Continuous-flow determination of manganese in natural waters containing iron

Analytica Elsevier

Chimicn Acta, Science

Publishers

199 (1987) 221-226 B.V., Amsterdam

-

Printed

in The Netherlands

Short Communication

CONTINUOUS-FLOW DETERMINATION OF MANGANESE NATURAL WATERS CONTAINING IRON

IN

D. J. HYDES

Institute of Oceanographic G U8 5 UB (Great Britain) (Received

17th

December

Sciences,

Brook Road,

Wormley,

Godalming,

Surrey

1986)

A re-evaluation of the use of formaldoxime to determine manganese in natural waters at concentrations of O-100 PM is reported. Addition of EDTA after formation of the manganese/formaldoxime complex removes interference from up to 100 PM iron. The extents of formation and destruction of the iron and manganese complexes with formaldoxime depend on the pH of the solution and on the time between reagent addition and measurement of absorbance.

Summary.

The formaldoxime method is widely used for the determination of manganese in natural waters [l-3]. It has the advantages of being rapid, based on simple reagents, and having sufficient sensitivity for determinations in natural water in which dissolved manganese levels have been enhanced by anoxia. The drawback of the method is that iron, which is frequently present in anoxic waters, also forms a complex with formaldoxime with a similar molar absorptivity. Removal of the iron interference with EDTA was originally suggested by Goto et al. [4] and then applied in an automated method by Henricksen [l], but the mechanism was uncertain. In this communication, the removal of iron interference in the formaldoxime method is reinvestigated in order to produce a method in which removal of the iron interference is reliable. Experimental Instrumentation. A Chemlab continuous-flow analyser was used for the development of the automated method; the manifolds are shown in Fig. 1. Manifold A was used for iron-free waters and manifold B for sediment pore water. A Pye SP500 spectrophotometer fitted with a 4-cm path length cuvette was used for measurements of the absorption spectra of the formaldoxime complexes of iron and manganese and measurements of the rate of colour development. Reagents. Distilled water was used throughout. The formaldoxime solution (stock) was prepared by dissolving 4 g of hydroxylammonium chloride in 60 ml of water; 2 ml of 37% formaldehyde solution was added and the solution was diluted to 100 ml. This solution can be stored for several 0003-2670/87/$03.50

o 1987

Elsevier

Science

Publishers

B.V.

222

Fig. 1. Single (A) and two-reagent (B) manifolds for the continuous-flow determination GY, of manganese. Flow rates are in ml min.’ 0, orange; Y, yellow; G-O, green/orange; grey; Bu-Y, blue/yellow; P-W, purple/white.

months. The working reagent was prepared by diluting 4.5 ml of stock formaldoxime solution in 200 ml of water; 9 ml of 10% (v/v) ammonia solution was added followed by 0.5 ml of Brij-35 solution (25% w/v) and the solution was diluted to 250 ml with water. For the EDTA solution, 10 g of ethylenediaminetetraacetic acid disodium salt, was dissolved in 100 ml of water. An aqueous 20% (w/v) solution of hydroxylammonium chloride was also prepared. Results Simple manifold. Manganese reacts rapidly with formaldoxime to give an orange-red solution. If a single reagent addition method is used [2], manganese can be determined by using the manifold shown in Fig. 1A. This manifold was used to evaluate the reproducibility of the method as well as its sensitivity to variations in the pH of the mixed reagent and dissolved iron concentration. The calibration was found to be linear up to a concentration of 60 PM. The precision (relative standard deviation, r.s.d.) for ten determinations on a solution containing 20 PM manganese was I%, the standard deviation being 0.2 MM. Under the same conditions, a solution containing 100 PM iron produced an interference equivalent to 4.7 PM of manganese. The effect of varying pH was tested by adjusting the pH of the formaldoxime solution with the hydroxylammonium chloride solution. The results TABLE

1

Peak heights formaldoxime PH Mn peaka Fe peaka

produced solutions 4.7 _b

1.9

aIn cm. bNot detectable.

by solutions containing of different pH 5.8 _b

2.2

7.0 _b

3.0

80 bM Mn or 1 mM Fe reacting

8.7 9.2 4.8

9.4 11.5 5.6

with

9.9 11.8 6.0

223

(Table 1) show that formation of the manganese complex is more sensitive to changes in pH than formation of the iron complex. Therefore, simply reducing the pH of a single mixed reagent cannot be used as a means of decreasing the iron interference. Addition of 4 ml of 10% (w/v) EDTA solution to the working formaldoxime solution (and then adjusting the pH back to 9.3 with ammonia solution) gave a mixture which allowed neither the manganese nor the iron formaldoxime complex to form. Discrete sample measurements. The first step in deciding how best to remove the iron interference was to compare the absorption spectra of the formaldoxime complexes of iron and manganese. The spectra measured were similar to those reported by Goto et al. [4]. These measurements showed that formation of the iron complex was much slower than that of the manganese complex. At pH 9.9, formation of the iron complex was essentially complete only after 2.5 h and a 100 PM iron solution gave an absorbance of 0.17, whereas formation of the manganese complex was complete in
2

Effect of decreasing the pH of the reaction formaldoxime complex at pH 9.9 pHa

Absorbance After

9.9 9.4 9.0 7.7 aAfter TABLE

mixture

5 min

0.318 0.318 0.321 0.289 addition

pHa After

100

min

of increasing

amounts

9.9 7.4 7.2 6.6 6.3

of the manganese/

Absorbance 5 min

After

100 min

0.223 0.136 0.050

0.260 0.242 0.227

of hydroxylammonium

chloride.

