# Re: [APD] Interesting idea

```Based on the info from George Booth and Roger Miller and a few others -- but mostly Roger, I prefer these formula, roughly adapted with rounding from Celsius to Fahrenheit:

CO2 ppm = 15.696*KH*10^(6.37-pH) for 70 degrees F
CO2 ppm = 15.696*KH*10^(6.34-pH)  for 78-80 degrees F
CO2 ppm = 15.696*KH*10^(6.333-pH) for 83 degrees F

If you want to do the math yourself, the exponent is the pka-pH (so the pka is 6.37 in the first example).

Basically, the pka varies by temp according to
0     6.580
5     6.520
10     6.470
15     6.420
17     6.410
18     6.400
19     6.390
20     6.380
22     6.370
23     6.365
23.5     6.360
24     6.355
25     6.350
26     6.345
26.6     6.340
27     6.338
28     6.333
29     6.332
30     6.330

where the left column is degress C and the right column is pka.

* * * * * * * * *
The aquatic plant convention is coming in November:

http://www.aquatic-gardeners.org/convention.html

----- Original Message ----
From: Jerry Baker <jerry at bakerweb_biz>
To: aquatic plants digest <aquatic-plants at actwin_com>
Sent: Tuesday, October 17, 2006 6:05:44 PM
Subject: Re: [APD] Interesting idea

Stuart Halliday wrote:
> I believe Thomas Barr wrote this email section below:
>> Scott et al,
>>
>> The good idea from all this is the fact it gets around the KH
>> issue in the tank and variation it can cause.
>>
>> We have folks claiming 50-200 ppm ranges of CO2 based on the
>> pH/KH test readings and the fish are just fine, others have
>> 35-50ppm and dead fish...........we know there's screwy issues
>> with our assumptions about KH.
>>
>> By using a referenced solution for the KH, we eliminate one
>> variable/assumpotion that has been causing issues for sometime,
>> but by using a pH color change to measure, we lose some of the
>> accuracy.
>>
>> ==>>>
>>
>> So this **obviously** leads us to make a referenced KH solution,
>> that we can __insert a pH probe into__ so we can get very
>> accurate deterimination of CO2 ppms.
>>
>> ==>>>
>>
>> That way we have successfully removed the KH interference issues
>> from the tank water and still have saved the accuracy of the pH
>> meter.
>>
>> You simply add the pH meter inside the solution of the drop
>> checker.
>
> So once you know it's in a KH of a known value you could design a ph meter
> that doesn't just display the ph but the CO2 values too?
>
> After all it's just a mathematical formula isn't it? Or does temperature
> come into it?
>
> This would make for an interesting CO2 meter wouldn't it?
>
> Hmm I wonder how long it would take to get a design knocked up?
>
> My company is just about to design a ph meter that measures the ph of the
> flesh of human tissue. Don't ask. ;-)
>
> I'll ask the engineers at work tomorrow.
>
> Anyone got a spreadsheet that calculates the relationship of ph/KH/CO2?

You can use my Web-based calculator which solves for any unknown, but
the formula is CO2 = 3 x [degrees KH] x 10^(7-pH). If you were making a
spreadsheet formula, it would be something like:

=3*A1*(POWER(10,7-B1))

Where A1 was KH, B1 was pH and the formula is in C1.

--
Jerry Baker
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