Steps of complexometric titration to detect calcium ions in water

  Calcium ion is a common inorganic salt in water, and it and magnesium ion are also important components of water hardness. For example, the white precipitate formed when water is boiled is actually caused by calcium and magnesium ions in the water. Therefore, for many industries, the specific parameters of calcium ions in water, such as water in the boiler industry and textile dyeing industry. Today we will introduce to you how to detect calcium ions in water by complexometric titration.
Detect water quality calcium ion

Principle of calcium ion detection in water

  In fact, the principle of rapid detection of calcium ions in water quality is very simple. We use calcein-phenolphthalein as an indicator, and use EDTA to titrate the calcium ions in the water sample when the pH of the water sample is ≥12. The color of the end point changes from yellow-green to yellow-green under a black background. It is red, and magnesium forms a magnesium hydroxide precipitate under this pH condition.

Reagents and instruments used for testing

1. Micro burette, 10mL acid type
2.20% potassium hydroxide solution
3. Triethanolamine solution (1+2).
4. Hydrochloric acid solution (1+1).
5. Calcein-phenolphthalein mixed indicator. Weigh 0.2g of calcein and 0.07g of phenolphthalein in a glass mortar, add 20g of potassium chloride to grind it evenly, and store it in a ground bottle (or use the indicator to test paper) .
6. Weigh 1.000g of reference calcium carbonate (CaCO3) dried at 150℃ for 2h, dissolve it in 10mL hydrochloric acid solution (1+1), heat to boiling, cool and transfer to a 1L volumetric flask. Dilute to the mark with secondary reagent water and shake well.
7.0.01mol/L EDTA standard solution
a. Preparation Weigh 4.0g of disodium ethylenediaminetetraacetic acid, dissolve it in 200mL of secondary reagent water, dilute to 1L with secondary reagent water, and store in a plastic bottle.
b. Calibration accurately draw 25mL 0.01mol/L calcium standard solution into a 250mL Erlenmeyer flask, add 75mL secondary reagent water and 5mL 20% potassium hydroxide solution, and then add about 30mg calcein phenolphthalein mixed indicator (or 1 piece of test paper) ), titrate with EDTA standard solution on a black background until the yellow-green fluorescence of the solution disappears. When the solution is red, it is the end point of the titration. At the same time, make the blank test correction result.
Water sample with calcium yellow green reagent

Detection operation steps

  Accurately draw 50mL of the water sample filtered by medium-speed qualitative filter paper into a 250mL Erlenmeyer flask, add 3 drops of hydrochloric acid solution (1+1), mix well, heat it to boil for 0.5min, add 50mL secondary reagent water, add 50mL20 % Potassium hydroxide solution, then add about 30mg calcein phenolphthalein mixed indicator (or 1 piece of test paper), titrate with EDTA standard solution on a black background until the yellow-green fluorescence of the solution disappears, and the titration end point is when the solution is red. At the same time, make a blank test. Then get the calcium ion content in the water according to the corresponding parameters.