introduction

calcium

hydrogen phosphate

chloride

ammonium

nitrite

hydrogen carbonate

total water hardness

pH

conductivity and TDS

7. A TITRIMETRIC DETERMINATION OF THE TOTAL CONCENTRATION OF CALCIUM AND MAGNESIUM IONS IN A WATER SAMPLE – TOTAL WATER HARDNESS

7.1 Objective

Use a complexometric titration to determine the total concentration of calcium and magnesium ions in a water sample. The titration is simplified and miniaturised. Use a syringe instead of a burette, and use a Coulter-counter container as a replacement for an Erlenmeyer flask.

7.2 Background

? Write the reaction of calcium and magnesium ions with a solution of disodium salt of EDTA. For the latter use the simplified expression Na2H2Y.

7.3 Equipment and reagents

  • A device for miniaturized titration.
  • A standard solution of disodium salt of EDTA (N,N'-1,2-Ethanediylbis[N-(carboxymethyl)glycine], disodium salt, dihydrate), concentration 0.1 mol/L.
  • A solution of ammonium buffer, pH 10, concentration 19.8 mol/L.
  • A mixture of Eriochrome black T and NaCl.

Hazards

The ammonia solution should be handled carefully because of the unpleasant smell of the gas, which causes irritation of the eyes and skin.
R: 36/37/38, S 26-36/37/39-45-61


7.4 Procedure

  1. Fill the syringe with the standard solution of EDTA.
  2. Fill the Coulter-counter container with the water sample up to the first mark (V=10 ml).
  3. Add five drops of the pH-10 buffer solution and a spatula of the mixture of Eriochrome black T and NaCl.
  4. Titrate the examined solution carefully until the moment when the final blue coloration of the solution is obtained.
  5. Record the volume of EDTA solution consumed during the titration from the syringe.

Note. &
The procedure can be made easier by first estimating the volume of standard solution that is needed to reach the end-point of the titration. Start with a preliminary titration in which the standard solution of disodium salt of EDTA is added in 0.05-ml increments.

? Calculation of the result:

Calculate the total amount concentration of calcium and magnesium ions in the water sample.

 cCa2++Mg2+ =

 cEDTA . VEDTA

 Vsample

Find the result of the spectrometric determination of calcium in the water sample. Calculate the amount and mass concentration of the magnesium ions in the water sample.

 cMg2+  =  cCa2++Mg2+  -  cCa2+

g Mg2+ = cMg2+ . MMg

MMg = 24.31 g/mol

Write all the results in the table “Results – Major Components”, under the serial number of the sample.