3500-Ca CALCIUM*

3500-Ca A. Introduction

1. Occurrence and Significance

Calcium (Ca) is the third element in Group IIA of the periodic

table; it has an atomic number of 20, an atomic weight of 40.08, and

a valence of 2. The average abundance of Ca in the earth’s crust is

4.9%; in soils it is 0.07 to 1.7%; in streams it is about 15 mg/L; and

in groundwaters it is from 1 to 500 mg/L. The most common

forms of calcium are calcium carbonate (calcite) and calciummagnesium carbonate (dolomite). Calcium compounds are widely

used in pharmaceuticals, photography, lime, de-icing salts, pigments, fertilizers, and plasters. Calcium carbonate solubility is controlled by pH and dissolved CO2. The CO2, HCO3

, and CO32

equilibrium is the major buffering mechanism in fresh waters.

Hardness is based on the concentration of calcium and magnesium

salts, and often is used as a measure of potable water quality.

NOTE: Calcium is necessary in plant and animal nutrition and

is an essential component of bones, shells, and plant structures.

The presence of calcium in water supplies results from passage

over deposits of limestone, dolomite, gypsum, and gypsiferous

shale. Small concentrations of calcium carbonate combat corrosion of metal pipes by laying down a protective coating. Because

precipitation of calcite in pipes and in heat-exchangers can cause

damage, the amount of calcium in domestic and industrial waters

is often controlled by water softening (e.g., ion exchange, reverse osmosis). Calcium carbonate saturation and water hardness

are discussed in Sections 2330 and 2340, respectively.

Calcium contributes to the total hardness of water. Chemical

softening treatment, reverse osmosis, electrodialysis, or ion exchange is used to reduce calcium and the associated hardness.

2. Selection of Method

The atomic absorption methods (3111B, D, and E) and inductively coupled plasma method (3120) are accurate means of

determining calcium. The EDTA titration method gives good

results for control and routine applications, but for samples

containing high P levels (50 mg/L) only the atomic absorption

or atomic emission methods are recommended because of interferences often encountered with EDTA indicators.

3. Storage of Samples

The customary precautions are sufficient if care is taken to

redissolve any calcium carbonate that may precipitate on standing.

3500-Ca B. EDTA Titrimetric Method

1. General Discussion

a. Principle: 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.

b. Interference: Under conditions of this test, the following

concentrations of ions cause no interference with the calcium

hardness determination: Cu2, 2 mg/L; Fe2, 20 mg/L; Fe3, 20

mg/L; Mn2, 10 mg/L; Zn2, 5 mg/L; Pb2, 5 mg/L; Al3, 5

mg/L; and Sn4, 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.

c. Quality control (QC): The QC practices considered to be an

integral part of each method can be found in Section 3020.

2. Reagents

a. Sodium hydroxide, NaOH, 1N.

b. Indicators: Many indicators are available for the calcium

titration. Some are described in the literature (see Bibliography);

others are commercial preparations and also may be used. Murexide

(ammonium purpurate) was the first indicator available for detecting

the calcium end point; directions for its use are presented in this

procedure. Individuals who have difficulty recognizing the murexide end point may find the indicator Eriochrome Blue Black R

(color index number 202) or Solochrome Dark Blue an improvement because of the color change from red to pure blue. Eriochrome

Blue Black R is sodium-1-(2-hydroxy-1-naphthylazo)-2-naphthol4-sulfonic acid. Other indicators specifically designed for use as

end-point detectors in EDTA titration of calcium may be used.

1) Murexide (ammonium purpurate) indicator: This indicator

changes from pink to purple at the end point. Prepare by dissolving 150 mg dye in 100 g absolute ethylene glycol. Water

solutions of the dye are not stable for longer than 1 d. A ground

mixture of dye powder and sodium chloride (NaCl) provides a

stable form of the indicator. Prepare by mixing 200 mg murexide

with 100 g solid NaCl and grinding the mixture to 40 to 50 mesh.

* Approved by Standard Methods Committee, 1997. Editorial revisions, 2011.

Joint Task Group: 20th Edition—See 3500-Al.

