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Determination of molybdenum traces in grass using a catalytic method J. T. Kennedy and G. Svehla

Department of Analytical Chemistry, The Queen's University, Belfast BT9 5AG, Northern Ireland

Bestimmung von Molybd~inspuren in Gras mit Hilfe einer katalytisehen Methode

Zusammenfassung. Das beschriebene Verfahren beruht auf der Landolt-Reaktion zwischen Wasserstoffperoxid, Iodid und Ascorbins/iure und eignet sich zur Bestimmung yon 0,00005-0,0003% Mo in Gras. Eine Probe von 5 g wird trocken verascht, mit Salzsfiure und Wasserstoffperoxid be- handelt, und mit Benzoin-~-monoxim in Chloroform extra- hiert. Die organische Phase wird mineralisiert und schliel3- lich der Molybdfingehalt mit Hilfe der katalytischen Reak- tion (Reaktionszeitmessung mit potentiometrischer An- zeige) bestimmt. Die Ergebnisse zeigen gute fSberein- stimmung mit den Resultaten aus AAS und Plasma-Emis- sionsspektrometrie. Die relativen Standardabweichungen der Reaktionszeit-Messungen liegen unter 2%.

Summary. A catalytic method, based on the hydrogen peroxide-iodide-ascorbic acid Landolt reaction is described for the determination of 0.00005 - 0.0003 % molybdenum in grass samples. The sample (5 g) is dry-ashed, then treated with hydrochloric acid and hydrogen peroxide, extracted with benzoin-~-monoxime in chloroform, the organic phase mineralized and finally the molybdenum content determined by the catalytic method (measurement of reaction time) using potentiometric monitoring. Results are evaluated from a calibration graph; these agree well with those obtained with atomic absorption and plasma emission spectrometry. Relative standard deviations of reaction time measurements are below 2%.

Introduction

Molybdenum is an important trace element in agriculture; its presence in soil is important for the fixation of nitrogen by certain plants. Some of the molybdenum gets into pasture grass and is taken by grazing animals. Too much molyb- denum (especially if the copper levels are low) can be harmful to cattle and sheep; it is important therefore to keep the molybdenum contents of pasture grass within the optimal 1 - 3 ppm limits (in dry material). To organise a proper fertilising programme, it is essential to determine regularly the molybdenum content of grasses.

Offprint requests to: G. Svehla

The usual method for determining molybdenum in grass is atomic absorption spectrometry, or in some cases plasma emission spectrometry. Both methods require costly in- struments and specially trained personnel. The catalytic method, described here, is equally specific and sensitive and requires much less skill and relatively cheap equipment.

The method is based on the molybdenum-catalysed hydrogen peroxide-iodide-ascorbic acid reaction with potentiometric monitoring. We developed this from an earlier method, with simple visual end-point detection, combined with the manual operation of a stopwatch [6]. Kinetic and mechanistic explanation of the chemical reac- tions involved has also been published [5] and will not be repeated here. The potentiometric monitoring of Landor reactions has been applied first by Weisz [7]. By altering the composition of reagents, we optimised the method for grass analysis. The complexation of molybdenum with benzoin- ~-monoxime has first been described by Knowles [4], while the extraction of the complex was first suggested by Jones [2]. The extraction procedure is often used in plant analysis. An automated catalytic procedure, based on the colorimetric monitoring of iodine formed in the hydrogen peroxide- iodide reaction has been described and adapted for the AutoAnalyser by Bradfield and Stickland [1].

Experimental

Potentiometrie monitoring of reaction rates. Potentiometric measurements were made using the same experimental de- sign detailed earlier [3]. A bright platinum electrode and a saturated calomel electrode were immersed in the reaction mixture in a reaction cell thermostated at 25 + 0.2 ~ C. The electrodes were connected to an EIL 7050 pH-meter, which in turn was connected to a UNICAM AR25 linear recorder. The pH-meter was used in the mV mode, and the recorder was operated with a 1 cm/min chart flow rate, with a Full scale sensitivity of 1,000 mV. Any other pH-meter/chart re- corder combination may be used for the purpose.

