Recent advances in fundamental studies of hydride generation

76

trends in analytical chemistry, vol. 74, no. 2, 1995

Table 2 Computation

References of CH, and HI,

aL= 39.66254% CaO M;=96

655 g

m( h,) = 0 C1Y,=~(h,J=2144~10-~ ML = CH, M; = 207.7 g ~(h,)=46.3.10-~

erogeneity, CH,, and the heterogeneity HZ= have been computed in Table 2.

invariant

Acknowledgements The author would like to express his gratitude to Professor Dr. E. Pretsch, ETH Zurich, and to the TrAC reviewer, for their valuable advice and corrections. Thanks to them, this article should be clearer and easier to read than the original manuscript.

[ 11 P.M. Gy, Sampling of Heterogeneous and Dynamic Material Systems, Elsevier, Amsterdam, 1992. [ 21 P.M. Gy, He’te’rogdne’itb, Echantillonnage, HomogPne’isation, Masson, Paris, 1988. [3] R. Kellner, Anal. Chem., 66 (1994) 98A-101A. [4] G. Kateman and L. Buydens, Quality Control in Analytical Chemistry, John Wiley, New York, pp. 15-76. [5] F.F. Pitard, Pierre Gy’s Sampling Theory and Sampling Practice, CRC Press, Boca Raton, FL, 1989. [ 61 K.C. Ng, W.B. Whitten, S. Arnold and J.M. Ramsey, Anal. Chem., 64 ( 1992) 2914-2919. [7] S.A. Soper, Q.L. Mattingly and P. Vegunta, Anal. Chem., 65 (1993) 740-747.

Dr. Pierre M. Gy is a Sampling Consultant at 14 Avenue Jean-de-Noailles, 06400 Cannes, France.

Recent advances in fundamental studies of hydride generation Qiu De-ren Shanghai, China Advances in fundamental studies of hydride generation (HG) are described under three topics. These are a study of non-nascent hydrogen mechanism; of the alkaline mode of HG; and observation on the course of release of hydride and the reaction sequence for the hydride-forming elements.

Wang and Barnes studied the pH dependence of HG for As, Se, Sn and Pb, and considered the decomposition of the borohydride and hydrolysis of the four hydride-forming elements [9]. The author’s group is focusing on studies of the following three topics: 0 the non-nascent hydrogen mechanism; 0 the exploitation of an alkaline mode of the HG reaction; 0 observation of the reaction sequence and reaction course for the hydride-forming elements.

1. Introduction Hydride generation (HG) has shown great progress since NaBH4 was introduced as a reducing agent and more than 1200 papers, including several excellent reviews [l-S] have been published. There now seems to be a state of relative stagnancy whereas only a few analytical standards have established on the basis of HG, which is still not mature. Development of HG demands fundamental studies. 016%9936/95/$09.50

2. Study of the non-nascent mechanism

hydrogen

The nascent hydrogen mechanism has been accepted as the classical theory for HG for the explanation of AsH3 generation over a century. In 1979, when NaBHa was introduced for HG, Robbins and Caruso [ 1] followed the nascent hydrogen theory already available for explanation of the 0 1995 Elsevier Science B.V. All rights reserved

77

trends in analytical chemistry, vol. 14, no. 2, 7995

xlcf

I

‘,TT 2

12

/

5 8 10

200 ngSn/ml

I.

2 ;8 iij

~/

/

6 ,I,"

:

4t

/i

-0

,,A'

100 ngSn/ml

.

