitivity and accuracy. The most critical aspect of the method is the drying step, and the results will automatically be low by the amount of water not removed from the dried sample. LITERATURE CITED
(1) Chapman, D., Nacey, J. F., Analyst
83,377 (1958). (2) Cordes, H. F., Tait, C. W., ANAL. CHEM.29,485 (1957).
Deviation of NIR from KF, % as Ht0 +o. 02 +0.03 -0.01 -0.01 -0.03 + O , 04 -0.13 -0.01 -0.06 0.00 f0.02 -0.02 -0.01 +0.01 -0.01 0.00 +o. 01 -0.02 0.01 +0.02 +0.01 -0.04 -0.04
Relative deviation, yo
+
0.00 +o. 01
+4
-6 -4
$15 - 16 -6 -7 '+4 -8 -2
?; '+2
- 12 $2
+9
+2 - 10
-7
'+2
(3) Streim, H. G., Boyce, E. A., Smith J. R., ANAL.CHEM.33,85 (1961). Division of Analytical Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September 1962. R. L. MEEKER F. E. CRITCHFIELD E. T. BISHOP Research and Development Dept. Union Carbide Chemicals Co. Division of Union Carbide Corp. South Charleston, W. Va.
indirect Procedure for the Determination of Tin(l by Potentiometric Titration SIR: The quantitative determination of tin in the bivalent state is a rather tedious analysis as the tin(I1) ion is very sensitive t o air oxidation. To titrate tin(I1) directly with an oxidizing agent such as iodine, potassium iodate, potassium permanganate, or potassium dichromate, it is necessary t o remove all dissolved oxygen from the titrant and to maintain an oxygen-free atmosphere above the solution during titration. The need for these precautions is particularly acute when either permanganate or dichromate solution is used because each has the ability to induce the air oxidation of tin(I1) ions ( I , 14). A review of all direct methods of analysis for tin(I1) is given by Kolthoff and Elving (11). The need for titrating in an oxygen-
free atmosphere can be avoided by use of an indirect procedure. Indirect methods of analysis are often applied to strong reducing agents which are susceptible to air oxidation. By adding an excess of some readily reducible species, an equivalent amount of an intermediate is formed which can then be titrated in the presence of air. A typical reagent for this type of procedure is a solution of iron (111)chloride. Use of an indirect method for estimating tin(II), in which iron(I1) is produced as an intermediate, was first reported by Druce (3) ; however, his procedure, which involved titrating the iron(I1) intermediate with potassium dichromate and using diphenylamine as an indicator, was capable
of onlv ~ 2 %accuracv. Another procedire u t i l k i g the &me strategy was reported recently by Donaldson and Moser (g), who employ ceric sulfate and 1,lO-phenanthroline-ferrous sulfate as the titrant and indicator, respectively. An accuracy of approxtin(I1) content is imately *0.2% obtained by use of this procedure. The only other mention of indirect tin(I1) analyses, via the iron(I1) route, is found in the reference texts of Kolthoff and Sandell (la)and Laitinen (IS) ; however, no procedural details are given in either instance. This paper presents an alternate procedure which involves potentiometrically titrating the iron(I1) intermediate with a standard solution of potassium dichromate. VOL. 34, NO. 1 1 , OCTOBER 1962
151 1