Atomic absorption spectrometry. For the atomic absorption measurements a Perkin-Elmer Model 306 spectrophoto- meter was used, equipped with a HGA-72 graphite furnace, deuterium background corrector and a PE 56 recorder. The molybdenum hollow cathode lamp was operated with a current of 30 mA. Absorbance was measured at 313.3 nm, combined with a 314 UV-setting and a slit setting of 4

Fresenius Z Anal Chem (1986) 324:19-22 �9 Springer-Verlag 1986

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(0.7 nm). The graphite furnace was operated at 128 ~ C drying temperature for 30 s, then at 1,889~ thermal dissociation temperature for 30 s and finally at 2,659~ atomising temperature for 15 s. Nitrogen was used as purge gas.

Plasma emission spectrometry. A Spectrospan III instru- ment with an inductively coupled Spectrojet III plasma was used; the plasma jet was operated at 28 lb/sq.in, at the nebuliser and 50 lb/sq, in. at the electrode sleeves. The input slit dimensions were: 300 ~tm (height) and 50 gm (width). Emissions were measured at 379.8 nm.

Procedure for the catalytic determination

Reagents. Analytical grade reagents and double distilled wa- ter from an all glass still were used.

For the digestion and extraction procedure the following re- agents were made up:

Hydrochloric acid, 2 M and 5.5 M. Hydrogen peroxide, 100 vol (30% w/v). Benzoin-c~-monoxime solution in absolute ethanol, 2%

w/v. The solution keeps for several months if stored in a refrigerator.

Nitric acid-perchloric acid mixture, 3 + 1 v/v.

For the catalytic determination use the following solutions: Hydrochloric acid, 1% v/v and 5% v/v. Solution A: Dilute 1.5 ml of 100 volume hydrogen

peroxide with 1% hydrochloric acid to 1 1. Solution B: Dissolve 23.24 g potassium iodide, 13.61 g

sodium acetate trihydrate in about 500 ml water, then add 2.79 g disodium dihydrogen ethylenediamine tetraacetate dihydrate and 0.52 g ascorbic acid. After complete dissolu- tion dilute the mixture with water to 1 1. The solution keeps for 72 h.

Molybdenum standards. A stock solution, containing 1,000 ppm molybdeniam, can be prepared by dissolving 1.840 g hexammonium heptamolybdate tetrahydrate in 1% hydrochloric acid, and diluting with the same acid to 1 1. From this, a 10 ppm stock solution can be made up. The latter can be diluted further, using a burette and volumetric flasks, to obtain standards containing 0.1, 0.2, 0.3, 0.4 and 0.5 ppm molybdenum. All the dilutions have to be made with 1% hydrochloric acid.

Sample collection and treatment. The herbage samples used in this study were collected from field plots of pasture grasses. The samples were finely ground through a 1 mm steel screen in a Tecator mill and were then stored in sterile plastic jars: Prior to digestion, the samples were dried in an oven at 100~ for 12 h.

Digestion and extraction. About 5 g of the herbage sample was weighed into a silica crucible, fire-ashed, then ashed overnight in a muffle furnace at 450 ~ C. The cooled ash was transferred into a 100 ml wide-necked borosilicate flask, 10 ml of 2 M hydrochloric acid added and the mixture was heated gently on a sand bath for about 30 min. After adding 2.5 ml of 100-vol hydrogen peroxide and one anti-bumping granule the solution was evaporated to dryness. The re- sulting solid was dissolved in 10 ml of 5.5 M hydrochloric acid and, after covering the flask with a watch-glass, it was boiled under reflux for a few minutes. When the solution had cooled it was diluted to about 100 ml, transferred into a

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~ 2 0 0 iii

a star t

t(O)=6.0Omin t

~ 200

I I Z, 8 0

t /min

b s tar t

t(O.3)=2.63min

I

f /min

Fig. 1. Evaluation of potentiograms a trace obtained with 1% HC1; b trace obtained with 0.3 ppm Mo with 1% HC1