I

,‘%

1

OS

2.5

1.5

Feed-rate

of NaSH4-Na20H.

ml/min

Fig. 1. Critical amount in SnH, generation. The acidity of the Sn solution (100 or 200 ng/ml Sn) is 0.5 M HCI and the feed-rate is 1 .Ol ml/min. The reagent is 0.25 MNaBH,.-0.25 MNaOH. (Adapted from Ref. [lo].)

mechanism. Our first study on the non-nascent hydrogen mechanism was performed for stannane generation, and an induced or catalysis mechanism was proposed [ lo]. The experimental background can be described as follows. We started our study of the mechanism by investigating the molarity relationship between reactants. The experimental design uses a continuous hydride generator and an ICP spectrometer. The tin-containing solution was fed into the generator at a fixed rate by a peristaltic pump, and the reagent was fed at a rate regulated by another pump. The millimoles of the reactants participating in stannane generation per unit time can thus be calculated from the concentration and the feed-rate. The yield of stannane produced was measured via inductively coupled plasma-atomic emission spectrometry (ICP-AES) . Table 1 Molarity relationship Experiment

Changing NaBH,conc.

Changing NaOHconc.

millimoles of HCl = millimoles of (NaBH, + NaOH) exists per unit time. The equation primarily indicates the positive function of NaOH, but we still cannot differentiate NaBH4 from NaOH. Before the critical point is reached, acid is always present in excess and all the introduced NaBH, and NaOH are being used. In order to make further discrimination between the functions of NaBH4 and NaOH we generated stannane from an alkaline solution, by reacting an alkaline Sn-NaOH-NaBH, mixture with acid. The second three sets of experiments, changing one of the three reactants NaBH,, NaOH, and acid, yielded another relationship, namely millimoles of HCl = millimoles

Sn-HCI

of NaOH

The critical point appeared to be independent of NaBH, (Table 2). The experimental findings regarding the lack of dependence of the critical point on NaBH, (Fig. 2), led us to conclude that neutralization is the preferential reaction compared to decomposition of

at the critical point, for the case of Sn-HCI reacting with NaBH,-NaOH

Acidity

Changing HCIconc.

We found that there are critical amounts of NaBH,-NaOH for stannane generation. An excess of NaBH,-NaOH reagent over the critical amount is useless (Fig. 1) . The critical point depends on the molarity relationship of three reactants, namely acid, NaBH4, and NaOH, and is independent of the amount of tin. After a series of experiments in which one of the three reactants was changed while keeping the other two constant, calculation (Table 1) showed that at the critical point, a molarity relationship

NaBH.,-NaOH

(adapted from Ref. [lo])

Millimoles of reactants, per min HCI

NaBH,

NaOH

(NaBH, + NaOH)

(IM)

Critical value (ml/min)

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.12 0.25 0.50 1.00

0.70 1.06 1.86 1.38 1.05 0.70 0.40 1.23 1.04 0.73 0.43

0.30 0.50 1.00 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.175 0.265 0.465 0.17 0.26 0.35 0.40 0.31 0.26 0.18 0.11

0.175 0.265 0.465 0.34 0.26 0.18 0.10 0.15 0.26 0.37 0.43

0.35 0.53 0.93 0.51 0.52 0.53 0.50 0.46 0.52 0.55 0.54

NaBH, (M)

NaOH

(W

Feed-rate (ml/min)

0.30 0.50 1.00 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.25 0.25 0.25 0.12 0.25 0.50 1.00 0.25 0.25 0.25 0.25

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trends in analytical chemistry, vol. 14, no. 2, 7995

Table 2 Molarity relationship Experiment

at the critical point for the case of Sn-NaOH-NaBH,

Sn-NaOH-NaBH,

Changing HCI cont. Changing NaBH, cont.

Changing NaOH cont.

NaOH (AI)

NaBH, (AJ)

Feed-rate

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.12 0.25 0.50

0.25 0.25 0.25 0.12 0.25 0.50 1.0 0.25 0.25 0.25

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

(ml/min)

1.o 1.0

the NaBHa, when acid comes into contact with the reagent NaBH,-NaOH. A similarly important finding is that stannane is generated synchronously with the neutralization, and that its yield is proportional to the amount neutralized (Fig. 2). Therefore, we deduced a mechanism of induced or catalytic reaction for stannane generation. Since the publication of Ref. [ lo] another study on the mechanism of GeH, generation has been completed by Luo with the same result and conclusions [ 111. The study of the stoichiometry led to the establishment of optimized conditions for SnH4 [ lo] and GeH, [ 1 l] generation, Experimentally, it was x104

