Standization of NaOH (aq)

alkali, especially string base like sodium hydroxide and potassium, can corrode glass slowly, and is faster when concentrated. silicate glass mainly contain silicon dioxidem which reacts with alkali, forming sodium silicate:
SiO2 + 2NaOH ------> Na2SiO3 + H2O

this reaction etches the inner wall of pipette, changing the volume of liquid that pipette contains, thus affect the accuracy of transferring fixed volume of liquid.
moreover, after drying with sodium silicate reamained, the sodium silicate is difficult to be redissolved. this blocks the opening of the pipette, making it unusable.

therefore, pipette used to transfer alkali is rinsed with weak acid like succinic acid, to remove the hydroxide and form soluble salts:
2NaOH + HOOC-CH2CH2-COOH ------> 2H2O + NaOOC-CH2CH2-COONa

after that, water is used to rinse for removing any acid / salt remained.


2010-09-23 01:25:04 補充：
not necessarily washed with succinic acid, but can be other common acids like hydrochloric acid.