Why my new RODI give me low ph water?

[tanker]

I just get my new RODI Puratek Deluxe 100GPD. I modified it to add a second stage for carbon filter.

So after 45min the unit was doing water with 00ppm reading TDS.

I test the pH :

• before the RO 7.4
• after the RO 7.4
• the output (after the DI) 6.2

Look like the DI drop the pH to 6.2.

Have you ever had this problem?

The unit work great and normally 00 ppm. But a pH of 6.2

Need help. Thanks.

[Feivel]

I never bothered to take the time notice what the ph was beforehand. I k.ow when i mix a fresh batch of salt i get 8.2 and was suggested to add kalk to my top off to keep the ..... Ahhhh what are those called again it keeps the ph at a steady 8.2 for longer time capacities ..... Buffering capabilities maybee i think it was?

hope this helps?
Cheers

[Feivel]

And it does make sense that the di would or could drop the ph. Its design is to remove impirities that are most likely creating a acidic reaction causing the higher ph. Not sure for sure but just my thoughts. It is possible.

[fischkopp]

How do you test pH? Electronic testers can not be used in RO/DI water; eventually the electrode will be damaged.

Keep in mind that the water has no buffer capacity and can easily swing in either direction.

[JetJumper]

How do you test pH? Electronic testers can not be used in RO/DI water; eventually the electrode will be damaged.

Interesting, what is the reason for the damage when its simply water?


Also, if you have a look at this (http://reefkeeping.com/issues/2008-09/nftt/index.php) its the process to recharge DI.  In the process Acids are used.  So this would most likely explain the PH. (I think?)

[bt]

A pH drop is to be expected:

http://wiki.answers.com/Q/How_the_PH_of_deionized_water_differs_from_ionized_water

And it's not really a problem, either.  Salt mix will bring it right back up.

[fischkopp]

Interesting, what is the reason for the damage when its simply water?

I guess simply dipping it will not damage it. But you shouldn't keep the pH electrode for too long in deionized water.

As you said, it's simply water and doesn't contain anything else but water. The pH electrode contains two chambers with different solutions, each of which containing an electrode. The potential between these electrodes is proportional to your pH readings. One of the chambers is closed, but the other one contains a junction to allow the flow of electrones. It is though this junction that the solutions in the chamber gets diluted, and this happens faster in deionized water as the salts try t reach an equilibrium with the water that contains no salts at all.

That's probably not a very good summery on what's going on. This site has a lot info if you are interested in it:
http://www.ph-meter.info

[Hookup]

If it was just water wouldn't the pH be 7.0?  Isn't that what the pH scale is based upon. Water being neither basic or acidic? 

I vaguely remember something in highschool science on this.

[JetJumper]

Here we go:
I never knew this.  That explains why the PH Probe I had from a long time ago doesn't work anymore.  I also let it dry out which destroys them as well.

A glass electrode alone (i.e., without combined reference electrode) is typically stored immersed in an acidic solution of around pH 3.0. In an emergency, acidified tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water "sucks" ions out of the probe membrane through diffusion, which degrades it.

http://en.wikipedia.org/wiki/PH_meter

[tanker]

Thanks every one for the help.

I was testing my RODI with drops in a tube.

But like you say after adding the salt mixed the ph should come back to the normal level.

But if is for the evaporated water top off.
Is it to much acid?
Can cause the ph in the sump to drop?

[bt]

If you're adding that much water for top-up, you've probably got bigger issues (like a salinity spike).  The pH should adjust pretty quickly.