Determination of Hardness of Water for Painting

Hardness (H) indicates the total concentration of high-valent metal atoms in water. In general natural water, there are mainly calcium ions and magnesium ions, and there are few other high-priced metal ions, so the sum of calcium ions and magnesium ions in water is usually called hardness. They are bicarbonates, carbonates, chlorides, etc. of calcium and magnesium, and need to be softened when the water hardness is high. There are two types of hardness:

Temporary hardness: refers to the part of hardness that can be precipitated after long-term boiling of water. The chemical reaction formula is as follows:

Ca(HC03)2-CaC03↓tenH20tenC021

M9(HC03)2-M9C03↓10H2010C021

MgC03+H20→Mg(0H)2↓

Long-term hardness: refers to the hardness that cannot be removed when water is boiled, called long-term hardness (refers to calcium, magnesium chloride and sulfate).

The unit of hardness: usually expressed by the milligram equivalent of cac03 contained in water. The method of expressing the hardness of water varies from country to country and will not be detailed here.

(1) Determination steps of total hardness

(1) Take a 50mL sample and pour it into an Erlenmeyer flask, add 1mL of buffer solution (NH40H and NH4cl 1:1 mixed), use chrome black T (EBT) as indicator, and use 0.01mol/L disodium edetate Salt (EDTA) solution was fixed, and the end point was changed from reddish purple to cyan.

(2) In order to remove other harmful high particles, 3~5% KcN solution with a mass fraction of 10% can be added.

The total hardness (H) calculation formula is

(2) Long-term hardness measurement steps

(1) Take 50mL of sample and pour it into Erlenmeyer flask, boil for 30min.

(2) Cool and filter after boiling.

(3) Dilute the filtrate to 50 mL with water that removes CO ₂ .

(4) Use the same method to measure the total hardness.

The formula for calculating the permanent hardness (H) is

(3) Calculation of temporary hardness

total hardness_permanent hardness=temporary hardness