Understanding Changes in Tap Water pH Over Time

[caliban07]

Why does tap water ph rise or decline after it has been set aside for a period of time.

I've heard that Co2 or chlorine 'gas off' and the result in loss if CO2 can raise ph?

 

[Borek]

Both are definitely a factor here.

Carbon dioxide can work both ways - depending on the concentration of CO₂ in the air near the water source and and in your house it can either leave the water, or dissolve, changing pH up or down.

 

[caliban07]

Both are definitely a factor here.

Carbon dioxide can work both ways - depending on the concentration of CO₂ in the air near the water source and and in your house it can either leave the water, or dissolve, changing pH up or down.

I suppose the rate in which it changed would also depend on whether there was surface agitation provided by some kind of air flow or water fall which would break the surface enabling better/easier gas transfer?

What part does chlorine play? I've tried to seek explanations online but have been unsuccessful so far.

 

[Borek]

I suppose the rate in which it changed would also depend on whether there was surface agitation provided by some kind of air flow or water fall which would break the surface enabling better/easier gas transfer?

Definitely.

What part does chlorine play? I've tried to seek explanations online but have been unsuccessful so far.

It should slightly acidify the solution, as it reacts with water producing two acid - HCl and HOCl. But chlorine concentrations in tap water are orders of magnitude lower than concentrations of carbon dioxide, so in most applications their effect on pH can be ignored.

 

[caliban07]

What about the temperature of the water? Would that have an effect on the way the gas is transferred ie does warmer water have a better gas transfer at the surface that would change ph faster or change the ph all together?

I'm only talking warm in the region of 26-30 degrees C

 

[Borek]

There are two different things here.

One, is what is happening at the equilibrium, the other is how fast we get to that equilibrium. Both depend on the temperature.

Note, that is much easier to calculate equilibrium (final) pH, that to predict pH changes in time (transport phenomena can get incredibly complicated even in what looks like a simple case).