introduction

calcium

hydrogen phosphate

chloride

ammonium

nitrite

hydrogen carbonate

total water hardness

pH

conductivity and TDS

1. A SPECTROMETRIC DETERMINATION OF
CALCIUM IN A WATER SAMPLE

1.1 Objective

Use the calibration-line method to determine the concentration of calcium ions in a water sample. Express the results in mmol/L and mg/L.

1.2 Background

Calcium ions form a complex with o-cresolphtalein complexon. The solution of the complex is violet.

1.3 Equipment and reagents*

• A Spektra^TM spectrometer.
• A colour-forming reagent for determining the concentration of calcium.
• A standard solution of CaCl₂.

*For reagent preparation please contact natasa.gros@fkkt.uni-lj.si.

Hazards

1.4 Procedure

For the blank, all five calibration solutions and the sample, follow the procedure as given below.

6 Mix thoroughly, measure after five minutes using the green LED.

? Write the measurements in the table below and calculate the absorbance.

? Calculation of the result:

Calculate the calibration-line equation and the mass concentration of calcium in the water sample. This can be done easily with MS Excel by drawing a scatter plot. The mass concentration of calcium ions is an independent variable and the absorbance is a dependent variable. After drawing the graph, select the option “Add trend line” and then the option “Show equation in a graph”. Next, insert the sample absorbance in the equation as y, and then calculate x, which represents the mass concentration of calcium ions in the water sample. The final result should also be expressed as an amount concentration. Write both results in the table “Results – Major Components”, under the serial number of the sample.

M_Ca = 40.08 g/mol