ISSN 00201685, Inorganic Materials, 2011, Vol. 47, No. 15, pp. 1635–1639. © Pleiades Publishing, Ltd., 2011. Original Russian Text © L.P. Eksperiandova, A.I. Fedorov, N.A. Stepanenko, T.A. Blank, 2010, published in Zavodskaya Laboratoriya. Diagnostika materialov, 2010, Vol. 76, No. 12, pp. 8–11.
Method for Water Determination Using an Elemental Analyzer L. P. Eksperiandova, A. I. Fedorov, N. A. Stepanenko, and T. A. Blank Institute for Single Crystals, National Academy of Science of Ukraine, pr. Lenina 60, Kharkiv, 61158 Ukraine email:
[email protected] Received November 16, 2009
Abstract—The possibility of determining the total water content in inorganic salts and their crystal hydrates using a EuroEA3000 elemental analyzer (EuroVector, Italy), designed for CHNS analysis of organic com pounds, is shown, and the metrological characteristics of such determination are assessed. To confirm the correctness, the results obtained are compared to independent data obtained by Fisher titrimetric and gravi metric analyses. The accuracy of determination with the elemental analyzer is comparable with that of the titrimetric method. The essential advantages of the method suggested over the conventional techniques are its high speed (only 5–6 min), use of milligram charges, and simplicity of performance. The simultaneous assessment of sulfate sulfur that is part of the analyzed salt is also possible. Keywords: water in inorganic salts, CHNS elemental analyzer, nonconventional analysis method; express water determination. DOI: 10.1134/S0020168511150052
It is known that the purity of starting compounds or reagents and their correspondence to stoichiometric composition affect the physicochemical and opera tional properties of the resulting functional materials. For example, the number of molecules of crystalliza tion water in inorganic substances affects their physi cal parameters such as density and crystal structure (the density of anhydrous copper sulfate with a rhom bic structure is 3.6 g/cm3, while its fivehydrated crys tal hydrate has a density of 2.3 g/cm3 and triclinic structure) [1, 2]. Water in inorganic substances is determined, as a rule, by the conventional gravimetric procedure [3– 5], based on drying of the analyzed sample to constant weight at a temperature lower than its melting or decomposition point. Drying is performed in a tem perature range from 100 to 300°C, keeping the work ing temperature slightly higher than the reference value. The process takes typically a lot of time (several days) and requires conditions excluding sample pollu tion from the environment. Often, water in inorganic salts (after its extraction with methanol) is determined by the Fisher titration method [3–6]. This method has some restrictions; for example, it can not be used to analyze compounds that interact with the Fisher reagent [compounds of iron (III), cerium (IV), and copper (II), chromates, bichromates, permanganates, and so on]. The dura tion of the titrimetric method for water determination depends on the kinetics of water extraction from the samples, since the titration itself takes only several minutes. In determination of water in salts from which
water cannot be extracted, as well as in the cases where the period of quantitative water extraction exceeds several hours, the analyzed samples are subjected to pyrolysis in a quartz tube, and then the Fisher titration of the distillate is performed. In so doing, the total time for the analysis is about 1 h [5]. This technique allows water to be determined in such functional materials as KCl; Mg, Ca, and Ba fluorides; and Mg, Al, Y, and Nd oxides [7]. Sometimes, dielcometry is used to determine water. This method is based on the fact that the dielec tric permittivity of most compounds is considerably lower than the corresponding value for water. How ever, the analysis of solid and freerunning substances by this method is complicated, since the dielectric per mittivity depends not only on the nature of the com pound under analysis and its humidity but also on the particle form, packing density, and many other factors [5, 8]. The realization of this method is associated with the construction of special dielcometric cells whose geometrical and physical parameters depend on the nature of the analyzed material and set problem; the method was used, in particular, in the analysis of alu minum oxide and alcohols [9, 10]. It should be noted that, if gravimetric and titrimet ric methods are designed to determine total water con tent (including chemically bound and crystallization water), dielcometry is applicable only for the determi nation of weakly bound water, for example, adsorbed water. The EuroEA3000 elemental analyzer from EuroVector (Italy) is designed to determine hydrogen,
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Table 1. Temperature characteristics of crystal hydrates [1, 2] Salt MnSO4 ⋅ H2O CuSO4 ⋅ 5H2O ZnSO4 ⋅ 7H2O MgSO4 ⋅ 7H2O Na3PO4 ⋅ 12H2O Na2MoO4 ⋅ 2H2O Na2WO4 ⋅ 2H2O
Crystal hydrate Anhydrous salt decomposition point melting point 280 (–7H2O)* 250 (–5H2O) 280 (–7H2O) 200 (–7H2O) 100 (–12H2O) 100 (–2H2O) 100 (–2H2O)
850 650 680 1127 1340 687 696
* There are no reference data for monohydrate MnSO4 ⋅ H2O.
