Analisis Del Agua

Standard Methods for the Examination of Water and Wastewater

4500-SO42– A.

Introduction

1. Occurrence Sulfate (SO42–) is widely distributed in nature and may be present in natural waters in concentrations ranging from a few to several thousand milligrams per liter. Mine drainage wastes may contribute large amounts of SO42– through pyrite oxidation. Sodium and magnesium sulfate exert acathartic action. 2. Selection of Method The ion chromatographic method (4110) and capillary ion electrophoresis (CIE—see Section 4140) are suitable for sulfate concentrations above 0.1 mg/L. The gravimetric methods (C and D) are suitable for SO42– concentrations above 10 mg/L. The turbidimetric method (E) is applicable in the range of 1 to 40 mg SO42– /L. The automated methylthymol blue methods (Fand G) are the procedures for analyzing large numbers of samples for sulfate alone when the equipment is available; over 30 samples can be analyzed per hour. Methods C, D, F, G, 4110, or CIE (4140) are preferred for accurate results. 3. Sampling and Storage In the presence of organic matter certain bacteria may reduce SO42– to S2–. To avoid this, store samples at 4°C. 4500-SO42– B. (Reserved)4500-SO42– C.

Gravimetric Method with Ignition of Residue

1. General Discussion a. Principle: Sulfate is precipitated in a hydrochloric acid (HCl) solution as barium sulfate (BaSO4) by the addition of barium chloride (BaCl2). The precipitation is carried out near the boiling temperature, and after a period of digestion the precipitate is filtered, washed with water until free of Cl–, ignitedor dried, and weighed as BaSO4. b. Interference: The gravimetric determination of SO42– is subject to many errors, both positive and negative. In potable waters where the mineral concentration is low, these may be of minor importance. 1) Interferences leading to high results—Suspended matter, silica, BaCl2 precipitant, NO3–, SO32– and occluded mother liquor in the precipitate are the principalfactors in
© Copyright 1999 by American Public Health Association, American Water Works Association, Water Environment Federation

Standard Methods for the Examination of Water and Wastewater
positive errors. Suspended matter may be present in both the sample and the precipitating solution; soluble silicate may be rendered insoluble and SO32– may be oxidized to SO42– during analysis. Bariumnitrate [Ba(NO3)2], BaCl2, and water are occluded to some extent with the BaSO4 although water is driven off if the temperature of ignition is sufficiently high. 2) Interferences leading to low results—Alkali metal sulfates frequently yield low results. This is true especially of alkali hydrogen sulfates. Occlusion of alkali sulfate with BaSO4 causes substitution of an element of lower atomic weightthan barium in the precipitate. Hydrogen sulfates of alkali metals act similarly and, in addition, decompose on being heated. Heavy metals, such as chromium and iron, cause low results by interfering with the complete precipitation of SO42– and by formation of heavy metal sulfates. BaSO4 has small but significant solubility, which is increased in the presence of acid. Although an acid medium isnecessary to prevent precipitation of barium carbonate and phosphate, it is important to limit its concentration to minimize the solution effect. 2. Apparatus a. Steam bath. b. Drying oven, equipped with thermostatic control. c. Muffle furnace, with temperature indicator. d. Desiccator. e. Analytical balance, capable of weighing to 0.1 mg. f. Filter: Use one of the following: 1) Filter paper,acid-washed, ashless hard-finish, sufficiently retentive for fine precipitates. 2) Membrane filter, with a pore size of about 0.45 µm. g. Filtering apparatus, appropriate to the type of filter selected. (Coat membrane filter holder with silicone fluid to prevent precipitate from adhering.) 3. Reagents a. Methyl red indicator solution: Dissolve 100 mg methyl red sodium salt in distilled water and dilute...