1
Titrate immediately after adding indicator because it is unstable

under alkaline conditions. Facilitate end-point recognition by

preparing a color comparison blank containing 2.0 mL NaOH

solution, 0.2 g solid indicator mixture (or 1 to 2 drops if a

solution is used), and sufficient standard EDTA titrant (0.05 to

0.10 mL) to produce an unchanging color.

2) Eriochrome Blue Black R indicator: Prepare a stable form

of the indicator by grinding together in a mortar 200 mg powdered dye and 100 g solid NaCl to 40 to 50 mesh. Store in a

tightly stoppered bottle. Use 0.2 g of ground mixture for the

titration in the same manner as murexide indicator. During

titration the color changes from red through purple to bluish

purple to a pure blue with no trace of reddish or purple tint. The

pH of some (not all) waters must be raised to 14 (rather than 12

to 13) by the use of 8N NaOH to get a good color change.

c. Standard EDTA titrant, 0.01M: Prepare standard EDTA titrant

and standardize against standard calcium solution as described in

Section 2340C to obtain EDTA/CaCO3 equivalence. Standard

EDTA titrant, 0.0100M, is equivalent to 1000 mg CaCO3/1.00 mL;

use titrated equivalent for B in the calculations in 4.

3. Procedure

a. Pretreatment of water and wastewater samples: Follow the

procedure described in Section 3030E or I if samples require

preliminary digestion.

b. Sample preparation: Because of the high pH used in this

procedure, titrate immediately after adding alkali and indicator.

Use 50.0 mL sample, or a smaller portion diluted to 50 mL so

that the calcium content is about 5 to 10 mg. Analyze hard waters

with alkalinity higher than 300 mg CaCO3/L by taking a smaller

portion and diluting to 50 mL. Alternatively, adjust sample pH

into the acid range (pH 6), boil for 1 min to dispel CO2, and

cool before beginning titration.

c. Titration: Add 2.0 mL NaOH solution or a volume sufficient

to produce a pH of 12 to 13. Stir. Add 0.1 to 0.2 g indicator mixture

selected (or 1 to 2 drops if a solution is used). Add EDTA titrant

slowly, with continuous stirring to the proper end point. When using

murexide, check end point by adding 1 to 2 drops of titrant in excess

to make certain that no further color change occurs.

4. Calculation

mg Ca/L  A  B  400.8

mL sample

Calcium hardness as mg CaCO3/L  A  B  1000

mL sample

where:

A  mL titrant for sample and

B  mg CaCO3 equivalent to 1.00 mL EDTA titrant at the

calcium indicator end point.

5. Precision and Bias

A synthetic sample containing 108 mg Ca/L, 82 mg Mg/L, 3.1

mg K/L, 19.9 mg Na/L, 241 mg Cl/L, 1.1 mg NO3

-N/L, 0.25

mg NO2

-N/L, 259 mg SO42/L, and 42.5 mg total alkalinity/L

(contributed by NaHCO3) in distilled water was analyzed in 44

laboratories by the EDTA titrimetric method, with a relative

standard deviation of 9.2% and a relative error of 1.9%.

6. Bibliography

DIEHL, H. & J.L. ELLINGBOE. 1956. Indicator for titration of calcium in

the presence of magnesium using disodium dihydrogen ethylenediamine tetraacetate. Anal. Chem. 28:882.

HILDEBRAND, G.P. & C.N. REILLEY. 1957. New indicator for complexometric titration of calcium in the presence of magnesium. Anal.

Chem. 29:258.

PATTON, J. & W. REEDER. 1956. New indicator for titration of calcium

with (ethylenedinitrilo) tetraacetate. Anal. Chem. 28:1026.

SCHWARZENBACH, G. 1957. Complexometric Titrations. Interscience Publishers, New York, N.Y.

FURMAN, N.H. 1962. Standard Methods of Chemical Analysis, 6th ed. D.

Van Nostrand Co., Inc., Princeton, N.J.

KATZ, H. & R. NAVONE. 1964. Method for simultaneous determination of

calcium and magnesium. J. Amer. Water Works Assoc. 56:121.

CALCIUM (3500-Ca)/EDTA Titrimetric Method

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