Analyst, October, 1966, Vol. 91, &5. 647-651 647 An Automatic, Modified Formaldoxime Method for Determining Low Concentrations of Manganese in Water Containing Iron BY A. HENRIKSEN" (Norwegian Institute for Water Research, Oslo 3, hrovwuy) An automatic method for determining manganese in fresh water with the AutoA4nalyzer is described, together with details of the analytical system. The method is based on a modification of the formaldoxime procedure pub- lished by Goto, Komatsu and Furukawa,l which consists in reacting man- ganese with formaldoxime in alkaline solution, and decomposing any iron formaldoxime formed with EDTA and hydroxylammonium chloride. Aluminium, zinc, copper, iron, calcium, magnesium, chloride and phos- phate do not interfere a t concentrations above those normally found in natural water in Norway.The capacity of the method is 25 samples per hour, and the standard deviation is 5 p g of manganese per litre. A statistical comparison of the manual persulphate oxidation method with the automatic formaldoxime method indicated that both methods give identical results. THE manganese content of fresh water is usually determined colorimetrically by oxidation with persulphate or periodate to form permanganate ions2 Compared with most colorimetric methods, the sensitivity is poor. As the concentration of manganese in natural water in Norway is normally low, a more sensitive method was needed that could be easily adapted for use with the Technicon AutoAnalyzer. Formaldoxime, which forms an orange - red colour with manganese in alkaline solutions, has been used for determining manganese in plant material by Sideris3y4 and Bradfield,5 and in water by Goto, Komatsu and Furukawa,l and Morgan and Stumm.6 The sensitivity of the formaldoxime method is estimated to be about five times that of the permanganate m e t h ~ d .~ A disadvantage of the formaldoxime method is that iron interferes by forming a violet - red coloured complex with the reagent. Sideris* recommended precipitation of iron as iron(II1) phosphate. Bradfields observed that precipitation of iron, calcium and mag- nesium in alkaline solutions was prevented by the addition of N-hydroxyethylethylene diaminetriacetic acid (HEEDTA), and the coloured iron complex was decomposed by heating the sample at 65" C for 2 hours.These observations were confirmed by Morgan and Stumm.6 Goto, Komatsu and Furukawal showed that the formaldoxime complex of iron was rapidly decomposed by adding EDTA and hydroxylammonium chloride, while the manganese formaldoxime was stable under these conditions. The procedure described by Goto et al. seemed to be the simplest for routine determina- tions of manganese on the AutoAnalyzer. EXPERIMENTAL Analytical-reagent grade chemicals were used. A Technicon AutoAnalyzer with a phototube colorimeter was used for the colorimetric determinations. All procedures described in the subsequent text were converted to automatic methods. The flow scheme of these is essentially the same as that shown in Fig. 1, except that one or more reagents were omitted. All values found are given in micrograms of manganese per litre, and are obtained by reading against a calibration graph obtained with manganese standards.INTERFERENCE FROM IRON- The concentration of iron in natural water in Norway usually varies up to a maximum The effect of this concentration range of iron in the following three of 1 mg per litre. formaldoxime procedures was investigated- * Present address : The Institute of Paper Chemistry, Appleton, Wisconsin, U.S.A.648 HENRIKSEN : AN AUTOMATIC, MODIFIED FORMALDOXIME METHOD [Analyst, Vol. 91 Sampler I I %0° Y 0. I00 A i r Sample Formaldoxime Iron (I I ) sulphate Ammonia Re-i ntrod uction A i r EDTA H y d roxy I am i n e Fig. 1. Flow diagram of apparatus used in an automatic method for determining manganese in water (i) Addition of formaldoxime and ammonia only.