# phosphate and CO2

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Folks,

In the recent talk about measuring CO2 in the presence of phosphate at
least two of us said not to worry about phosphate unless phosphate levels
are very high.  That advise apparently didn't take, as the question was
repeated.

The phosphate content of water can be measured as part of the KH
(alkalinity), and thus cause an error when determining CO2 from the
pH-KH-CO2 relationship.  If you know the pH and phosphate content of your
water then you can correct your KH reading for phosphate.  The table below
gives the correction factor.  To use the table, measure you alkalinity
(KH) in degrees, your pH and the phosphate content of you water.  Use the
pH to look up the conversion factor on the table.  Divide the measured
phosphate concentration by the conversion factor.  That gives you the KH
contribution due to the measured phosphate level.  Subtract that KH value
from the measured KH, and use the adjusted KH in the pH-KH-CO2 chart or
table.

pH    conversion
ppm per degree
---    ----------
5.0    341
5.1    312
5.2    286
5.3    262
5.4    240
5.5    221
5.6    203
5.7    187
5.8    172
5.9    159
6.0    147
6.1    136
6.2    127
6.3    118
6.4    110
6.5    103
6.6    96
6.7    90
6.8    85
6.9    80
7.0    75
7.1    72
7.2    68
7.3    65
7.4    62
7.5    59
7.6    57
7.7    55
7.8    53
7.9    51
8.0    49
8.1    48
8.2    46
8.3    45
8.4    44
8.5    43
8.6    42
8.7    41
8.8    41
8.9    40
9.0    39
---    ---------

For example let's say that you have pH=6.8, KH=3 degrees and PO4=3 ppm.
The conversion factor for a pH of 6.8 is 85.  The KH contribution from a
PO4 concentration of 3 ppm/85 or 0.035 degrees.  At this point you ask
yourself if it makes a difference, and the answer is "No".  Going on with
the example anyway, the correct KH is 3-0.035 or 2.965 degrees.  Use
KH=2.965 in the pH-KH-CO2 chart, table or formula.

Obviously, you have to have a *lot* of phosphate in solution to make much
of an impact on the measured alkalinity, and the pH in your tank has to be
pretty high for it to make any difference.

The chances of getting interference from organic acids appears to be far
higher than the chance of a problem arising from phosphate concentrations.

Roger Miller

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