250 ml separatory funnel, and 10 ml of benzoin-c~-monoxime solution added. After thorough mixing the solution was extracted with two 10 ml portions of chloroform. The ex- tracts were collected in the original flask, and the chloroform was evaporated. The residue of the extracted organic phase was then treated with 5 ml of nitric acid-perchloric acid mixture and heated gently on a sandbath until a colourless solution was obtained. The digestion was completed by heating the flask until white fumes of perchloric acid were evolved (if the digest began to char during the procedure, 1 ml of concentrated nitric acid was added, and boiling until the appearance of white fumes was repeated). The contents of the flask were then evaporated to dryness, the residue dissolved in 10 ml of warm 5% v/v hydrochloric acid and diluted to 50 ml in a volumetric flask. All catalytic and spectrometric determinations were made from this solution.

Determination of molybdenum. Transfer 10.00 ml sample solution into a thermostated potentiometric cell, add 10.00 ml of solution A. Switch on magnetic stirring and start the recording of potential against time. When an about 1 cm long trace is recorded (and the potential remains fairly steady), add 10.00 ml of solution B. Keep recording the potentiogram until the inflexion on the potentiogram appears (Fig. 1), and the solution turns yellow owing to the liberation of iodine. Repeat the procedure with 10.00 ml 1% hydrochloric acid and with 10.00 ml portions of the standards. Reaction times vary between 0.5 and 6min according to the amount of molybdenum present.

Evaluation of potentiograms. Characteristic potentiograms are shown in Fig. 1. The reaction times are measured from the instant of addition of solution B until the appearance of iodine, which coincides with the inflexion point on the curve, as shown on the figure. The reaction times are well reproduc- ible; the relative standard deviation of repeated time mea- surements (within the range of 1 to 7 min) is less than 2%. The measured Landolt reaction time is inversely pro- portional to the concentration of the catalyst (molybdenum) [5, 6]. For routine measurements it is enough to measure distances on the chart.

Construction of the calibration graph. Further evaluation of the results is carried out with the aid of calibration graphs. To construct this, the average of blank reaction times [t(0)] or distances [1(0)] is divided by the reaction time [t(c)] or

Table 1. Data of a calibration curve

c(Mo)/ppm t(c)/min t(0)/t(c)

0 6.00 [= t(0)] 1.000 0.1 4.23 1.417 0.2 3.23 1.858 0.3 2.63 2.285 0.4 2.20 2.727 0.5 1.90 3.162

Intercept with t(O)/t(c) axis: 1.000; Slope: 4.333/ppm; Coefficient of correlation: 0.991; Lowest determinable concentration ([3]): 0.006 ppm

Table 2. Interference studies on the uncatalysed reaction

Concentration of the interferent ion (ppm)

100 10 1 0.1

t(O)/t(c) values

Ion Ag(I) 1.02 AI(III) 1.02 Au(III) 1.16 1.01 Ba(II) 1.02 Ca(II) 1.03 Cd(II) 1.19 1.03 Ce(III) 1.02 Co(II) 0.97 Cr(III) 0.95 0.97 Cu(II) 1.01 Fe(II) 1.09 1.03 Fe(III) 1.13 1.03 Hg(II) 0.98 La(III) 1.00 Mg(II) 1.00 Mn(II) 1.00 Ni(II) 1.05 1.03 Pb(II) 1.00 Sb(III) 1.00 Se(VI) 1.00 Sn(II) 0.98 Sn(IV) 1.00 Sr(II) 1.00 Ti(III) 1.04 V(V) 1.92 1.08 1.03 W(VI) 5.08 2.91 1.22 Y(III) 1.00 Zn(II) 1.03 Zr(IV) 1.07 0.99

1.03

distance [l(c)] obtained with each sample and standard, and the t(O)/t(c) ratio is plotted as the function of the concentra- tion of molybdenum. A straight line is obtained with an intercept on the t(O)/t(c) axis at unity. Although, according to the law of the propagat ion of errors, experimental errors are accumulated in calculating the t(O)/t(c) ratios, we still recommend this method, because we found that when new reagent solutions are prepared and used, the individual reac- tion times [t(0) and t(c) values] may vary, but their ratios are reproducible within a few per cent. For accurate work it is advisable to make up a calibration graph daily.