3

Effect of EDTA on the absorbances of the iron doxime, added after complex formation at pH 9.9 PH

formation

After 6.7 6.5 6.3

0.332 0.332 0.323 0.286

after

Mn absorbance

and manganese

complexes

of formal-

Fe absorbance

5 min

60 min

5 min

60 min

0.342 0.278 0.267 0.215 0.213

0.341 0.249 0.196 0.189 0.189

0.166 0.065 0.032 0.011 0.014

0.309 0.007 0.005 0.007 0.007

224

the results obtained by forming the iron and manganese complexes from 12 ml of working reagent solution and 1 ml of iron (1 mM) or manganesespiked (80 PM) acidified seawater, and then after 2 min adding 4 ml of the hydroxylamine or EDTA solutions of various dilutions to give the final pH recorded in the tables. Initially, the effects of adding the hydroxylammonium solution after formation of the manganese complex were studied; the results of measurements taken 5 and 100 min after addition of the hydroxylammonium chloride are shown in Table 2. There was obviously considerable hysteresis in the kinetics of the formation and decomposition of the manganese complex but, once formed, it appeared to be stable at pH values above 7.7. Addition of EDTA solution had, at the same pH, a similar effect to the addition of hydroxylammonium chloride. This suggests that the decomposition of the manganese complex of formaldoxime is principally controlled by the pH of the solution. For the iron/formaldoxime complex, there was a significant difference in the effects of hydroxylamine and EDTA (Table 3). Additional hydroxylamine simply lowers the pH of the solution, so that its effects on the stability of the iron and manganese complexes were

0.0

i

I

0

IO

I

I

I

20

30

40

Time (min) Fig. 2. Variation in the absorbance of solutions containing the iron or manganese complex with formaldoxime initially formed at pH 9.9, caused by addition of hydroxylammonium chloride or EDTA. Curves: (1) 100 PM manganese with EDTA added to give pH 7.7; (2) 1 mM iron with NH,OH. HCl added to give pH 8.3; (3) as for (2) but final pH 7.0; (4) 1 mM iron with EDTA added to a final pH of 7.7.

225

similar in the alkaline range. The chelating ability of EDTA enhanced decomposition of the iron complex in the pH region where the manganese/ formaldoxime complex was still stable. The rate of decomposition of the iron and manganese complexes after the addition of either hydroxylamine or EDTA as a secondary reagent was examined. Typical results (Fig. 2) show that, below pH 8, the change in pH caused by the addition of the secondary reagent initially caused a rise in the amount of iron/formaldoxime complex formed. At 2O”C, breakdown of the iron/formaldoxime complex by EDTA at pH 7.7 was effectively complete after 25 min. These measurements indicate that, for the addition of a second reagent to be successful, sufficient time has to be allowed after its addition for decomposition to occur. This was put into effect in building a continuous-flow manifold by running the reacting solution through a heating bath (37°C) after the addition of the second reagent. Two-reagent manifold. The manifold shown in Fig. 1B was constructed. The 5% EDTA solution used gave a final pH of 7.7. When this manifold was used, interference from
Henricksen [l] and Goto et al. [4] suggested that for effective inhibition of the formation of the iron/formaldoxime complex, addition of both hydroxylamine and EDTA was necessary. The results presented here show that the reaction of formaldoxime with iron is slower than that with manganese. The extent to which both complexes are formed depends on the pH of the solution, and complex formation is favoured by higher pH values. Once the complex has formed, lowering the pH leads to the destruction of TABLE

4

Comparison and without

of the peak heights measured for the manganese and iron complexes, the addition of EDTA and with and without heating Peak heights 100 PM Mn

(cm) 100pMFe

1mMFe

0.9 0.2

6.3 1.8

14.6 10.1

aNot detectable.

100 PM Mn Heating

No heating No EDTA EDTA

with

15.2 8.6

100 FM Fe

1 mM Fe

at 37°C 1.5 _a

8.4 0.4

226

both complexes but at near neutral pH this is slow. In slightly alkaline solutions, addition of EDTA accelerates the reduction of the iron but not the manganese complex; the conditions in the mixture may be such that iron is oxidised to Fe(II1) which forms a more stable complex with EDTA than do Mn(I1) or Fe(H). The stability constants (pK,) are 25,14 and 14, respectively [5]. On the basis of these results, it appears likely that the earlier suggestion [ 1, 41 that both hydroxylamine and EDTA were necessary to remove the iron interference was based on a lack of awareness of the controlling influence of pH on the stability of the two complexes. Henricksen [l] reported that EDTA was ineffective for removing iron interference at low levels of iron. In this work, no evidence was found for the removal of iron interfer’ ence being less effective at low iron concentrations. REFERENCES 1 2 3 4 5

A. Henricksen, Analyst, 91 (1966) 647. P. G. Brewer and D. W. Spencer, Limnol. Oceanogr., 16 (1971) 107. J. J. Sawlan and J. W. Murray, Earth Planet. Sci. Lett., 64 (1983) 213. K. Goto, T. Komatsu and T. Furukawa, Anal. Chim. Acta, 27 (1962) 331. L. G. Sillen and A. E. Martell, Chemical Society Special Publication No. 25: Stability Constants, Suppl. No. 1, The Chemical Society, London, 1970.