carbon, nitrogen, and sulfur in solid organic com pounds from n to n × 101 wt % [11, 12]. The qualitative and quantitative analyses, which are carried out using an original packed column in a complex with the reac tor filled with a catalyst, are based on the identification and measurement of chromatographic peaks. After burning the probe in the reactor at 980°C and passing the gaseous products through the chromatographic column, H2O, CO2, SO2, and N2 are determined with a catharometer. Since in the case of hydrogen determi nation water serves as an analyte, it is logical to assume that its direct determination is feasible. The analyzed probes can be inorganic salts including crystallization water in their composition. Since the contribution of free water content in inorganic salts to its total con tent, as a rule, is insignificant, the results of such anal ysis can be used for approximate assessment of water content in inorganic crystal hydrates. The goal of the present work is to investigate the possibility of express determination of total water in inorganic salts using an elemental analyzer. Equipment and reagents. The elemental analysis was performed with an EA3000 CHNS analyzer from EuroVector s.p.a. (Italy). The samples for gravimetric measurements were dried to constant weight in an SNOL 7,2/1300 laboratory muffle oven (Lithuania). The titration by the Fisher method was carried out with the apparatus described in [6], which was pro tected from atmospheric moisture. Weighing was per formed with a Sartorius CP2P microbalance (Ger many) (elemental analysis) and a Kern ABJ 2204M analytical balance (Germany) (gravimetric analysis). All the salts for investigations were of analytical grade; Lcystine (E11009) used as a reference sample for elemental analysis was calibrated by the manufac turer of the unit. CHNS elemental analysis. The charge of the ana lyzed (or standard) sample with the weight ranging from 1 to 5 mg, taken with an accuracy of 0.001 mg, was packed in a tin capsule, which was introduced into the elemental analyzer autosampler. Then the probe was burned in an automatic mode, and the resulting gaseous products were analyzed chromatographically.
The chromatogram obtained was processed using an original computer program Callidus. The calibration with respect to the reference sample was carried out for each series of analysis. Gravimetric analysis. The charge of the analyzed compound with the weight of ~5 g was placed in a pan, brought to constant weight, and then transferred into a thermostat and dried to constant weight. The pan with the analyzed substance was cooled in an exsicca tor with calcined calcium chloride for 30 min prior to each weighing (with an accuracy of up to 0.0001 g). Titrimetric analysis. The determination of water in the samples (with the weight from 0.01 to 0.3 g, accu racy 0.0005 g) was carried out by the Fisher method with visual indication of the final point in the presence of a background dye with a unit protected from atmo spheric moisture. Methanol was used as an extracting agent, the water in which was bound by the Fisher reagent in the preliminary titration [13]. The resulting mixture was stirred using a magnetic stirrer at room temperature for the period of time required for the qualitative extraction of water from the sample, and then the suspension was titrated by the Fisher tech nique. Prior to the beginning of investigation, the metro logical characteristics of the original procedure [14, 15] for hydrogen determination in organic substances (n = 10; P = 0.95) were estimated using the reference Lcystine sample enclosed in the analyzer. Mean value found, wt % Calculated content, wt % sr* ±δ**, wt % tP,f = n – 1 tfound
5.04 5.03 0.02 0.1 2.26 0.29
* sr was estimated from the data of an analytical archive for a long period (n = 100). ** The confidence interval ±δ was calculated from ten parallel determinations.
