RO produce very low pH water. The pH is even lower after the DI

[barrowkwan]

What happen to my RO unit?

tap water pH : 7.8
pH after RO ( no DI ) : 4.9
pH after RO + DI : 1.5

is it a problem with my RO's membrane ?

[johnnathan]

it is brand new unit ?

[barrowkwan]

it is a 1.5 years old unit.

[Ronald]

As far as I know you cannot measure the pH of Pure R/O.
I had the same expierience and found this information :

"pH of RO water (was APD V3 #749)
by George Slusarczuk <yurko/warwick.net> 
Date: Sun, 03 Jan 1999

You are absolutely right! Even among chemists it is a very little known
fact, that the pH of distilled (and RO) water can not be measured. Let
me correct that: It CAN be measured, but the value is meaningless! (That
does NOT mean that distilled water has "no pH" -- just that we can not
mesure it using methods available to a hobbyist -- and that includes
"research grade" electronic pH meters!)

I know of only one commonly available reference (the ASTM volume on
water) where this is spelled out in plain English (if only in a
footnote). I will not go into the technical reasons for it.

The pH of RO water probably will not be the same as that of starting
water, because the exclusion of ions by the RO membrane is not uniform
accross the periodic table and the residual ionic equilibrium will be
different from that in the source water. But, as you mentioned, the RO
water is not buffered, so it really does not matter: anything added to
it -- fish, plants, water, etc -- will change its pH.

Best,

George

[Ronald]

And part 2:
"by George Slusarczuk <yurko/warwick.net> 
Date: Mon, 04 Jan 1999
Hello Joseph,

You are, obviously, right that the dissociation constant of pure water
is 10^-14. But do you think that a *hobbyist* can determine such a
dissociation constant in the fish-room? I don't think so!

As I mentioned in my original post, The ASTM volume on water (I think
that it is volume 18 A & B, but am not certain) in the section on
measuring pH, gives that warning about measuring pH of pure water. Right
now I don't have access to that work, so can not cite the exact page
reference. The classic reference is Clark's "Determination of Hydrogen
Ions", from the 1920's.

If we take a medium-grade distilled water, resistivity of, say, 1 megohm
or better, then in a colorimetric determination one drop of an indicator
solution will be the dominant source of hydrogen ions. You will get a
measurement, but it will be that of the resulting diluted indicator
solution, not of the original water.

An electronic pH meter is just a high impedance Volt meter. If the
resistance of the medium is too high, and one is using a *glass*
electrode, (not quinhydrone/sat. calomel half-cells) -- the reading will
drift -- because the instrument can not reliably measure the 59
mV/decade that it is required to do, to measure pH. This is easily
provable, if one has access to *fresh* (without dissolved CO2) 10 megohm
(or better) water then the drift is very obvious. (The quinhydrone/sat.
calomel half-cells are not something that a hobbyist would want to try
either.)

I know of one way to circumvent this problem but, again, that is not
something a hobbyist should try in his/her fishroom (because of danger
of contamination and subsequent false measurements): Per 25 mL of sample
add 1 drop of a 10 mM KCl solution of 99.9999% (6N) purity, prepared in
an acid-washed Teflon volumetric flask, using 10 megohm (or better)
water. It will lower the resistivity sufficiently, that a "research
grade" (very high impedance) pH meter, equipped with a quality glass
electrode, will be able to measure it with some assurance of accuracy
and precision.

Hope this explains it somewhat.

Best,

George"

[Ronald]

After using various methods of reconstitution R/O the results of each method when repeated were the same regarding pH.
You can Target a pH with the reconstitution but you will have to find the right balance for your specific needs e.g. Breeding, Holding, Planted and so on.
To see if the R/O unit is producing the way it should one needs to use a TDS (most common) or Conductivity reader.
But that you already know I guess.

HTH
Regards,
Ronald

[TURQ64]

What happen to my RO unit?

tap water pH : 7.8
pH after RO ( no DI ) : 4.9
pH after RO + DI : 1.5

is it a problem with my RO's membrane ?

What was the basis for your numbers? new or old ph chemical reagents? A meter?....Never seen those kind of numbers in decades of RO use....Your membrane may be old and needing replacement due to storage, but it's not the culprit here..more info please..Gary

[barrowkwan]

thanks everyone for the information. I think the problem is with the digital pH meter. I did the pH test with the API test kit and the RO ( without DI ) show pH around 6.8-7. With DI, it dropped to between 6-6.2. Then I used the digital pH meter and test the RO ( without DI ) water with some tape water added ( about 1 : 50 ratio ), the pH was around 6. The digital pH meter just not good for water test without enough ion IMO.

again thanks everyone.