*OF------l NaBH4 0.12M

reacting with HCI (adapted from Ref. [lo])

HCI

Millimoles of reactants, per min

cont. (flJ) Critical value (ml/min)

HCI

NaOH

NaBH,

1.oo 0.50 0.30 0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.23 0.25 0.27 0.24 0.25 0.25 0.20 0.14 0.25 0.53

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.12 0.25 0.50

0.25 0.25 0.25 0.12 0.25 0.50 1.0 0.25 0.25 0.25

0.23 0.49 0.90 0.47 0.49 0.47 0.40 0.28 0.50 1.06

demonstrated that the largest yield of SnH, or GeH4 can be obtained when the final pH is in the range of 2-l 1.5 after reaction (Fig. 3). This means that the acidity of the sample solution should match the NaOH concentration in the NaBH4 reagent solution, or that a weak acid or NaBH4 should be in excess after the HG reaction. Judging by this criterion, we find that there is no contradiction between the apparently contradictory results that have appeared in the literature. Some examples [ 12-161 illustrate this (Table 3). The similar mechanisms and optimized conditions for Sn and Ge are plausible. In the early stages of HG, analysts used active metal-acid systems to generate hydrides of the group V (As, Sb, Bi) and VI (Se, Te) elements, and the nascent hydrogen mechanism was never challenged. When NaBH4 was used instead for HG, the hydride-forming elements were extended to those in group IV (Ge, Sn,

x x

0.4

0.2 t

OV 0

*x

0.5

Feed-rate of hcl.

K

tnl/nk5

0.0 0

Fig. 2. The critical point is independent of NaBH,, in case of generating stannane by reacting a SnNaOH-NaBH, solution (200 ng/ml Sn, 0.25 MNaOH, NaBH, concentration as shown in the figure, feedrate 1 .O ml/min) with HCI (0.5 IV). (Adapted from Ref. [lOI.)

2

4

e

8

10

12

14

PH

Fig. 3. Dependence of SnH, yield on the final pH of the reactants after reaction. The data were obtained by reacting an acidic solution of tin in HCI (a) or in HAc (x) with NaBH,-NaOH, by using continuous HG-ICP-AES. (Adapted from Ref. [lo].)

trends in analytical chemistry,

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Table 3 Examples showing that the criterion described in the text can explain the apparently (in all these cases, NaBH, is in excess after reaction)

contradictory

optimized conditions

Comment

Reactant cont. in HG (M)

Author(s)

Vijan and Chan [12] Thompson and Pahlavanpour [ 131 Robbins et al. [14] Barnett [15] Fodor and Barnes [I 61

Acidity

NaOH

NaBH,

0.12 0.1 0.13 (tartaric acid) 1.2 0.12 0.6

0.1 0.1 0.25 0.5

0.25 0.25 0.25 1 0.5 0.5

Pb). We cannot explain why the same nascent hydrogen in the metal-acid system cannot reduce group IV elements to hydrides. In addition, the first report by Thompson et al. [ 171 on HG-ICP for multielement analysis included only five elements of groups V and VI, while group IV (Ge, Sn, Pb) was excluded. If we notice that the parameters for SnH, and GeH, generation in Thompson and Pahlavanpour’s second report [ 131 were very similar, the similar results and conclusion for Sn and Ge in our reports [ IO,1 1] are not surprising. To hydrogenate Pb, one needs the presence of an exceptional oxidant such as dichromate, ferricyanide or H,02. The next subject of our non-nasHz volume,ml

4o-,-_w

in literature

For For For For For For

Sn only Sn,Ge Sn,Ge Sn,Ge Sn only Sn only

cent hydrogen mechanism study will therefore be plumbane generation. Preliminary results demonstrate that plumbane generation may be induced by the redox reaction involving neutralization. Recently we completed another experiment to prove that the neutralization occurs prior to the decomposition of NaBH4 when acid comes into contact with the NaBH,-NaOH reagent. We reacted an acid solution, containing no hydrideforming elements, with NaBH,-NaOH reagent and the amount of decomposed Na13H4 was estimated via a measurement of the volume of hydrogen produced. The experiment demonstrated that only a small amount of hydrogen is produced from the decomposition of NaBH, owing to the local reaction, in the case where an amount of acid insufficient for the NaOH neutralization was introduced (Fig. 4).

