Anal. Chem. 1997, 69, 4728-4730
Catalytic Reduction of Water to Hydrogen for Isotopic Analysis Using Zinc Containing Traces of Sodium Juha A. Karhu
Geological Survey of Finland, P.O. BOX 96, FIN-02151 Espoo, Finland
AnalaR zinc shot manufactured by BDH Chemicals has been widely used for the conversion of water to hydrogen for isotopic analysis. New chemical analyses indicate that AnalaR zinc contains relatively high quantities of sodium as impurity. This compositional feature may characterize AnalaR zinc in general, having made it a successful reagent for water reduction. Experiments using Zn-Na mixtures demonstrate that trace quantities of sodium in zinc may act as a catalyzing agent significantly promoting the reduction of water by zinc. Re-formed zinc with ∼200 ppm admixed sodium may be used for quantitative reduction of water to hydrogen for isotopic analysis. Isotopic measurements of waters using this re-formed zinc are insensitive to moderate excess of reagent relative to water. Reproducibility of the isotopic composition of waters processed in sealed quartz ampules is (0.7‰ for δD. The technique using zinc metal for the conversion of water to hydrogen for stable isotope analysis was introduced by Coleman et al.1 They also identified the reagent, AnalaR zinc metal shot, supplied by BDH Ltd., Poole, U.K., that could be used in the reaction. Since then the method has been studied extensively, and although several other zinc types have been tested, AnalaR zinc has been the only commercial product shown to reduce water quantitatively and reliably.2,3 It has not been resolved why under similar conditions AnalaR zinc reduces water quantitatively while other zinc types produce incomplete reactions. The issue has turned out to be more important now that BDH Ltd. has discontinued the supply of the AnalaR zinc shot. It has been speculated that the surface texture of zinc, which in AnalaR is smoother and more homogenous than in the other brands, may be the critical factor.2,4 An alternative explanation is offered by compositional differences among various zinc types, but only limited variation in composition has been reported. Kendall and Coplen2 noted that AnalaR contains at least 5 times more lead (36 ppm) than the other brands they studied. Recently, a new type of zinc for the conversion of water to hydrogen was prepared by the Biogeochemical Laboratory of the University of Indiana.5,6 Unfortunately, details on the composition or surface texture of this zinc have not been published. (1) Coleman, M. L.; Shepherd, T. J.; Durham, J. J.; Rouse, J. E; Moore G. R. Anal. Chem. 1982, 54, 993-995. (2) Kendall, C.; Coplen, T. B. Anal. Chem. 1985, 57, 1437-1440. (3) Tanweer, A.; Hut, G.; Burgman, J. O. Chem. Geol. 1988, 73, 199-203. (4) Florkowski, T. Int. J. Appl. Radiat. Isot. 1985, 36, 991-992. (5) Venneman, T. W.; O’Neil, J. R. Chem. Geol. 1993, 103, 227-234. (6) Deme´ny, A. Chem. Geol. 1995, 121, 19-25.
4728 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
This investigation started after an unsuccessful attempt to replace AnalaR with SHG (superhigh grade) zinc supplied by Outokumpu Zinc Co., Kokkola, Finland. The level of impurities in Outokumpu zinc is guaranteed to remain below 50 ppm, which suggested that AnalaR zinc may contain impurities critical for the conversion of water to hydrogen. To explore this possibility, the trace element composition of AnalaR zinc was reanalyzed. The analytical data implied that AnalaR zinc contains very high concentrations of Na relative to Outokumpu zinc. This compositional feature is utilized in this paper to provide a zinc reduction method for the determination of D/H ratios in water samples. EXPERIMENTAL SECTION Reagents. AnalaR zinc metal shot with a grain size between 0.5 and 2.0 mm was supplied by BDH Limited. SHG zinc was supplied by Outokumpu Zinc Co. Zinc-sodium mixtures were prepared from Outokumpu zinc and metallic sodium (Merck). Weighed aliquots of both metals were inserted in a 25 mm o.d. borosilicate glass tube, and the mixture was fused by a resistance furnace under a constant flow of argon. The presence of an inert gas, such as Ar, is essential in order to prevent oxidation and eventual ignition of Na during heating. After cooling, the fused Zn-Na cylinders were lathed. Relatively thick lathings, ∼1.0 × 1.5 mm in cross section, were preferred in order to minimize the surface area of the reagent. The lathings were cut to ∼15 mg chunks, which were small enough to fit easily into 6 mm o.d. glass tubes. These Zn chunks were used without any chemical pretreatment for reduction of water. Procedure. Water samples were converted to hydrogen using a sealed tube technique.5 About 30 mg of Zn for 1 µL of H2O was loaded into 6 mm o.d. quartz tubes sealed from the other end. The tubes with the Zn reagent were pumped to high vacuum for 3 h and heated during evacuation to ∼250 °C by a heat gun. Using a microsyringe, 1-5 µL volumes of water were injected into the evacuated