pH Testing

Daniel and Rebecca wrote:

>In an attempt at greater accuracy we purchased an Oakton PH meter (w\.01
>resolution and temperature compensation), Hach Total Alkalinity, Total
>Hardness, CO2 and PH 6.5 to 8.5 Phenol Red. Now, instead of greater
>accuracy, we have acheived total confusion :-).

I've never used the Oakton pH meter, but have used highly accurate
electrode type meters in the Quantitative Analysis lab at UWEC.  If the
buffer solution is at the same temperature as the water to be tested, and
the electrodes have been stored and cleaned in DI water, and you use
extreme care with them,then the best you can realistically expect from them
is +/- 0.02 pH.  The limits of the Hach phenol red test depend on whether
you use a colorimeter or color wheel.  Using a color wheel and incadescent
light, you can expect reasonable results to be +/- 0.2 pH.

If your meter reads pH 8.3 and the water under test mixed with a drop of
phenophthalein was not pink after being titrated with the Hach sodium
hydroxide solution, then the meter should be recalibrated with 7.0 buffer
and rechecked.  The electrode might be damaged, or contaminated or not
properly stored or the buffer solution is not at the right temp.  They are
extremely fragile and finicky.

When using the Hach CO2 test kit, care should be taken when removing the
water from the tank.  When you remove the water try not to pour it into the
test bottle.  I have used a plastic measurement syringe successfully for
this purpose.  You are measuring carbonic acid.  Your end point is the
first point at which the solution turns a light pink.  I use a reference
solution of DI water in a second test bottle.  I place both bottles on a
white surface, side by side.  The endpoint is obvious when you compare the
two bottles - even though it is light in color.

When using the Hach alkalinity test, make sure the phenophthalein /
bromocresol green - methyl red solution turns fully pink at end point.  The
total hardness does not matter when determining CO2 levels.

If your total alkalinity is between 3 and 4 dkh and your pH is between 6.5
and 6.7, a CO2 measurment of 21 ppm is lower than the chart, but not
unreasonalble.  However, the lack of pink in an 8.3 solution indicates the
meter in not operating correctly.

If you want to calibrate your eye to the phenol red color wheel, try adding
several drops of your buffer 7.0 pH solution to RO water and procede with
the phenol red indicator test.  This will let you see what an exact color
match for 7.0 pH looks like given your backlighting.

I can't comment on the necessary breeding conditions for your chocolate
gouramis, but the accuracy you stated (pH 6.6 to 6.7 and a CO2 level of 20
- 25) is going to be practically impossible to maintain and monitor given
the dynamics of a CO2 injected tank with water changes, etc...  Since your
plants are doing well, and your fish are not showing signs of stress, you
have obviously succeeded at achieving a good balance in your tank.

Augustine Rodriguez
Elk Mound, WI, USA