Anal. Chem. 1993, 65, 1210-1212
1210
Determination of Oxide Ions in Aluminum Chloride-Alkali Chloride Melts by Karl Fischer Titration Hirofumi Kurayasu' Iron and Steel Research Laboratories, Sumitomo Metal Industries, Ltd., 1-8 Fusocho Amagasaki 660, Japan
Yasuo Inokuma Sumitomo Metal Technology, Inc., 1-8 Fusocho Amagasaki 660, Japan
The precise determination of oxide ions in acidic aluminum chloride-alkali chloride melts is important for the control of alloy composition in the electrodeposition of aluminum-manganese alloys. Karl Fischer titration was applied to the determination of oxide ions in the acidic melts (57-63 mol 90AlCl,). Oxide ions were found to be capable of reacting quantitatively with the Karl Fischer reagent when a melt sample was directly injected into the reagent. Oxide ions in eutectic AlC13NaCl-KCl melts (61:26:13mol '3%) were determined with a relative standard deviation of less than 4%. The absorbances of infrared absorption bands observed over the range from 1200 to 1800 cm-I in the spectra of eutectic melts were proportional to the oxide ion concentration determined by this method. INTRODUCTION Acidic aluminum chloridealkali chloride melts (AlC13rich) have been recently used as electrolytes for the electroplating of amorphous aluminum-manganese alloys. 1-3 Oxide ion impurities, which are difficult to avoid owing to the hygroscopic nature of the melts, affect the alloy composition and smoothness of the electrodeposits. The precise determination and control of oxide ions in the melts are, therefore, important for the successful electrodepositionof aluminum-manganese alloys. The behavior of oxide ions in aluminum chloride-alkali chloride melts has been investigated.4-7 At present it is generally recognized that oxide ions exist as AlOCl instead of 0 2 - although the solvated form of AlOCl is not known completely. A few methods for quantifying oxide ions in these melts have been reported. Laher et al. proposed a voltammetric method employing Ta(V) as a probe s01ute.~ The method is complicated and applicable only to AlC13-NaCl melts saturated with NaC1. Flowers and Mamantov suggested an infrared emission methoda9The method requires a special spectrometric cell with a diamond window. (1) Tsuda, T.; Seto, H.; Uchida, J.; Yamamoto, Y.; Usuki, N.; Shiota, T.; Shibuya, A.; Noumi, R. SAE '90 Congress, 1990; Paper 900719. (2) Uchida, J.; Tsuda, T.; Yamamoto, Y.; Seto, H.; Abe, M.; Shibuya, A. Tetsu to Hagane 1991, 77, 931. (3) Stafford, G.R. J . Electrochem. SOC.1989, 136, 635. (4) Tremillon, B.; Bermond, A.; Molina, R. J . ElectroanaL Chem. 1976,
74, 53. (5) Gilbert, B.; Osteryoung, R. A. J . Am. Chem. SOC.1978,100,2725. (6) Berg, R. W.; Hjuler, H. A.; Bjerrum, N. J. Inorg. Chem. 1984, 23, 557. (7) Berg, R. W.; Ostvold, T. Acta Chem. Scand. 1989, A40, 445. (8)Laher, T. M.; McCurry, L. E.; Mamatov, G.Anal. Chem. 1985,57, 500. (9) Flowers,P. S.; Mamantov, G.Anal. Chem. 1987,59, 1062.
0003-2700/93/0385-12 10$04.00/0
Karl Fischer titration is well-known as a method for the determination of H20 in various materials. The method is based on the fact that H20 reacts quantitatively with 12, SO2, RN (pyridine or imidazole), and methanol according to eq 1.
