PH Reading increasing over time.

[MrJackArra]

Hi all,

 

I have always wondered about this and thought well why not ask the the group! When filling a barrell / nutrient mix, reaching your desired EC then go to regulate your ph using a 'PH Down' Hydrochloric Acid.

 

Once the ph is set the number will slowly continually rise over time and you can go back and regulate again. My question is; why does it take considerably less PH Down to bring the numbers down again that second time round? ie. It may of take 5 mls to bring the initial reading down .5 but the second time you might only need say 1ml.

Anyone thought this? I couldn't find any light reading on this.

Cheers!

[bufo marinus]

Couple of things, firstly, if your using Hydrochloric acid to lower your pH that's just really funny however I'm sure you meant Phosphoric acid.

Secondly, seeing as we are discussing the pH, it's always best practise to drop the pH of the water before adding the nutrient so as not to cause a chemical reaction which can ruin the nutrient. This is trial and error but if you have to add more pH down, or up for that matter, dilute the acid in water first and add to the nutrient mix slowly.

... and the closest thing I have in my arsenal on pH literature regarding your question is that

 

the pH scale is logarithmic, which means that each 1 unit variation in pH represents a 10 fold change in the hydrogen ion concentration. For example, water with a pH of 4.0 is 100 times more acidic than one of pH 6.0.

 

.... make sense?

 

Also, just because I'm a nice guy this is the only Hydroponic reference you will ever need, trust me The Flairform Growers Guide   http://www.flairform.com/hints/pH_control.htm

 

That is all, carry on

[nici]

I believe it's to do with changing the percentage composition of the original solution.....  Think about it this way you have a glass half full with water and you add 1/3 of the volume with dishwashing liquid, then add another 1/3 dishwashing liquid of the new TOTAL volume of the glass.  Your new % (and concentration) of dishwashing liquid is much higher than if you had of originally added 2/3 dishwashing liquid to the glass of water.

 

EDIT: OR you could listen to bufo (I would!!!!)

Edited May 5, 2013 by nici

[MrJackArra]

Ha yes, thanks for pointing that out I should of said Phosphoric. Thank you both bufo & nici, that information has helped me better understnad the working behind it which makes me happy and I'm glad I asked.

 

As for that link.... Holy Mumma, thanks for sharing I really appreciate the mixing advice.