pH changing overnight

Hi All,

Quick question about pH. I have a digital pH meter which I calibrated twice and tested in calibration solutions just to make sure it is working properly. My tap water runs about 7.4 straight out of the tap. I pH about 3 gallons of my tap water to 6.8 by adding the pH down. I have an airstone in there to keep things oxygenated. Then the weird part is I let it sit overnight and when I check the pH in the morning, its back up to 7.4. Any ideas why this is happening?

Thanks!

water temp can affect ph.

the second time I must agree with insan3....
from yahoo answers: (though I should remember this from the time I worked in a biology lab... but we always used RO/DI water and some times distilled..)

"Yes, temperature does affect the pH of solutions.

pH is -log[H(+)] and so anything that affects the concentration of hydrogen ions in the solution will affect the pH.

The easiest example to look at is pure water which has a pH of 7 at 25 Celsius, a pH of about 6.6 at 50 Celsius and a pH of about 7.5 at 0 Celsius.

This is because the dissociation of pure water, H2O <=> H(+) + OH(-) is an endothermic process so heating it up increases the dissociation and increases the hydrogen ion concentration. At 50 C the conc. of H(+) is about 2.34 x 10(-7) compared to 1 x 10(-7) at 25 C and about 0.33 x 10(-7) at 0 C."

it is probably certain elements or buffers being off gassed. after it sits for a day and you adjust it does it stay?

superstoner1 said:

it is probably certain elements or buffers being off gassed. after it sits for a day and you adjust it does it stay?

Click to expand...

That sounds right. I was also thinking the chlorine gassing off, but chlorine has a ph of over 11. that is a fluctuation of .6 overnight. Is your res. clean? I know when mine gets gunky the ph starts to dip.

This is just in a brand new plastic bucket so I have dechlored water on hand in case I need it. I do have an airstone in it bubbling. Come to think of it, I did soak the airstone in a mild bleach solution prior to placing it in the bucket.. but I rinsed it thoroughly and its been a couple of days.

I tested it again last night w/o the airstone. Water was 7.4 so I pH'ed to 6.8. Let it sit overnight, and now its back to 7.1. No airstone, etc. Temperature might have changed by a degree or something but not much. Really weirding me out.. cause I keep adding acid so I want to make sure the pH isnt below 6.8 actually and burn my seedlings.

hummm that has me puzzled. A degree or two will not change the ph that dramatically. Make sure you are using a quality ph pen/stick, and a buffered up/down solution. BTW never use anything that could possibly leave a residue behind like bleach. Ya know, chlorine/ bleach has a high ph. That may be your prob. That plastic bucket may be a culprit as well. Also are you putting nutes in your res when you test, or just water?