(ii) Addition of formaldoxime and ammonia, followed by addition of EDTA and hydroxylammonium chloride according to the procedure given by Goto, Komatsu and Furukawa. (iii) Addition of iron( 11) sulphate, formaldoxime and ammonia, followed by addition of EDTA and hydroxylammonium chloride. The results obtained are given in Table I. The interference from iron is high if no EDTA and hydroxylammonium chloride are added. It is considerably reduced if these reagents are added, but a significant interference still occurs at concentrations of iron below 1 mg per litre. These observations indicated that the interference from iron could be further reduced by adding a sufficient amount of iron as a reagent in the second procedure.The last column in Table I shows the results obtained when a solution of iron(I1) sulphate was added as a reagent, making the sample 2 p.p.m. with respect to iron. It is seen that the interference is then reduced to an insignificant level. TABLE I EFFECT OF IRON IN THE FORMALDOXIME PROCEDURE FOR DETERMINING MANGANESE Concentration, p g of iron(I1) per litre 60 120 250 500 1000 2500 Without hydroxyl- amine and EDTA, pg of manganese per litre 20 60 190 360 7 00 1600 Method of Goto et al., pg of manganese per litre 20 45 55 35 20 ( 5 Method of Goto et al. + 2 mg of iron(I1) per litre, p g of manganese per litre < 5 < 5 < 5 ( 5 (5 <5October, 19661 FOR DETERMINING MANGANESE IN WATER CONTAINING IRON 649 A manganese standard of 100 pg per litre was analysed in the presence of different concentrations of iron.The results, given in Table 11, indicate that iron does not influence the determination of manganese. TABLE I1 EFFECT OF IRON IN THE PRESENCE OF MANGANESE 1OOpg of manganese per litre Concentration, pg of iron(I1) per litre 0 50 100 250 500 1000 Without iron added, pg of manganese per litre 100 120 142 150 130 115 With 2 mg of iron per litre added, p g of manganese per litre 100 102 100 104 99 100 The rather surprising effect of the additional iron has not been further investigated. However, from the experimental evidence given in Table I, it is reasonable to assume that the EDTA and hydroxylammonium chloride do not decompose the iron - formaldoxime complex when this is present below a certain concentration.The beneficial effect of the additional iron may then be to raise the concentration of the complex to a level at which decomposition will occur. INTERFERENCE FROM COLOURED SUBSTANCES IN WATER- The coloured substances in water are potential interferences in the formaldoxime method. The standard manual method for determining them consists in measuring the absorption of the water a t 420mp in a filter photometer. However, a significant absorption is still observed at 480mp. Thirty samples from different localities in Norway were analysed for manganese with the manifold shown in Fig. 1. The samples were passed through the apparatus for a second time, replacing the iron(I1) sulphate and formaldoxime solutions with distilled water. The absorption arising from the colour of the samples measured under these experi- mental conditions was correlated to the absorption measured manually at 420 mp. The mean value of the “apparent” manganese content due to the coloured substances was found to be 35 pg of manganese per litre.The mean value of the colour was 37” H. However, no correlation was found between these two sets of results. Consequently, coloured water samples that are low in manganese content should be run through the apparatus for a second time, with the iron(I1) sulphate and formaldoxime solutions replaced by distilled water. The “apparent” manganese content caused by the natural colour of the water samples must be deducted to give the net concentration of manganese. EFFECT OF pH ON COLOUR FORMATION- Morgan and Stumm have shown that the intensity of colour of manganese formaldoxime depends upon the pH; it reaches its maximum at a pH of 9.2.This observation has been confirmed, and in the recommended procedure given above a sufficient amount of ammonia solution is added to give a pH of 9.0 to 9.4. EFFECT OF OTHER IONS- Morgan and Stumm also point out that large amounts of iron and alkaline earths may be precipitated in alkaline solutions (as phosphates, hydroxides and carbonates). The effect of these and other ions in concentrations above those normally found in natural water in our country has been studied. Solutions containing 100 pg of manganese per litre, and different concentrations of other ions were prepared according to Table 111. Calcium and magnesium (up to 100 mg per litre) alone, and in the presence of 3 p.p.m.of phosphate, do not have any effect on the determination of manganese, neither do concen- trations of iron up to 5 p.p.m. alone, or in the presence of 3 p.p.m. of phosphate. Phosphate, however, interferes at high concentrations in the presence of calcium. Nickel and cobalt also interfere. When equimolar concentrations of cobalt or nickel and manganese are present the interference is 7.5 and 3 per cent., respectively. If the sample is heated to 60” C during650 [Analyst, Vol. 91 the decomposition with EDTA and hydroxylammonium chloride, the interference from cobalt and nickel is reduced to 1 and 3 