3. Alkaline mode of HG

0

02

04

0.6

08 cont.

1

of acid&f.

Fig. 4. Reaction sequence: neutralization is in preference to the decomposition of borohydride. Both acid and 0.2 M NaBH,-O.2 M NaOH reagent are fed into a continuous reactor by using a multichannel peristaltic pump. Feed-rate 1.O ml/min; time 120 s; room temperature 30°C; (X) HCI and (0) HAc. The amount of NaBH, decomposed is expressed in terms of the volume of produced H,. At the stage before the equivalent point of neutralization (acid cont. 0.2 M), a minor amount of H, is produced by the local reaction.

The traditional experimental approach to the HG is to react the acidic sample solution containing hydride-forming elements with a NaBH,-NaOH reagent. In order to distinguish this approach from that described below, we call such a traditional reaction mode as the ‘acidic mode of HG’ [ lo] for the study of the mechanism for tin. The reaction mode that is used to generate the hydrides by reacting an alkaline sample solution with an acid is called the ‘alkaline mode of HG’. By using both modes, we have discriminated the function between NaBH, and NaOH, as described above. Luo also applied the alkaline mode to a study of the mechanism for Ge [ 111. All the hydride-forming elements can form soluble salts of their respective oxy-acids such as germanate, stannate, plumbite, arsenate, antimonate,

80

Table 4 Generation Element

Ge Sn Pb As Sb Bi Se Te

trends in analyticalchemistry

conditions

and yield comparison

for two HG modes (adapted from Ref. [18])

Cont. of the reactant (MJ

Signal (mean rf: S.D.)

HCI

NaBH,-NaOH

0.5 0.5 0.5, + 3% H,O, 3 3 3 3 3

0.25 0.25 1.25 0.5 0.5 0.5 0.5 0.5

selenate, selenite, tellurate, and tellurite, in a strong alkaline medium. All these can produce volatile hydrides by the alkaline-mode reaction [ 181, and in Table 4 the yields are compared for the two modes with the same amounts of reactants participating. The yields in Table 4 were listed in terms of spectroscopic signals and the data expressed as (mean + standard deviation) taken from ten measurements, by using continuous HG-ICP-AES as the experimental system. It must be pointed out that this is preliminary work and the working parameters have not been optimized yet. However, the alkaline mode enables one to carry out HG while avoiding or decreasing the chemical interferences from group VIII and IB metals. As an example of an application, the determination of Ge in copper ores and in zinc residues was reported [ 191. Other workers have reported the generation of H,Se from an alkaline solution and give an explanation of the mechanism [20]. In alkaline solution, borohydride is a strong reductant: H,BO;

+ 5Hz0 + 8e --+BH, +80H-

vol. 14, no. 2, 7995

E= - 1.24V

and the hydride-forming element, for example the selenium, is reduced to the corresponding anion, 4SeOz- i- 3BH; + 4Se2- i- 3H,BO,

i- 3H,O

When the alkaline Se-containing solution reacts with an acid, H,Se is thus formed and released. The alkaline mode of HG provides a new approach for speciation analysis of the hydrideforming elements.

