⚗️ Manganese in water and waste water — the colorimetric method with formaldoxime (a brownish-red complex, measurement at 450 nm)

Physicochemistry PN-92/C-04590/03

Overview

The formaldoxime method is one of the two classical routes of the colorimetric determination of manganese — alongside oxidation with persulfate to permanganate. Formaldoxime is formed from formalin and hydroxylamine hydrochloride and forms with manganese in an alkaline medium a stable brownish-red manganese tetraformaldoxime complex, measured at a wavelength of 450 nm against distilled water.

The removal of two obstacles is essential for the correctness of the result. Iron forms its own coloured complex with formaldoxime and is masked with triethanolamine. The addition of sodium potassium tartrate prevents the precipitation of aluminium hydroxide, which would make the solution turbid. The complexing solution is prepared as a mixture of 20 g of sodium potassium tartrate and 300 g of triethanolamine in 1 dm³ of distilled water, and the formaldoxime solution by adding 73 cm³ of 38 % formalin to 700 cm³ of ten-per-cent hydroxylamine hydrochloride and making up with water to 1 dm³.

The course of the determination is strictly sequential and the order of addition of the reagents matters: to a portion of the solution ascorbic acid is added, after five minutes the complexing solution and the formaldoxime, immediately it is made alkaline with 1 N sodium hydroxide with an excess, after mixing the EDTA solution is added, it is made up to the mark and the absorbance is measured after 30 minutes. The EDTA added after the development of the manganese colour decomposes the iron-formaldoxime complex without affecting the manganese one — it is precisely this operation that makes the method useful for ferruginous waters.

Status of the standard — read this before relying on it: PN-92/C-04590/03:1992 has been WITHDRAWN by the Polish Committee for Standardization. A laboratory which has it in its scope of accreditation relies on a withdrawn standard and must be able to justify this at an assessment; the current routes of determining manganese in water are atomic emission spectrometry with plasma excitation according to PN-EN ISO 11885 or flame atomic absorption spectrometry. The withdrawal of the standard does not invalidate the method itself, but it changes the way in which it has to be described in the documentation of the quality system.

A note on the sources, because there is something unusual here: the full description of the chemistry and of the procedure below comes from the Polish industry standard BN-83/0520-22 on the determination of total manganese in peat and in products made of peat — the matrix is different from water, but the reaction, the reagents and the manner of removing the obstacles are the same. All the volumes, concentrations and times given below are parameters of the peat variant. None of us has read the water variant from PN-92/C-04590/03, because its text is not publicly available — the parameters for water (the range of determination, the volume of the portion tested, the manner of preparing the sample) the laboratory must take directly from the standard. Treat this entry as a description of the formaldoxime method and not as a summary of the water standard.

An alternative, if this particular standard is not needed: the method with ammonium persulfate according to PN-C-04590-01 (oxidation of manganese(II) to permanganate in the presence of silver nitrate as a catalyst, measurement at 525 nm, range 0.005–1.0 mg/dm³) or flame atomic absorption spectrometry at 279.5 nm.

Method principle

Formaldoxime, obtained from formalin and hydroxylamine hydrochloride, forms with manganese in an alkaline medium a stable brownish-red manganese tetraformaldoxime complex; the intensity of the colour is proportional to the manganese content and is measured at 450 nm against distilled water. The obstacles are removed by two additions: triethanolamine masks iron, which also forms a coloured complex with formaldoxime, and sodium potassium tartrate prevents the precipitation of aluminium hydroxide. The order of the operations: to 10 cm³ of the solution tested in a 50 cm³ flask add 2 cm³ of two-per-cent ascorbic acid, after 5 minutes 5 cm³ of the complexing solution (20 g of sodium potassium tartrate and 300 g of triethanolamine per 1 dm³) and 2 cm³ of the formaldoxime solution (73 cm³ of 38 % formalin per 700 cm³ of ten-per-cent hydroxylamine hydrochloride, made up to 1 dm³), immediately make alkaline with 1 N sodium hydroxide solution, adding 2 cm³ in excess, after mixing add 5 cm³ of five-per-cent EDTA solution, make up with distilled water to the mark and mix; after 30 minutes measure the absorbance at 450 nm. The calibration curve is determined from working solutions prepared from a stock solution containing 100 µg of Mn in 1 cm³ (91.2 cm³ of 0.1 N potassium permanganate solution, 30 cm³ of concentrated sulfuric acid and decolourisation with drops of ten-per-cent sodium sulfite, made up to 1 dm³) or from a ready-made manganese standard solution suitably diluted. In parallel a blank is carried through the whole course of the procedure and its result is subtracted.

Applications

• Drinking water, spring water and mineral water — manganese as an organoleptic and technological parameter
• Groundwater with a naturally elevated manganese content — control of the effectiveness of demanganisation
• Waste water and process water — determinations in matrices with an elevated iron content, where EDTA removes its influence
• Laboratories without an atomic absorption spectrometer — only a spectrophotometer is needed
• Control of demanganising filters and catalytic beds in water treatment stations

Key parameters

Standard

Standard number

PN-92/C-04590/03:1992

Title (PL)

Woda i ścieki — Badania zawartości manganu — Oznaczanie manganu metodą formaldoksynową

Title (EN)

Water and waste water — Tests for manganese content — Determination of manganese by formaldoxime method

Step-by-step procedure

1. Preparation of the solution tested

   In the determination of total manganese mineralise the sample (combustion in the liquid or in the solid phase) in accordance with the procedure; dilute the solution after mineralisation in the solid phase with distilled water in the ratio 2:3. For dissolved manganese filter the sample.

2. Measuring out the portion

   Measure out into a 50 cm³ volumetric flask 10 cm³ of the solution tested (for a solution after combustion in the solid phase — 4 cm³ and make up with water to 10 cm³).

3. Ascorbic acid

   Add 2 cm³ of two-per-cent ascorbic acid solution and wait 5 minutes.

   ⏱ Time: 5 min

4. Complexing and formaldoxime

   Add 5 cm³ of the complexing solution (sodium potassium tartrate with triethanolamine) and 2 cm³ of the formaldoxime solution.

5. Making alkaline

   Immediately after the addition of the formaldoxime make alkaline with 1 N sodium hydroxide solution, adding 2 cm³ in excess. A delay at this stage spoils the repeatability.

6. EDTA and making up

   After mixing add 5 cm³ of five-per-cent EDTA solution, make up with distilled water to the mark and mix again.

7. Measurement

   After 30 minutes have elapsed measure the absorbance at 450 nm against distilled water. Read off the manganese content from the calibration curve.

   ⏱ Time: 30 min

8. Calibration curve and blank

   Determine the curve from working solutions carried through the full procedure (10 cm³ of each working solution measured out). Carry the blank along the same route and subtract its result from the result of the determination.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Formalin is toxic, irritant and carcinogenic — prepare the formaldoxime solution only under a fume hood, in gloves and goggles.
• Hydroxylamine hydrochloride is harmful and sensitising; avoid raising dust when weighing it out.
• Triethanolamine and sodium hydroxide are corrosive to the skin and the eyes — making alkaline with an excess calls for caution.
• Potassium permanganate is a strong oxidising agent — do not mix it with organic substances; carry out the preparation of the standard with sulfuric acid with cooling.
• Collect waste containing formaldehyde, EDTA and manganese separately and hand it over to an authorised receiver.

Frequently asked questions