per cent., respectively. No interference was observed from aluminium, zinc, copper and chloride at the con- HENRIKSEN : AN AUTOMATIC, MODIFIED FORMALDOXIME METHOD centrations given in Table 111.TABLE I11 EFFECT OF VARIOUS IONS ON THE COLOUR FORMATION OF 100 pg of manganese per litre Ion Aluminium . . .. Zinc . . .. .. Chloride . . . . Copper . . .. Copper . . . . Copper . . .. Magnesium . . .. Magnesium . . .. Iron(I1) . . .. Iron(I1) . . . . Iron(I1) . . .. Iron(I1) . . .. Iron(I1) . . .. Iron(II1) . . .. Phosphate . . . . Phosphate . . . . Calcium . . .. Calcium . . . . Cobalt . . * . Nickel . . Phosphate and iron(I1) Phosphate and calcium Phosphate and calcium Phosphate and magnesium .. .. . . .. .. . I . . . . .. . . .. .. .. .. .. . . . . . . . . .. . . . . .. . . . . . . . . .. . . .. . . . . . . .. .. . . . . . . .. .. . . .. . . . . .. .. .. Concentration, mg per litre 4 2000 300 0.04 0.4 1-6 50 100 50 100 0.05 0.1 0.5 1.0 5 5 3 60 1 1 3 f 5 3 + 80 60 + 80 3 + 80 MANGANESE FORMALDOXIME Manganese found, p g per litre 100 103 100 100 104 102 102 102 98 100 100 102 98 100 102 104 100 98 130 175 100 98 20 102 COMPARISON OF THE MANUAL AND AUTOMATIC METHODS- The standard deviation of the automatic formaldoxime method was found to be 5 p g of manganese per litre in the range 20 to 500 pg of manganese per litre, determined from 20 samples. In order to compare this method with the manual persulphate oxidation method formerly used in our laboratory, 22 samples were analysed by both methods. The results were statistically compared by calculating their F-values and t-values. These were 1-16 and 0.126.Comparison of the t-value with a statistical table for t-distribution indicates a 90 per cent.probability that differences in the mean values are caused by random errors. REAGENTS- Iron(l1) sulphate-Dissolve 140 mg of ammonium iron(I1) sulphate, FeS04(NH4),S0,. 6H,O, in water containing 1 ml of concentrated sulphuric acid and dilute the solution to 1 litre. This solution contains 20 mg of iron per litre. Formaldoxime-Dissolve 20 g of hydroxylammonium chloride in 500 ml of distilled water. Add 10ml of formaldehyde solution (37 per cent. w/w). Ammonia solution (3 + 1)-Dilute 3 parts of concentrated ammonia solution with 1 part of water. EDTA, 0.1 M-Dissolve 37.2 g of ethylenediaminetetra-acetic acid disodium salt in distilled water and dilute the solution to 1 litre. Hydroxylammoni$m chloride-Dissolve 100 g of hydroxylammonium chloride in distilled water and dilute the solution to 1 litre. STANDARD SOLUTIONS- Dissolve 143.8 mg of potassium permanganate in 50 ml of water. Add 2 ml of concen- trated sulphuric acid.Add a 10 per cent. solution of sodium bisulphite drop-wise until the colour of the permanganate disappears. Boil the solution to expel excess of sulphur dioxide. Cool, and dilute to 1 litre. This solution contains 50 mg of manganese(I1) per litre. Dilute the solution to appropriate standards.October, 19661 FOR DETERMINING MANGANESE IN WATER CONTAINING IRON 651 PROCEDURE- The method finally adopted for determining manganese on the AutoAnalyzer is shown in Fig. 1. The sample is segmented with air and successively mixed with solutions of iron(I1) sulphate, formaldoxime and ammonia.After passing the solution through a quarter-length, time-delay coil, which allows for the colour to develop, the air bubbles are removed through an extended T-junction, the stream is re-introduced and again segmented with air. Solutions of EDTA and hydroxylamine are then added to destroy any iron formaldoxime formed. After passing the solution through a double-coil heating bath (25” C) and a half-length, time-delay coil, the colour is measured at 480 mp. The sampling rate is 25 per hour, lt minutes are allowed for both washing and sampling. The range expander is used at x 4 and x 10 expansions, giving an operative range of 0 to 0.8 mg of manganese per litre. A linear relationship between optical density and manganese concentration is obtained in this range. The shape of the AutoAnalyzer traces is nearly symmetrical. REFERENCES 1. 2. “Standard Methods for the Examination of Water, Sewage and Industrial Wastes,” Eleventh 3. 4. __ , Ibid., 1940, 12, 307. 5. 6. Goto, K., Komatsu, T., and Furukawa, T., Analytica Chim. Acta, 1962, 27, 331. Edition, American Public Health Association, Inc., New York, 1960, p. 155. Sideris, C. P., I d . Engng Chem. AnaZyt. Edn, 1937, 9, 445. Bradfield, E. G., Analyst, 1957, 82, 254. Morgan, J. J., and Stumm, W., J . Amer. Wat. Wks Ass., 1965, 57, 107. Received November 26th, 1965