quartz tubes. Between samples, the microsyringe was dried by pressurized air in order to avoid cross-contamination. After loading, the quartz tubes were sealed off and the ampules were placed into a 480 °C furnace for 20 min. At the mass spectrometer, the lower part of the ampule with Zn was heated to 250 °C using a heat gun, the ampule was broken by a tube cracker, and H2 gas was released into the mass spectrometer (Finnigan MAT 251) for isotopic analysis. This moderate heating to 250 °C instead of a higher temperature of 480 °C, suggested by Deme´ny,6 was found sufficient in reducing the small systematical bias resulting from the absorbtion of H2 by Zn. Trace element concentrations in zinc reagents were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) and S0003-2700(97)00446-0 CCC: $14.00
© 1997 American Chemical Society
flame atomic absorption spectroscopy (FAAS) at the Geological Survey of Finland. Before analysis, any surface oxidation on zinc was removed by dilute HNO3 and the grains were rinsed thoroughly by distilled water. RESULTS AND DISCUSSION Trace Elements in Zinc. Analyses of trace element concentrations in AnalaR and Outokumpu zinc using ICP-MS and FAAS techniques revealed that only two trace elements, Pb and Na, occur at levels exceeding 10 ppm in either zinc type. AnalaR zinc contained 24 ppm Pb (ICP-MS), which agrees well with the results given by Kendall and Coplen.2 Nevertheless, the concentration of Pb in AnalaR zinc is only 2-fold compared to Outokumpu zinc containing 14 ppm Pb. A more striking difference between the two zinc types was, however, in the Na content. AnalaR zinc contained 310 ppm Na, while the Na content in the Outokumpu zinc was below the detection limit of 15 ppm (ICP-MS). Accordingly, it could be proposed that trace quantities of Na in zinc might be a critical catalyzing agent for the reduction of water. The catalytic effects of several metals, such as Rh, Pt, Ru, Pd, Au, and Fe, have been reported to enhance the decomposition of water by Zn,7 but the possible promoting effects of Na have not been investigated before. Therefore, a new type of zinc was prepared by adding metallic Na to Outokumpu zinc. Na and Zn were mixed in proportions calculated for 100, 500, and 1000 ppm Na. As a comparison, one batch of Zn was melted without any admixed Na. Under the normal reaction conditions at 480 °C for 20 min, turnings lathed from the zinc cylinders with 500 and 1000 ppm Na reduced water quantitatively, while reactions using zinc with 100 ppm Na and those without any Na remained incomplete. Incompletely reacted samples were easily detected on the basis of moisture condensing on the walls of the cooling reaction tubes. Accordingly, a sufficiently high Na content in zinc seems to be essential for catalyzing the reduction of water by Zn. The possibility that corrosion of borosilicate glass vessels during preparation of the Zn-Na mixtures would be critical can be excluded, because both the re-formed zinc without any extra Na and that with 100 ppm Na produced incomplete reactions. These results suggest that relatively high concentrations of Na may be a general compositional feature of AnalaR zinc shot, which has made it a successful reagent for water reduction. The zinc type with 500 ppm admixed Na was chosen for further experiments to test whether it could be used as a reagent for the conversion of water to H2 gas for stable isotope determination. A sodium concentration of 190 ppm was measured for this zinc type by ICP-MS, indicating that the Zn-Na mixture was not completely homogenized during fusion. Possibly, Na was partitioned preferentially into a thin layer of slag, which formed at one end of the Zn cylinder. Analysis of Reference Waters. The reproducibility of the δD analyses made using the new zinc reagent with 190 ppm Na was estimated on the basis of repeated analyses of reference waters processed in quartz ampules. The isotope data on in-house distilled reference water (G-1/5), Vienna standard mean ocean water (VSMOW), standard light Antarctic precipitation (SLAP), and Greenland ice sheet precipitation (GISP) are given in Table 1 relative to a laboratory reference H2. The reproducibility of the δD determinations for 2 µL samples is ∼0.7‰ (1 SD, Table 1), which is comparable to that reported for other techniques. For (7) Jeong, K. M.; Swift, H. E. J. Catal. 1986, 101, 246-252.
Table 1. δD Analyses of H2 Gas Prepared from Different Water Standards Using Zn Reagent with 190 ppm Na
a
sample
δDb (‰)
1 SD
n
G1/5a SMOW SLAP GISP
13.0 67.6 -386.3 -133.6
0.6 0.8 1.7 0.8
20 11 11 8
Laboratory Standard. b δD given relative to a reference H2 gas.
Table 2. δD Results from 1 µL Samples of Reference Waters Reduced with Varying Amounts of Zinc with 190 ppm Na
a
sample
Zn (mg)
δDa (‰)
TLN-3 TLN-3 TLN-3 TLN-3 TLN-3 TLN-3 W-1 W-1 W-1 W-1 W-1
30 30 60 120 180 240 30 60 120 180 240
-357.6 -357.8 -357.8 -359.8 -353.4 -357.9 61.6 62.8 61.6 62.4 66.4
δD given relative to a reference H2 gas.