Random errors characterized by the relative stan dard deviation sr are small, the confidence interval estimated from ten experiments is 2% relative, and the systematic errors estimated using t criterion are insig nificant. The choice of the objects of investigation was restricted to compounds the temperature of water release from which does not exceeds the reactor tem perature, as well as salts containing no halogens, since halogenide compounds form halogen hydrides upon decomposition, which irreversibly poison the chro matographic columns of the elemental analyzer (Table 1). INORGANIC MATERIALS
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METHOD FOR WATER DETERMINATION USING AN ELEMENTAL ANALYZER
The reference sample for analysis of inorganic salts was original LCystine containing 5.04 wt % hydro gen, which is used in the analysis of organic com pounds. It was shown that, to make the absolute con tent of hydrogen in the probe equal to its absolute con tent in the reference sample, in the choice of the charge weight m, one must take into account the approximate content of crystallization water (the number of its molecules N) in the crystal hydrate. Thus, at N = 1–5 m can range from 2 to 5 mg, while at N ≥ 5 it can range from 1 to 2 mg (otherwise, the hydrogen and sulfur peaks coalesce). A typical chromatogram is presented in the figure. The results of analysis for selected crystal hydrates are listed in Table 2. It is obvious that the actual number of water molecules N satisfactorily agrees with its nomi nal number for all the crystal hydrates, except for ZnSO4 and Na3PO4, for which the found number of crystallization water molecules is much higher than the nominal ones. Apparently, a fraction of the crystal lization water was disintegrated upon prolonged (mul tiyear) storage of the open package of these reagents. To confirm the correctness, the results were com pared to the data of independent Fisher titrimetric and gravimetric analyses. The results obtained using the elemental analysis are in good agreement with the data of the titrimetric analysis (Table 3). The observed dif ferences in the results for magnesium sulfate and sodium molybdate crystal hydrates can be explained in the following manner. In the first case the, during the titration, some of the water molecules are bound by magnesium salts (such a technique is used, in particu lar, for deep dehydration of alcohols [16]), and the results appear to be understated. The overstated results derived by the Fisher method during sodium molyb date analysis are apparently related to the interaction of this salt with the components of the Fisher reagent over prolonged extraction (more than a day). It is also obvious that the elemental analyzer allows determina tion of water in CuSO4 ⋅ 5H2O, for which the Fisher titration method is impossible, as was already men tioned. Results closer to the elemental analysis data were obtained by conventional gravimetric analysis (see Table 4). Despite some dissimilarities, the suggested method based on elemental analysis can be recom mended for approximate assessments of the number of crystallization water molecules (after rounding the results to whole values of N) in inorganic crystal hydrates. Let us note that the experience of determining inorganic hydrogen using the elemental analyzer [17] was used to estimate the content of crystallization water in such a complex compound as phosphomolyb dic acid (PMA), often used as a reagent to determine reducing agents. The number of water molecules in PMA can range from 12 to 60. The result obtained INORGANIC MATERIALS
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C E, μV 90 80 70 N 60 50
(a)
40 30
H
20 10 0 E, μV 60 50 40 30 20 10
1637
100
S
200
300
400
t, s
400
t, s
(b) S
H
0
100
200
300
Chromatograms of the (a) reference (Lcystine) and (b) analyzed (MnSO4 ⋅ H2O) samples.
