|Equivalent Weight||40.00 (Molar = Normal)|
Sodium Hydroxide is a strong base in terms of chemical ionization and solutions of it can be assayed using a strong acid, such as Hydrochloric Acid or Sulfuric Acid, or by using an acidimetric standard such as Potassium Hydrogen Phthalate (KHP).
Typical neutralization reactions are:
NaOH + HCl → NaCl + H2O
NaOH + KHPhthalate → NaKPhthalate + H2O
The endpoint can be determined potentiometrically or by using one of a number of pH indicators with a color change anywhere in the pH 3 - 10.5 range. The selection of the indicator used depends on the initial concentration of the Sodium Hydroxide and the strength of the acid used. This is due to the fact that the Normalities of the acid and the base affect the shape of the neutralization curve that results during the titration.
When both acid and base are 1 Normal in concentration, indicators with effective ranges between pH 3 and 10.5 can be used. Suitable indicators include Methyl Orange, Bromocresol Green, Methyl Red, Methyl Purple, Bromothymol Blue, Phenol Red, Phenolphthalein and Thymolphthalein.
When both the acid and base are 0.1 Normal, the ideal pH range is reduced to 4.5 to 9.5. Thus, Methyl Orange and Thymolphthalein are not as suitable as the others.
With 0.01 Normal solutions, the ideal pH range is further limited to 5.5 - 8.5.
Sodium Hydroxide solutions tend to absorb Carbon Dioxide and other acidic gases from the air and care should be exercised to minimize this occurrence.