H,O
+ I, + SO, + 3RN + CH,OH
-
SRNHI + RNHSO,CH, (1)
In this paper we demonstrate that oxide ions in acidic aluminum chloride-alkali chloride melts (57-63mol 3'6 AlC13) can be simply and precisely determined by Karl Fischer titration because it was proved that oxide ions in the melts react quantitatively with the Karl Fischer reagent. In addition, the absorbances of infrared bands observed over the range from 1200 to 1800 cm-l in the spectra of the melts are shown to be proportional to the oxide ion concentration determined by Karl Fischer titration. EXPERIMENTAL SECTION Materials. AlC1, (Toyama Chemicals, anhydrous) was purified by sublimation. NaCl and KCl were dried at 500 "C for 4 h. The reagents were mixed and transferred to the vessel, which has been mentioned elsewhere.'O The mixture was then heated to 200 "C. Oxide ion concentration in the melts was adjusted by the addition of NazC03 or AlC13.6H20. NaZC03 was quantitatively dissolved in the melts with the evolution of COz to generate AlOCl according to eq 2.4 The H20 of A1Cl3.6H20 Na,CO,
+ AlCl,
H,O
-
+ AlCl,
AlOCl + 2NaCl+ CO,
-
AlOCl + 2HC1
(2) (3)
was decomposed with the release of HCl according to eq 3 . 6 The Mn concentration in the melts was changed by the addition of MnC12, which was received in the form of MnC12.4H20. It was heated at 250 "C for 4 h under reduced pressure to remove the water. The Karl Fischer reagent (Hydranal coulomat)was purchased from Riedel-de Haen. Instrumentation. Karl Fischer titrations were carried out by using a CA-01 Karl Fischer analyzer (Mitaubishi Chemicals, coulometric titrator), in which 1, was generated electrolytically to make the Karl Fischer reaction proceed. Infrared spectra of the melts were obtained by using a 270-50 infrared spectrometer (Hitachi) with a quartz- or NaC1-windowed cell heated with heating tape. Procedures. A portion of a melt (1.5 g) was sampled by the use of a pipet heated at 100 O C to remove adsorbed water. The sample was then introduced into the cell of the Karl Fischer analyzer filled with the Karl Fischer reagent. These operations were so quickly performed that the sample was not solidified in the pipet. After stirring, a coulometrictitration was carried out. (10) Kurayasu, H.; Inokuma, Y.Proceedingsofthe Thirdlnternational Conference on Progress of Analytical Chemistry in the Iron and Steel Industry (1991), 1992; p 484.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1, 1993 1211
0'31 0
1800
0
0
0.1
0.2
0.3
AlOCl added (mollkg)
Figwo 1. Karl Flscher tltrlmetrlc results of AlOCl in eutectic AI&NaCI-KCI melts wlth added Na2C03(melt temperature, 200 "C).
1600
1400 Wavenumber ( c r n - l )
1200
Flgure 2. Infrared absorption spectra of eutectlc AICI&aCI-KCI melts wlth (A) no addltlon of Na2C03and (B) addltlon of 0.08 mollkg Na2C03(melt temperature, 200 "C; window, NaCI; path length, 20 mm).
The amount of oxide ion was calculated from the quantity of electricity used to reach the end point. One mole of oxide ion consumed 1.93 X lo5 C, which could generate 1 mol of Iz. The handling of reagents, the operationsfor determinationof oxide ion by Karl Fischer titration, and the transfer of the melt sample from the vessel to the cell of the infrared spectrometer were done in a drybox with an Ar atmosphereto prevent moisture contamination.
RESULTS AND DISCUSSION Figure 1 shows the Karl Fischer titrimetric results of eutectic A1C13-NaC1-KC1 melts (61:2613 mol 76 ) with added Na2C03 after the evolution of CO:! ceased. A linear relationship with a slope of unity is obtained between the analytical values of AlOCl and the concentrations of AlOCl generated by the addition of Na2C03. This means that AlOCl reacts quantitatively with the Karl Fischer reagent. It is known that some oxides react with the Karl Fischer reagent; strongly basic oxides such as CaO and MgO react quantitatively." Similar to the case with these oxides, AlOCl can presumably liberate H20 as given in eq 4. The H20 is then titrated. AlOCl
+ ZRNHI
-
AlClI,
+ 2RN + H,O
(4)
Infrared absorption spectra of a eutectic A1C13-NaC1-KCl melt with no addition of Na2C03,melt A, and a melt with the addition of 0.08 mol/kg Na2C03, melt B, are shown in Figure 2. Absorption bands at ca. 1570,1470,1360, and 1240 cm-1 are observed in the spectra of both melts, although the measurements were not very intensive because the NaCl windows were attacked by the melts. The absorbance ratios of melt B to melt A at 1570, 1470, 1360, and 1240 cm-1 are 2.3,2.0,2.3,and 2.2, respectively. These values are approximately equal to the ratio of AlOCl concentration determined by Karl Fischer titration; the AlOCl concentration of melt A was 0.065 mol/ kg and that of melt B was 0.148 mol/kg. These absorption bands can be, therefore, attributed to AlOCl although the assignment of the vibrational modes was not attempted. In addition, the value of the AlOClconcentration found in melt A must represent the initial oxide ion contamination. This value is higher than that reported in the literature.8 The residual H20 in the drybox might cause this higher oxide ion concentration because the moisture level (11) Hydranal Manual; Riedel-de Haen: Seelze, Germany, 1987; Chapter 10.