Table 3. Interference studies on the catalysed reaction [c(Mo)= 0.3 ppm]

Concentration of the interferent ion (ppm)

100 10 1 0.1

t(O)/t(c) values

Ion Ag(I) AI(III) 1.03 Au(III) 1.12 Ba(II) 1.00 Ca(II) 1.04 Cd(II) 1.04 Ce(III) 1.00 Co(II) 1.01 Cr(IlI) 0.92 Cu(II) 1.05 Fe(I1) 1.06 Fe(III) 1.14 Hg(II) 1.02 La(III) 1.00 Mg(lI) 0.98 Mn(II) 1.00 Ni(II) 1.09 Pb(II) 1.00 Sb(IlI) 0.95 Se(VI) 1.00 Sn(II) 0.37 Sn(IV) 0.47 Sr(II) 1.02 Ti(III) 1.05 V(V) 1.66 W(VI) 2.23 Y(II1) 1.02 Zn(II) 1.04 Zr(IV) 1.00

1.03

1.02

0.98 1.03 1.01 1.05 1.02

1.01

1.01

0.68 0.96 0.79 0.97

1.04 1.64 1.07 1.00

Typical data of a calibration curve, together with statisti- cal values are collected in Table 1.

Calculation

From the c(Mo)/ppm concentration, obtained from the calibration graph, the percentage molybdenum content is obtained with the formula:

Mo(%) = c(Mo)/lOO/m(s)

where m(s) is the sample weight (g). The formula is correct only if the same dilutions are made as described above.

Intetferences

With the extraction of molybdenum with benzoin-c~-mon- oxime all the interferences are eliminated. Before we included this step into the procedure, we carried out a systematic study of interferences, the results of which are interesting to those, who would try to apply the method for different matrices.

We tested 29 different ions, at 100, 10, I and 0.1 ppm concentration levels, both in the absence (Table 2) and in the presence of (0.3 ppm) molybdenum (Table 3). Ratios of t(O)/t(c) values are tabulated (if the ratio was 1.00, the value is shown only at the highest concentration of the particular

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Table 4. Comparison of results obtained with atomic absorption (AA), plasma emission (PE) and catalytic (CA) methods

Sample Mo(%) x 104 Differences n u m b e r in dry herbage (rel. %)

AA PE CA AA-PE AA-CA PE-CA

1 1.13 1.20 1.17 - 6.19 - 3.65 2.50 2 1.38 1.40 1.40 - 1.45 - 1.45 0 3 1.26, 1.21 1.27 3.97 - 0.79 - 4 . 9 6 4 1.88 1.73 1.73 7.98 7.98 0 5 1.93 2.05 2.03 - 6.22 - 5.18 0.98 6 1.85 2.10 2.07 -13 .51 - 1 1 . 8 9 1.43 7 1.96 1.97 1.97 - 0.51 - 0.51 0 8 0.95 1.00 0.97 - 5.26 - 2.11 3.00 9 1.97 1.81 2.00 8.12 - 1.52 - 6 . 9 5

