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STANDARD METHODS: FOR THE EXAMINATION OF WATER AND WASTEWATER

18TH EDITION 1992

Prepared and published jointly by:
AMERICAN PUBLIC HEALTH ASSOCIATION
AMERICAN WATER WORKS ASSOCIATION
WATER ENVIRONMENT FEDERATION

Joint Editorial Board
Arnold E. Greenberg, APHA, Chairman
Lenore S. Clesceri, WEF
Andrew D. Easton, AWWA

Managing Editor
Mary Ann H. Franson

Publication Office
American Public Health Association
1015 Fifteenth Street, NW
Washington, DC 20005

All rights reserved. No part of this publication may be reproduced, graphically or electronically, including entering in storage or retrieval systems, without the prior written permission of the publishers.

30M7/92

The Library of Congress has catalogued this work as follows:
American Public Health Association.
        Standard methods for the examination of water and wastewater.
ISBN 0-87553-207-1

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3500-Mg MAGNESIUM^*

3500-Mg A. Introduction

1. Occurrence

Magnesium ranks eighth among the elements in order of abundance and is a common constituent of natural water. Important contributors to the hardness of a water, magnesium salts break down when heated, forming scale in boilers. Concentrations greater than 125 mg/L also can have a cathartic and diuretic effect. Chemical softening. reverse osmosis, electrodialysis, or ion exchange reduces the magnesium and associated hardness to acceptable levels. The magnesium concentration may vary from zero to several hundred milligrams per liter, depending on the source and treatment of the water.

2. Selection of Method

The four methods presented are applicable to all natural waters. Direct determinations can be made with the atomic absorption spectrometric and inductively coupled plasma methods. Magnesium can be determined by the gravimetric method only after removal of calcium salts (see Section 3500-Ca). These methods can be applied to all concentrations by selection of suitable sample portions. Choice of method is largely a matter of personal preference and analyst experience.

^* Approved by Standard Methods Committee, 1990.

3500-Mg B. Atomic Absorption Spectrometric Method

See flame atomic absorption spectrometric method. Section 3111B.

3500-Mg C. Inductively Coupled Plasma Method

See Section 3120.

3500-Mg D. Gravimetric Method

1. General Discussion

a. Principle: Diammonium hydrogen phosphate quantitatively precipitates magnesium in ammoniacal solution as magnesium ammonium phosphate. The precipitate is ignited to, and weighed as, magnesium pyrophosphate. A choice is presented between: (a) destruction of ammonium salts and oxalate, followed by single precipitation of magnesium ammonium phosphate; and (b) double precipitation without pretreatment. Where time is not a factor, double precipitation is preferable because, while pretreatment is faster, it requires close attention to avoid mechanical loss.

b. Interference: The solution should be reasonably free from aluminum, calcium, iron, manganese, silica, strontium, and suspended matter. It should not contain more than about 3.5 g NH₄Cl.

2. Reagents

a. Nitric acid, NHO_3, conc.

b. Hydrochloric acid, HCI, conc; also 1 + 1, 1 + 9, and 1 + 99.

c. Methyl red indicator solution: Dissolve 100 mg methyl red sodium saltin distilled water and dilute to 100 mL.

d. Diammonium hydrogen phosphate solution: In distilled water, dissolve 30 g (NH₄)₂HPO₄ and make up to 100 mL.

e. Ammonium hydroxide, NH₄OH, conc; also 1 + 19.

3. Procedure

a. By removal of oxalate and ammonium salts: To the combined filtrate and washings from the calcium determination, containing not more than 60 mg Mg, or to a portion containing less than this amount in a 600 or 800-mL beaker, add 500 mL. cone HNO₃ and evaporate carefully to dryness on a hot plate. Do not let reaction become too violent during the later part of the evaporation; stay inconstant attendance to avoid losses through spattering. Moisten residue with 2 to 3 mL cone HCl; add 200 mL distilled water, warm, filter, and wash. To the filtrate add 3 mL cone HCI, 2 to 3 drops methyl red solution, and 10 mL (NH₄)₂HPO₄ solution. Cool and add cone NH₄OH, drop by drop, stirring constantly, until the color changes to yellow. Stir for 5 min, add 5 mL cone NH₄OH. and stir vigorously for 10 min more. Let stand overnight and filter through filter paper.^*Wash with 1 + 19 NH₄OH. Transfer to an ignited, cooled, and weighted crucible Dry precipitate thoroughly and burn paper off slowly, allowing circulation of air. Heat at about 500°C until residue is white. Ignite for 30-min periods at 1100°C to constant weight.

b. By double precipitation: To the combined filtrate and washings from the calcium determination, containing not more that 60 mg Mg, or to a portion containing less than this amount, add 2 to 3 drops methyl red solution; adjust volume to 150 mL and acidify with 1 + 1 HCl. Add 10 mL (NH₄4)₂HPO₄ solution. Cool. Add cone NH₄OH, drop by drop, stirring constantly, until the color changes to yellow. Stir for 5 min, add 5 mL cone NH₄OH, and stir vigorously for 10 min more. Let stand overnight and then filter through filter paper.^* Wash with 1 + 19 NH₄OH. Discard filtrate and washings. Dissolve precipitate with 50 mL warm 1 + 9 HCl and wash paper well with hot 1 + 99 HCl. Add 2 to 3 drops methyl red solution, adjust volume to 100 to 150 mL, add 1 to 2 mL (NH₄)₂HPO₄ solution, and precipitate as before. Let stand in a cool place for at least 4 h or preferably overnight. Filter through filter paper^* and wash with 1 + 19 NH₄OH. Transfer to an ignited, cooled, and weighed crucible. Dry precipicate throughly and burn paper off slowly, allowing circulation of air. Heat at about 500° until residue is white. Ignite for 30-min periods at 1100°C to constant weight.

4. Calculation

5. Precision and Bias

A synthetic sample containing 82 mg Mg/L, 108 mg Ca/L, 3.1 mg K/L. 19.9 mg Na/L, 241 mg Cl −/L, 1.1 mg NO₃−-N/L, 0.250 mg NO₂−-N/L, 259 mg SO₄²−/L, and 42.5 mg total alkalinity/L (contributed by NaHCO₃) was analyzed in eight laboratories by the gravimetric method, with a relative standard deviation of 6.3% and a relative error of 4.9%

* Carl Schleicher and Schuell Co.. S & S No. 589 White Ribbon, or equivalent.

6. Bibliography

EPPERSON, A.W. 1928. The pyrophosphate method for the determination of magnesium and phosphoric anhydride. J. Amer. Chem. Soc. 50:321.

KOLTHOFF, I.M. & E.B. SANDELL. 1952. Textbook of Quantitative Inorganic Analysis, 3rd ed. Macmillan Co., New York, N.Y., Chapter 22.

HILLEBRAND. W.F. et al. 1953. Applied Inorganic Analysis, 2nd ed. John Wiley & Sons. New York. Chapter 41 and pp. 133-134.

3500-Mg E. Calculation Method

Magnesium may be estimated as the difference between hardness and calcium as CaCO₃ if interfering metals are present in noninterfering concentrations in the calcium titration (Section 3500-Ca.D) and suitable inhibitors are used in the hardness titration (Section 2340C).

mg Mg/L = [total hardness (as mg CaCO₃/L)
                            – calcium hardness (as mg CaCO₃/L)] × 0.243

See Section 3120.