x)
Figure 3. Infrared absorption spectra of OH in a eutectlc AIC13NaCI-KCI melt: (a)before addltion of 2.5 mmollkg AIC13.6H20 (b) 10 rnln after: (c) 20 min after; (d) 30 rnln after; (e) 40 mln after; (5) 120 min after (melt temperature, 200 "C; window, quartz; path length, 10 mm). of the atmosphere in the drybox was not controlled as carefully as that described in the literature. Figure 3 indicates the infrared absorption spectral change of a eutectic AlCl3-NaCl-KC1 melt with the passage of time after the addition of AlCl3-6HpO. The absorption band of OH appears on addition of AlC13.6H20and decreasesgradually with time. This implies that H20 reacts with the melt to generate AlOCl as a final step. In addition, transient intermediates containing a OH group such as Al(OH)C12might be present as shown in eq 5. Hydroxychloroaluminate species b
t
H@
+
AI&
A
HCI
HCI
AI(OH)CC
Awl
(5)
have been recognized in 1-ethyl-3-methylimidazoliumchloride-aluminum chloride melts.12 Karl Fischer titrations of eutectic AlCl3-NaCl-KCl melts with added A1C13.6H20were carried out after no OH band was observed. Similar to the case with Figure 1, a linear relationship with a slope of unity is obtained between the analytical concentrations of AlOCl and the concentrations of (12)Zawodzinski, T.A., Jr.; Osteryoung, R. A. h o g . Chem. 1990,29, 2842.
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ANALYTICAL CHEMISTRY, VOL. 65, NO.
9. MAY 1, 1993
Table I. Karl Fischer Titrimetric Results of a Eutectic AlCls-NaCl-KCl Melt as a Function of Time after Addition of 0.04 mol/ke A1Cls.6HzOa ~~
time after addition (min)
oxide ion concn (molikg) 0.312 0.305 0.314 0.300 0.310
10 30 60 90 1206
*
a Melttemperature,200OC. NoOHabsorption bandwasobserved in the melt.
Table 11. Precision of Karl Fischer Titrimetric Results of Eutectic AlCls-NaCl-KCl Melts (a = 6)
0
0:5 1 .o AlOCl added (mollkg) Figure 4. Karl Fischer titrimetric results of AlOCl in eutectic AIC13NaCI-KCI melts with added AtC13.6H~0(melt temperature, 200 O C ) . 0
AlOCl generated by the addition of AlC13-6Hz0,as shown in Figure 4. Table I gives the Karl Fischer titrimetric results of AlOCl in a eutectic AlC13-NaCl-KCl melt as a function of time after the addition of AlCl3.6H20. In spite of the existence of the OH band, the analytical values are constant. These results suggest that Al(OH)C12 as well as AlOCl reacts quantitatively with the Karl Fischer reagent. The effect of melt compositionon the Karl Fischer titrations was examined. The concentration of in AlCl3-NaCl melts was varied from 63 to 57 mol % by the addition of NaCl in a melt with 63 mol % the composition of the melts was calculated from the concentrations of A1 and Na deter-
melt no.
mean (molikg)
re1 std dev ( % )
1 2
0.054 0.406
3.8 2.2
mined by atomic absorption spectrometry. The Karl Fischer titrimetric results of AlOCl were found to be constant over the range after correction for dilution with the addition of NaCl. In addition, no change of the analytical values of AlOCl was observed over the Mn concentration range of eutectic AlCl3-NaCl-KCl melts from 0 to 0.08 mol/kg. Table I1 shows the precision of this method. AlOCl in eutectic AlCl3-NaCl-KCl melts can be determined with a relative standard deviation of less than 4%. These results demonstrate that the Karl Fischer titration is a simple and precise method for the determination of AlOCl in the melts without special equipment.
RECEIVEDfor review October 29, 1992. Accepted January 25, 1993.
