water chem/ co2

[mooman]

Water chem question. I had a customer at the LFS I work at tell me that his water params were gh=6 kh=5 and ph=6.0 (adjusted using peat and blackwater extract). I on the other hand cannot seem to lower my ph below 7.0 despite the fact that my kh is nearly undetectable due to the use of RO water (i've emptied many times the recommended dosage of blackwater into this tank and the ph always bounces back). Whats the deal?

PS the question the customer was asking was in relation to the CO2 content of his tank (if his parameters were correct then his co2 levels should have been off the chart, no?) He does not use co2 injection, but does have some surface aggitation. Does the chart refer to actual co2 levels or just potential co2 levels if surface is not aggitated? Is this just a case of him testing incorrectly? Have I totally confused you all yet?

 

[tjudy]

Hmmm... what 'chart'? Excess CO2 will cause a decrease in pH (CO2 + H2O --> carbonic acid --> slow release of hydrogen ion), but a low pH is not necessarily an indicator of a high CO2. A pH of 6.0 using the products he lists does not surpise me. No clue why your pH is staying up.

 

[AquaAlbatros]

I on the other hand cannot seem to lower my ph below 7.0 despite the fact that my kh is nearly undetectable due to the use of RO water (i've emptied many times the recommended dosage of blackwater into this tank and the ph always bounces back). Whats the deal?

That's weird. How is your mix of water ?, How much percent of RO Water to Normal water ?. Does it take long to get to pH 7 ?, wich blackwater mix are you dosing ?.

Does the chart refer to actual co2 levels or just potential co2 levels if surface is not aggitated? Is this just a case of him testing incorrectly? Have I totally confused you all yet?

The chart relates to actual levels of CO2, not potential. If the surface is aggitated the pH will go up, any aggitation only sends co2 out of the tank. Try measuring after and before of aggitation (let a reasonable time pass, like 2 hs), the pH will go up BUT it depends how are you measuring. With color charts it's too dificult to note a .1 change in the pH, use an electronic meter.
Best regards, Juan.-

 

[AquaAlbatros]

WOW, just checked the co2 ph-kh table.
It's weird, with KH 5 and pH 6 the co2 concentration is 147 ppm ....too high !
How precise is your customer pH measurement ?
Just look at this extract from the table post in the APD FAQ's

-----------------------------------------------------------------------
\ pH | 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 8.0
KH\ |
-----------------------------------------------------------------------
4.0 | 118 75 47 30 19 12 6 5 1.2
5.0 | 147 93 59 37 23 15 9 6 1.5
-----------------------------------------------------------------------
| CO2 milligrams/liter
-----------------------------------------------------------------------

A difference of .2 degrees of pH makes a lot !, to determine CO2 concentration with this indirect measurement really needs precise pH measurement. Forget about color charts !

 

[Mike Wise]

I on the other hand cannot seem to lower my ph below 7.0 despite the fact that my kh is nearly undetectable due to the use of RO water (i've emptied many times the recommended dosage of blackwater into this tank and the ph always bounces back). Whats the deal?

Could your substrate or some decoration be releasing CO3 into your water?

 

[kingborris]

i believe the pH / KH / CO2 tables assume that KH is the only buffer in the system, and that CO2 is the only acid. if the pH has been lowered by peat, or blackwater extract etc, then this throws out the charts, and hence how the charts say the CO2 value is > 100ppm but the fish are still alive.

 

[AquaAlbatros]

I'm not a chemical engineer, but i know that this method is pretty useless without very precise pH measurements, a .1 error changes the numbers a 25 %, if my memory doesn't fail, roger miller in the APD list explained it. Buffers doesn't change the measurement.
Here is the original post of Roger Miller :

The method is not generally accurate. It assumes that there's a straight line relationship between CO2 content and pH; there isn't. Things might not be too bad if there is only a small pH difference between the high pH you get on the aerated water and the low pH you get from the breathed on water. If the difference is more than a pH unit or so, then the error can
get pretty big. I think that for common conditions this method could
easily overestimate CO2 levels by a factor of two. And that doesn't even
consider the possible problems that come up if the CO2 content of the
aerated water isn't 0.6 ppm or the CO2 content of the breathed on water
isn't 60 ppm.