Traditional 0.25 0.25 0.12 0.1 0.1 0.1 0.1 0.1

mode

(3.42 f 0.06). 1O5 (2.24+ 0.07). 1O5 (1.68 + 0.08). 1O5 (1.44 f 0.05). 103 (2.52 f 0.08). 1O5 (2.24 f 0.07). 1O5 (6.94f0.21).103 (3.99*0.15). 103

Alkaline mode (3.34 + 0.05) (2.17 + 0.08) (1.4OkO.06) (1.41 f 0.05) (1.63kO.05) (1.47f0.05) (6.98kO.17) (4.34*0.17)

,105 .105 .I05 ,103 .I05 .105 .103 .103

4. Observation of reaction sequence and

reaction course of hydride-forming elements in HG It is commonly impossible to observe the reaction sequence of the hydride-forming elements in HG. In HG-AAS or HG-AFS experiments, only one spectral line is measured. The other elements cannot be monitored simultaneously. In HG-ICPAES, a continuous generator combined with an ICP spectrometer is usually used to stabilize the torch, and a dynamic equilibrium between reactants and produced hydrides is established. All the hydrideforming elements give steady signals and the reaction sequence has accordingly been hidden, even if a multichannel spectrometer is used for the measurement. Earlier, the author made observations on the course of generation of stannane for various reaction modes, including Sn-NaOH-NaBH, or SnEDTANa4-NaBH4 with acid (HCl, HAc or H,BO,) [ 211. The experimental system consists of an ICP spectrometer and a hydride generator similar to the batch one, but the reagent was introduced step by step using a peristaltic pump instead of introducing all the reagents at one time. Fig. 5 illustrates the release course of stannane when HCl is successively fed into an alkaline Sn-NaOHEDTA-NaBH4 solution. At the end of the sequence, the solution is still alkaline (pH 9.02). The results show that stannane can be generated from an alkaline medium, even from a well complexed EDTA solution, as such chemical interferences can be avoided by adding sufficient EDTA. The importance of the above mentioned preliminary experiment lies in the fact that it led us to the use of the alkaline mode for the study of the mechanism for tin and also to the observation of the

trends in analytical chemistry,

81

vol. 14, no. 2, 1995

to that described above, but a multichannel spectrometer was used instead of a monochromator. The reagent was fed into a solution of mixed hydride-forming elements and the HG sequence of the elements observed by recording the progressive instantaneous spectroscopic signals of individual elements with the multichannel ICP spectrometer. As an example, Fig. 6 shows that differences in HG behaviour were revealed. Only a preliminary result has been reported, as we are limited by our spectrometer. However, the experiments have verified our idea that the HG sequence for the individual hydride-forming elements occurs. We plan to improve the software of our spectrometer and to make further experimental observations of the reaction sequence and thereby to study further the fundamentals of HG.

Signal 70

1

60

. -

50

. (a)

.

40

.

30

a

:

20

10

** *. . ..**...

0 i‘

20

1C

3s

. *. .* .*.a

..=

40

. 50

Time,s Signal jr‘.

60 *

: .

(b) .. 40.

5. Conclusion

*.

30

Time,s

Fig. 5. Release course of stannane when HCI (a) or HAc (b) are fed portionwise into an alkaline SnEDTANa,-NaBH, solution. (Adapted from Ref. [21].)

sequence for the hydride-forming elements. We have made a preliminary report on the latter [ 221. The experimental system was similar reaction

Signal

I

‘Igo

0

10

20

30

40

The HG conditions for Sn and Ge, established on the basis of a mechanistic study, have explained the contradictions which appeared in the literature. All the hydride-forming elements can generate the hydrides from an alkaline medium. The prospect in applications is attractive, because the chemical interferences from VIII and IB metals can be avoided. However, the conditions have not been optimized, except for St-r, Ge and Se. Experimental observation of the release course of the hydride for different hydrideforming elements, or speciation of the elements under a variety of media could provide a new approach to speciation analysis. The full details of the HG reaction remain to be clarified. Fundamental studies should push HG development forward, and a new stage couId be in prospect.

50

Time,s

Fig. 6. Release course of several hydrides when NaBH, reagent is portionwise fed into an acidic solution of mixed hydride-forming elements and drops of H,O,. (Adapted from Ref. [22].)