10 0.93 0.96 1.00 - 3.23 - 7.53 - 4 . 1 7 11 1.75 1.72 1.63 1.71 6.86 5.23 12 2.60 2.70 2.60 - 3.85 3.70 0 13 1.55 1.62 1.50 - 4.52 3.23 7.41 14 2.05 2.07 2.10 - 0.98 - 2.44 - 1 . 4 5 15 0.55 0.52 0.53 5.45 3.64 - 1 . 9 2 16 0.80 0.78 0.73 2.50 8.75 6.41 17 2.17 2.35 2.27 - 8.29 - 4.61 3.40 18 0.58 0.56 0.53 3.45 8.62 5.36 19 1.83 1.91 1.80 - 4.37 1.63 5.76 20 2~00 2.07 1.96 - 3.50 2.00 5.31 21 1.88 1.83 1.86 2.66 1.06 - 1.64 22 0.86 0.87 0.87 - 1.16 - 1.16 0 23 2.16 2.19 2.20 - 1.39 - 1.85 - 0 . 4 6 24 2.56 2.29 2.39 10.55 6.64 - 4 . 3 7 25 2.21 2.16 2.20 2.26 - 1.85 - 0 . 4 6 26 2.00 1.99 2.00 0.50 0 0.50 27 1.87 1.94 1.93 - 3.74 - 3.21 0.52 28 1.04 1.12 1.07 - 7.69 - 2.80 4.46 29 1.88 1.86 1.90 1.06 - 1.06 - 2 . 1 5 30 1.50 1.54 1.50 - 2.67 0 - 2 . 6 0 31 0.78 0.73 0.77 6.41 2.18 - 5 . 4 8 32 1.25 1.22 1.27 2.40 - 1.60 - 4 . 1 0 33 1.10 1.13 1.17 - 2.73 - 5.98 - 3 . 5 4 34 1.10 1.06 1.13 3.64 - 2.73 - 6 . 6 0 35 1.54 1.59 1.60 - 3.25 - 3.90 - 0 . 6 3 36 1.72 1.72 1.73 0 - 0.58 - 0 . 5 8 37 1.64 1.48 1.47 9.76 10.37 0.68 38 1.28 1.30 1.27 - 1.56 0.78 2.31 39 1.55 1.57 1.53 - 1.29 1.29 2.55

ion) . Al l va lues b e t w e e n 0.95 a n d 1.05 r e p r e s e n t negl ig ib le i n t e r f e r ence (i.e. a n e r r o r less t h a n 5%) . Resu l t s i nd i ca t e t h a t t u n g s t e n , v a n a d i u m , c a d m i u m a n d i ron ( I I I ) a re the p r i n c i p a l in te r fe ren t s . O f these ions , i r o n ( I I I ) a n d c a d m i u m are re- m o v e d by the e x t r a c t i o n process . V a n a d i u m a n d t u n g s t e n a re n o t r e m o v e d ; however , t he i r levels in p a s t u r e h e r b a g e w o u l d a lways be low e n o u g h n o t to cause in te r fe rences a t the d e t e r m i n a t i o n o f m o l y b d e n u m .

Results

We tes ted 39 h e r b a g e s amp le s ; resu l t s o f the ca ta ly t i c m e t h o d (CA) , t o g e t h e r w i t h resul t s o f a t o m i c a b s o r p t i o n ( A A ) a n d p l a s m a e m i s s i o n (PE) s p e c t r o m e t r i c m e t h o d s are s h o w n in Tab le 4. As the resul t s ind ica te , o n average , the re is g o o d a g r e e m e n t b e t w e e n the ca ta ly t i c a n d the two s p e c t r o m e t r i c p r o c e d u r e s ; the d i f fe rences are, o n the w h o l e lower t h a n the d i f ferences b e t w e e n a t o m i c a b s o r p t i o n a n d p l a s m a emis s ion values.

Acknowledgements. The assistance of the Department of Agri- culture, Government of Northern Ireland, by providing funds and samples, is gratefully acknowledged. We also wish to thank Dr. E. L. Dickson for helping with the plasma emission determinations.

References

1. Bradfield EG, Stickland JF (1975) Analyst 100:1 2. Jones GB (1954) Anal Chim Acta 10:584 3. Kennedy JT, Svehla G (1982) Fresenius Z Anal Chem 311:218 4. Knowles HB (1932) Bur Standards J Res 9:1 5. Svehla G, Erdey L (1963) Microchem J 7:206 6. Svehla G, Erdey L (1963) Microchem J 7:221 7. Weisz H, Rothmaier K (1974) Anal Chim Aeta 68:93

Received July 22, 1985

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