If you do have the case where there is a small difference between the two
extremes (implying a large buffer capacity), then you will need to use a
pH meter to get numbers that are accurate enough to use.

If you're willing to depend on the pH and CO2 content of aerated water,
then you might try something else. Take a sample of your tank water and
measure its pH (call it pH(t)), then aerate the sample so it's pH rises
and remains constant. At that pH (call it pH(a)) it should contain about
0.5 ppm CO2 - in equilibrium with air. Now put those values into this
equation:

log(CO2) = pH(a) - pH(t) - 0.3

This is based on the assumption that the bicarbonate content of the water
doesn't change significantly when the sample is aerated. It would be
possible to make a simple graph of this formula comparable to the
pH/KH/CO2 charts.

I tried this in one of my tanks but it didn't work because the pH of the
aerated sample rose above the range of my test kit. So I made up this
example.

A sample of the tank water has a pH of 7.5. That's pH(t). After the
sample was aerated its pH rose to 8.9. That's pH(a). Putting those
values into the equation above, I get:

log(CO2) = 8.9 - 0.3 - 7.5 = 1.1

using a calculator, CO2 is 10^(1.1), or about 13 ppm.

The biggest problem with using this equation is that your pH numbers need
to be precise; if any pH measurement is off by 0.1 unit up or down, the
resulting CO2 estimate is off by 20% to 25%. The estimate that the aerated
water will contain 0.5 ppm CO2 is also a source of problems. I don't
think there's a problem with my assumption that the bicarbonate content
doesn't change significantly when the sample is aerated, but heck, maybe
there is.

You can't use this method to split hairs, but you should be able to use it
in the presence of other buffers.


Roger Miller

Best regards, Juan.-

 

[fishgeek]

the chart you are talking about is to be used to estimate how much CO2 is being added to a particular system by measuring pH , it is just an estimate

it does not mean that your pH will tell you your CO2 level always

as others have said the pH is a factor of free hydrogen ions and free hydroxly ions

the difference between your waters is that less hydrogen ions bind to something else in his system than in yours , what that is i do not know

i have straight tap water that it hard approx 10kH and 15 Gh, pH 8.0-8.5 from tap
even after running it through r/o it will still measure (with a hannah meter- recently calibrated) as alkaline and nothing dissolved(all new membranes and di recently)

when i asked about things like this oxygen content and a few other things where suggested but in general we didn't find out what it was
irrespective it is easy to manipulate as most of the buffer has been removed by filtering

as and aside does anyone know of r/o membrnes pre filters that can cope with chloramine?

andrew

 

[kingborris]

as and aside does anyone know of r/o membrnes pre filters that can cope with chloramine?

andrew

ther has been a massive amount of debate on this on a discus forum i am a member of, many members said that yes, standard carbons used in RO prefilters are sufficient to remove chloraimines from tap wter. Others said that they will handle chlorine, but for chloramine you need a much larger surface area / interaction time. Speciallist carbon filters that are used in kidney dialysis are now available from some RO retailers for this very purpose. There doesnt seem to be a definative answer that I am aware of.

personally i just use standard granular carbon filters for my RO, and a CBR2 cartridge for any water that doesnt pass through the membrane itself. ive had no issues with chlorine or chloramine that i am aware of with this setup

 

[AquaAlbatros]

Chloramines can be removed from water with very low flow rates (5 to 10 minutes contact time) through shell-base activated carbon, followed by mineral zeolite media for residual ammonia adsorption.
I don't think a normal prefilter will help. Industrial installations to clean biocides from water before RO are really big. Maybe a batch system to treat water with some chloramine removal agent before the RO device, or a dosing system of the same agent to the RO water tank, if you can live with the residual ammonia.
Read this link from GE, it would be helpful
http://www.gewater.com/library/tp/813_Chloramines_.jsp
Best regards,
Juan.-

 

[mooman]

Thanks for all the replies. I knew I could count on the forum for expert advice.