from three parallel experiments (13.1 wt %), corre sponding to N = 16, was confirmed by the Fisher titri metric analysis (13.6 wt %). Along with the investigation of the possibility to determine water, it was established that the tempera ture mode of this elemental analyzer is also suitable for Table 2. Results of hydrogen determination in crystal hydrates according to the elemental analysis data, wt % (n = 10, P =0.95)* Calculated (by the chemical formula)
Found
MnSO4 ⋅ H2O
1.20
1.21 ± 0.08
CuSO4 ⋅ 5H2O
4.05
4.4 ± 0.3
ZnSO4 ⋅ 7H2O
4.90
2.6 ± 0.3
MgSO4 ⋅ 7H2O
5.73
6.0 ± 0.4
Na3PO4 ⋅ 12H2O
6.36
4.3 ± 0.4
Na2MoO4 ⋅ 2H2O
1.67
1.7 ± 0.1
Na2WO4 ⋅ 2H2O
1.22
1.16 ± 0.07
Salt
* The confidence intervals were estimated from ten parallel deter minations.
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Table 3. Comparison of the results of water determination in crystal hydrates (P = 0.95, f = n – 1)using the elemental ana lyzer (A, n =10), Fisher titrimetric method (B, n = 5), and gravimetric method (C, n = 5)* Found H2O, wt %
N calculated from the results of analysis
Salt A**
B
C
A
B
C
MnSO4 ⋅ NH2O
10.6 ± 0.7
8.8 ± 0.8
10.57 ± 0.05
0.99 ± 0.06
0.81 ± 0.07
0.997 ± 0.004
CuSO4 ⋅ NH2O
39 ± 2
35.92 ± 0.01
5.6 ± 0.2
ZnSO4 ⋅ NH2O
24 ± 2
24 ± 1
27.3 ± 0.3
2.9 ± 0.2
2.83 ± 0.09
3.37 ± 0.03
MgSO4 ⋅ NH2O
49 ± 4
42 ± 1
46.0 ± 0.1
6.4 ± 0.3
4.84 ± 0.07
5.70 ± 0.01
Na3PO4 ⋅ NH2O
38 ± 4
37 ± 2
35.3 ± 0.1
5.7 ± 0.4
5.34 ± 0.03
4.99 ± 0.01
Na2MoO4 ⋅ NH2O
14.7 ± 0.9
25 ± 3
14.8 ± 0.1
1.98 ± 0.1
3.8 ± 0.4
2.00 ± 0.01
Na2WO4 ⋅ N2H2O
10.4 ± 0.6
11.9 ± 0.4
10.6 ± 0.1
1.89 ± 0.1
2.20 ± 0.07
1.96 ± 0.01
–
–
4.963 ± 0.001
Notes: * The confidence intervals for each analytical method were estimated from n parallel determinations. ** The water content was calculated according to the found hydrogen content (see Table 2, column 3).
REFERENCES Table 4. Results of the determination of sulfate sulfur using the elemental analyzer, wt % (n = 10, P = 0.95)* Salt
Calculated (by the chemical formula)
Found
MnSO4 ⋅ 0.99H2O CuSO4 ⋅ 5.6H2O ZnSO4 ⋅ 2.9H2O MgSO4 ⋅ 6.4H2O
18.93 12.26 15.01 13.61
21.0 ± 0.5 13.4 ± 0.4 14.4 ± 1.1 13.1 ± 1.0
Notes: * The confidence intervals were estimated from ten par allel determinations. ** The numbers of water molecules for crystal hydrates are taken from this study.
the estimation of the content of sulfate sulfur. The results of the analysis of some sulfates are presented in Table 4. Let us note that, in the case of MgSO4 ⋅ 6H2O, the melting point of its anhydrous salt exceeds the reactor temperature. However, the real temperature of the reaction zone, as well as in the case of other salts, is significantly higher than the temperature of the reactor itself owing to the additional heat that is released upon burning of the tin capsule in which the analyzed sample is packed in an oxygen atmosphere. The results of the performed investigations indicate that the temperature mode of the elemental analyzer, designed first of all for burning of organic probes, is suitable for the determination of total water and approximate assessment of sulfate sulfur in inorganic compounds. Therewith, the advantages of the sug gested method are as follows: it allows using very low charges of the analyzed sample (1–5 mg), and the period of its performance is very short, running just several minutes.
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