[c3062079]

sodium hydroxide is utilised is acid/base titrations.

care must be taken when weighiing out NaOH as exposure to the atmosphere prior to weighing, and during weighing, may result in the solution having a lesser concentration than it is supposed to.

could someone explain this please.

 

[Alberto_Kravina]

It attracts water from the air, it may also react with CO₂.

[c3062079]

Cheers.

Is it also true that it is the water vapor in the air that lowers the pH of thymolphthaleiin solution when brushed onto a piece of cloth with a paintbrush and exposed to the atmosphere.

 

[Alberto_Kravina]

Could be, maybe air humidity, however, I think that the carbon dioxide (which is present in the air) could also cause the change of pH.

[jdurg]

For NaOH, the problems are that the solid is very hygroscopic and will pick up water readily, and the solution will react with CO2 in the air forming sodium bicarbonate.  Either way, you want to keep all solid and aqueous NaOH tightly capped.

[pantone159]

Is it also true that it is the water vapor in the air that lowers the pH of thymolphthaleiin solution when brushed onto a piece of cloth with a paintbrush and exposed to the atmosphere.

I think it is CO2 here.