Standard Methods: 3500-Ca B:  Calcium by EDTA

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  • Analytes
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  • Data and Sites
Official Method Name
3500-Ca B. EDTA Titrimetric Method
Current Revision
Standard Methods Online
Media
WATER
Instrumentation
Titration with Color Indicator
Method Subcategory
Inorganic
Method Source
  Standard Methods
Citation
  Standard Methods Online - Standard Methods for the Examination of Water and Wastewater
Brief Method Summary
When EDTA (ethylenediaminetetraacetic acid or its salts) is added to water containing both calcium and magnesium, it combines first with the calcium. Calcium can be determined directly, with EDTA, when the pH is made sufficiently high that the magnesium is largely precipitated as the hydroxide and an indicator is used that combines with calcium only. Several indicators give a color change when all of the calcium has been complexed by the EDTA at a pH of 12 to 13.
Scope and Application
The EDTA titration gives good results for control and routine applications, but for samples containing high P levels (>50 mg/L) only the AA or atomic emission methods are recommended because of interferences encountered with EDTA indicators.
Applicable Concentration Range
Interferences
Under conditions of this test, the following concentrations of ions cause no interference with the calcium hardness determination: Cu(II), 2 mg/L; Fe (II), 20 mg/L; Fe (III), 20 mg/L; Mn (II), 10 mg/L; Zn (II), 5 mg/L; Pb (II), 5 mg/L; Al (III), 5 mg/L; and Sn (IV), 5 mg/L. Orthophosphate precipitates calcium at the pH of the test. Strontium and barium give a positive interference and alkalinity in excess of 300 mg/L may cause an indistinct end point in hard waters.
Quality Control Requirements
See Section 3020 Quality Assurance/Quality Control.
Sample Handling
Plastic or glass containers. Add HNO3 to pH <2. See Sectio 3010 for more details.
Maximum Holding Time
6 months (See Section 1060)
Relative Cost
Less than $50
Sample Preparation Methods