SLAP, however, the standard deviation of repeated measurements is 1.8‰, which is considerably higher than that for the other reference waters. The great spread of δD values may, to some extent, be related to analytical errors associated with the large difference in the isotopic composition between the sample and the reference gas. The average δD value for SLAP relative to VSMOW, calculated from the data in Table 1, is -425.2‰. The average δD for GISP normalized to a value of -428‰ for SLAP relative to VSMOW is -189.8‰. Excess Zinc. In order to test for the possible isotopic effects of excess Zn, a constant amount of water was reacted in quartz ampules with varying quantities of zinc. The results of this experiment are sensitive to contaminating hydrogen-bearing compounds included in zinc2,8 and also to isotopic effects related to absorbtion of hydrogen into zinc.4,6 Because the isotope shifts caused by these processes may for some water compositions be opposite and cancel each other, two isotopically different reference waters were used. The first, TLN-3, represents Antarctic precipitation with very light hydrogen (δD ) -403.5‰, VSMOW), and the second, W-1, is Atlantic seawater (δD ) -5.5‰, VSMOW) from the Norwegian coast. The results of the D/H determinations are presented in Table 2. It can be observed that the δD values of do not change systematically when the quantity of excess Zn in the reaction tube is increased. Accordingly, measured δD values are not sensitive to moderate excesses of the reagent. Because this is true for both the light and heavy standard water, it may be concluded that the effects of both the hydrogen impurities in zinc and fractionation associated with the dissolution of hydrogen in zinc are not significant under these analytical conditions. Borosilicate vs Quartz Vessel. In addition to quartz, borosilicate glass tubing was also tested as material for the reduction (8) Schimmelmann, A.; DeNiro, M. J. Anal. Chem. 1993, 65, 789-792.
Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
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Such isotope systematics are compatible with the existence of a contaminating hydrogen component having a δD value between the two reference waters. The quantity of this component appears to be significantly higher for analyses made in borosilicate tubes compared to those made in quartz tubes, although H2 yields showed no differences between the two preparation methods. This seems to support the findings of Kendall and Coplen2 and Venneman and O’Neil,5 who suggested that there is a significant reservoir of exchangeable hydrogen in the walls of borosilicate tubing. δD values for hydrogen are generally given normalized to a value of -428‰ for SLAP relative to VSMOW. If sample volumes are constant, a linear normalization correction will remove the bias resulting from the use of borosilicate vessels. However, if the sample volumes vary, a separate normalizing equation would be needed for each volume. For these situations, reduction in quartz tubes would be preferable.
Figure 1. Comparison of the δD values of hydrogen prepared from reference waters in sealed borosilicate (+) and in quartz (b) ampules. The quantities of H2O and Zn were varied while the Zn/H2O ratio was kept constant at 30mg/1 µL. The error bars represent (1 standard deviation of the δD values estimated from repeated measurements of standard waters.
ampules. To constrain the possible exchange of H2 gas with glass walls of the ampules, the quantities of zinc and water were varied, keeping the Zn/H2O ratio constant at 30 mg/1 µL. Two important observations can made from the results of this experiment (Figure 1). First, the δD values seem to vary systematically according to the amount of water so that with decreasing sample size the difference between the δD values of the two reference waters becomes smaller (Figure 1). This shift is more pronounced for samples reacted in borosilicate glass than for those processed in quartz ampules. Second, there is a distinct difference in the δD values between samples prepared in quartz and borosilicate ampules (Figure 1). For the heavy reference water (W-1) the δD values are heavier for the samples prepared in quartz tubes, but for the light reference water (TLN-3) the shift is opposite.
4730 Analytical Chemistry, Vol. 69, No. 22, November 15, 1997
CONCLUSION I conclude that it is possible to prepare a useful reagent for the conversion of water to hydrogen for isotopic analysis by admixing ∼200 ppm Na with pure zinc metal. Trace quantities of Na in zinc act as a catalyzing agent significantly promoting the reduction of water by zinc. Chemical analyses imply that AnalaR zinc shot manufactured by BDH Chemicals contains significant quantities of Na. Possibly, this is a general compositional feature of AnalaR zinc, having made it a successful reagent for water reduction. Although the effects of Na impurity were tested using only one zinc brand manufactured by Outokumpu Zinc, it can be expected that the selection of the zinc type is not critical for the method. ACKNOWLEDGMENT I thank J. Urpinen, Outokumpu Zinc, Kokkola, Finland, for supplying a zinc sample, E. Kallio and R. Juvonen, Geological Survey of Finland, for ICP-MS and FAAS determinations, and R. V. Krishnamurthy and an anonymous reviewer for helpful comments. Received for review April 30, 1997. Accepted September 5, 1997.X AC9704467 X
Abstract published in Advance ACS Abstracts, October 15, 1997.
