1. General: 

The acidity of water refers to the amount of substance contained in water can accept hydroxyl ions. In the determination of this method, with methyl orange as indicator, use the sodium hydroxide standard solution for titration to the end point of to orange-yellow (PH is about 4.2). The value measured only includes stronger acid. This acidity is called methyl orange acidity. 

Its reaction is: H++ OH - = H2O  

This method shall be applicable to the measurement of exchange water of  hydrogen ions. 

2. The reagent:

(1) C (NaOH) = 0.05 mol/L (or 0.1 mol/L) sodium hydroxide standard solution. (2) 0.1% methyl orange indicator (weight/volume). 

3. Measuring method:

(1) measure water sample of 100 ml to inject it into the conical flask of 250ml. (2)With the methyl orange as indicator, use NaOH standard solution of 0.05 mol/L (or 0.1 mol/L) for titration until the solution is orange-yellow, record the consumed volume of NaOH standard solution a.  Acidity of water sample (SD) is calculated as the following formula:  

SD = C.a. / V x 103 mol/L.

Where: 

C - Molar concentration of standard solution of sodium hydroxide. 

a - The consumed standard solution of sodium hydroxide for titration of acidity, ml. 

V - Volume of water samples.