High hardness: 

This method is applicable to the measurement of hardness of water sample of natural water and cooling water.

The measuring scope is 0.1 ~ 5 mmol/ L hardness. When the hardness is more than 5 mmol/L,  the sampling volume can be appropriately reduced ,  it can be measured after it is diluted to 100ml. 

1.Summary of the methods: In water solution with PH value of 10.0 + / - 0.1, use  chrome black T as the indicator, and use the standard solution of ethylenediamine tetraacetic acid disodium salt (EDTA for short) for titration until the end point is blue. According to the consumed EDTA volume, the hardness value can be calculated. 

In order to improve the sensitivity of end-point indication, it is possible to add a certain amount of EDTA in the buffer solution. If the acid chrome blue K as indicator, EDTA may not be added.

2. Reagent:

(1) Standard solution of calcium (1 ml containing 0.01mmol/ Ca + ). 

(2) Ammonia- ammonium chloride buffer solution. 

(3) 0.5% chrome black T indicator. 

(4) 5% sodium hydroxide solution. 

(5) Hydrochloric acid solution. 

(6) Standard solution of EDTA  (1 ml is equivalent to 0.01 mmol hardness).

3.Analyzing procedures: 

(1) Take water sample of 100 ml to inject it into the conical flask of 250ml. If the water sample is turbid, before the samples is taken, it shall be filtered. 

(2) Add 5 ml ammonia-ammonium chloride buffer solution, and add 2 to 3 drops chrome black T indicator. Note: As foe the water sample with higher hardness of carbonate, before it is added to the buffer solution, it shall be diluted or 80% to 90% of the required EDTA standard solution(recorded in the titration volume) shall be firstly added, otherwise after the buffer solution is added, the calcium carbonate is separated out, the end point will be longer. 

(3) Under  constantly shaking,  use EDTA standard solution for titration,  when the end point is approached, the titrating shall be slow, the solution turns blue from wine red, namely the end point.

(4) Then take 100 ml  level Ⅱ reagent water, measure the blank value  according to the operating procedures of 3.2, 3.3.

4.Calculation:  

The water hardness X (tendency/L) is calculated according to the formula: 

 X = (a - b) x T/Vx 1000  

Where:  

a, the consumed EDTA standard solution for titration of water sample, ml.

b - The volume of the consumed EDTA standard solution for titration of the blank solution, ml. 

T -  Titration of EDTA standard solution to calcium hardness degrees, mmol/ ml. 

V - Volume of water sample, ml.

Low hardness : 

This standard is applicable to the measurement of the soften water, water discharged from H -cation exchanger, the boiler feed water and condensate water. 

Measuring range is 1 ~ 100 umol/L water sample hardness. 

1. Summary of the methods:  In the water solution with PH value of PH 10.0 ±0.1 , use acid chrome blue K as indicator, and use ethylenediamine tetraacetic acid disodium salt (EDTA) standard solution for titration until the solution becomes end point of blue. According to the consumed EDTA volume, the hardness value is calculated.

2. Reagent:

(1) Standard solution of calcium (1 ml contains  0.1 umol Ca + ). 

(2) the borax buffer solution. 

(3) 0.5% acid chrome blue K indicator. 

(4) 5% sodium hydroxide solution; 

(5) the hydrochloric acid solution (1 + 4). 

(6) EDTA standard solution (1 ml is equivalent to 0.5 umol hardness). 

3. Measuring methods:

(1) measure water sample of 100 ml to inject it into the conical flask of 250ml. 

Note: When the Acidity or alkalinity of water sample are high,  it is possible to use 5% sodium hydroxide or hydrochloric acid solution (1 + 4)  for neutralizing the water sample and then add the buffer solution  to the water solution.  

(2) Add 1 ml borax buffer solution and add 2 ~ 3 drops 0.5% acid chrome blue K indicator to the water sample. 

(3) Under constantly shaking, use EDTA standard solution for titration, when the solution turns blue (end point) from red. All procedures shall be completed within 5 min, the temperature is not lower than 15 ℃. 

(4) In addition, take100 ml Level II reagent water, measure the blank value according to the operating procedures of 3.2 and 3.3. 

Water hardness X (umol/L) is calculated according to the following formula   X = (a - b) x T/V x1000. 

Where: 

a - the volume of the consumed EDTA standard solution for titration of water sample, ml. 

b - the volume of the consumed EDTA standard solution for titration of blank solution, ml. 

T --,   Titration of EDTA standard solution to calcium hardness, umol/ml. 

V --, Volume of water sample, ml.