Acknowledgements I wish like to express my gratitude to Professor Richard Dams at Gent University. His suggestions

82

trends in analyticalchemistry,

vol. 14, no. 2, 1995

and effective help have allowed me to initiate a new field of my research interests and to achieve some meaningful advances.

131 M. Thompson and B. Pahlavaupour, Anal. Chim. Acta, 109 (1979) 251. [ 141 W.B. Robbins, J.A. Caruso and F.L. Fricke, Analyst, 104 ( 1979) 35. [ 151 N.W. Barnett, Spectrochim. Acta, 42B (1987)

References

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[ 1]

W.B. Robbins and J.A. Caruso, Anal. Chem., 5 1 (1979) 889A. T. Nakahara, Prog. Anal. Atom. Spectrosc., 6 [2 (1983)

163.

[3 T. Nakahara, Spectrochim. Acta Rev., 14 ( 1991) 95. ]4 J.A.C. Broekaert and P.W.J.M. Boumans, in P.W.J.M. Boumans (Editor), ICP Emission Spectroscopy, Part I, Wiley, New York, 1987, Ch. 6, p. 336. [5 J.A. Caruso, K. Wolnik and F.L. Fricke, in A. Montaser and D.W. Golightly (Editors), ZCP in Analytical Atomic Spectrometry, VCH, New York, Weinheim, 1987, Ch. 13, p. 487. 161 M. Thompson and J.N. Walsh, A Handbook Of ICP Spectrometry, Blackie, Glasgow, 1983, Ch. 6, p. 149. [7] D. Beauchemin, J.C.Y. Le Blat, G.R. Peters and J.M. Craig, Anal. Chem., 64 ( 1992) 442R. [ 81 K.W. Jackson and H. Qiao, Anal. Chem., 64 ( 1992) 50R. [ 91 X.-R. Wang and R.M. Barnes, Spectrochim. Actu, 42B (1987) 137. D.-R. Qiu, C. Vandecasteele, K. Vermeiren and 110 R. Dams, Spectrochim. Actu, 45B (1990) 439. [ll X.-W. Luo, D.-R. Qiu, Z.-J. Chen and S.-S. Zhu, Fenxi Huuxue, 21 (1993) 1241; Abstract of Pittcon’94, No. 3 1OP. [121 P.N. Vijan and C.Y. Chan, Anal. Chem., 48 (1976) 1788. \-~-I

[

Actu,

38B (1983) 229. [ 171 M. Thompson, B. Pahlavanpour, S.J. Walton and G.F. Kirkbright, Analyst, 103 ( 1978) 568. 118 ] D.-R. Qiu, Z.-J. Chen and X.-W. Luo, Proceedings of the 4th Chinese National Conference on Atomic Spectrometry, 1992, p. 103; Guangpuxue Yu Guangpu Fenxi, 14

]19 ]

( 1994) 77; Abstract of Pittcon’94, No. 994. Z.-J. Chen, D.-R. Qiu, and X.-W. Luo, Fudun

[ 201

T. Wickstrom, W. Lund and R. Bye, J. Anal. Atom.

Xuebao Zirun Kexueban, 32 ( 1993) 361. Spectrom., 6 (1991) 389. D.-R. Qiu, Fudan Xuebo Ziran Kexueban, 31 (1992) 23. [22] D.-R. Qiu, B. Lin and S. Ouyang, Proceedings of International 4th BCEIA, 1991, p. C47; ICP Information Newslett., 18 ( 1992) 231.

[ 211

Qiu De-ren is Professor of Analytical Chemistry at the Department of Chemistry, Fudan University, Shanghai 200433, China. Qi; is the author of more tfian 50 pub&a tions including a comprehensive ‘2 A/mm and 4 A/mm A t/as For Gra ting Spectrograph’ which was prepared together with his wife Cheng Wan-xia. His research interests in recent years lie main/y in hydride generation atomic spectrome tv, and high resolution spectrometry.

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