# dKH in drop checker



## corina savin (Jul 11, 2012)

Why do we need a particular dKH in the drop checker solution? 2 or 4 dKH, why can't we use zero KH of the RODI?
Is is dictated by the bromothymol's ability to change colors at higher pH numbers? Or is it something else?


NOTE: I understand why it has to be known, just not why "zero" is not an option.


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## Mikaila31 (Dec 18, 2008)

its due to buffering capacity since co2 is acidic in water. It is 4dKH will cause a color change at approx. 30 ppm. Lower kh and it will change color at a lower ppm. Straight RO will go yellow very easily, possibly without any CO2 addition.
_Posted via Mobile Device_


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## corina savin (Jul 11, 2012)

Yes, thanks. It is because of the pH indicator we use. That means that if I have a different indicator (nothing comes to mind), one that changes colour at much lower pH value, I can use RODI.
And you are right about RO and bromothymol...Long ago, I purchased RO water (I have a system now). I was not sure if I paid for the right stuff. I checked GH and KH and both showed zero. I then checked pH to see if its 7. Fresh from the bottle, it was almost....Went to look for the color chart, it was not 7 for sure. Looked again, definitely not 7. Wait few minutes....and it became yellow while I was thinking why....That experience was worth a thousand words.....


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## Mikaila31 (Dec 18, 2008)

Even with a different indicator RO/DI is so unstable you wouldn't want to use it without some ability to buffer. 

And yeah RO/DI may come out at about neutral, but again no buffers. As soon as it is exposed to air it will start taking up CO2 in small amounts and that will make it acidic quickly. You can boil it in glass and let it cool covered to drive the CO2 back off of it.


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