Re: Humic Acids
No problem. It is just that humic acids are analytically very slippery
because they aren't One Thing. There are more types of humic acid
molecules than can be enumerated. That means that almost all of the
time, we have to work with approximations and correlations.
The response of the test kit that you have is proportional to the product
of the concentration and reactivity of the phenolic -OH groups present in
solution. When they standardize the reagents, it is probably done to
tyrosine or phenol, which are two pure, monodisperse, discrete chemical
entities that can be dried down and weighed on a balance and are always
What you have in the aquarium is a melange of different compounds, some
of which have abundant, reactive -OH groups, and some of which are
perfectly "valid" humic acids that might be almost or completely devoid
of these groups. The functional groups can be hindered and hence react
more slowly in some compounds than others.
It is all rather maddening to think about.
Whatever correlations that you develop for your system will be very
interesting. What will be even more interesting is to see if they are
valid across systems. For example, we have a large piece of driftwood in
the lab planted tank. It is loaded with lignins, and they are very
reactive in your test method. Another tank may not have driftwood,
and may be relatively lignin poor.
What may be important for plant growth might be the chelating ability of
the humic acids, and how efficiently those chelates are absorbed by
plants. That might or might not be related in a simple way to the
concentration of reactive phenol groups present when cross-system
comparisons are made. It might or might not be related in a simple way to
the amount of yellow color in the water for x-system comparisons. God
only knows what they put in commercial blackwater extracts, and how that
relates in chelating power and -OH reactivity to the humic acids formed
naturally in various planted tanks.
I'm a little surprised and disappointed that at least the fundamentals of
the chemistry and the differential reactivity issues were not explained in
some detail in the LaMotte writeup for their test kit.
If you want to learn more about humic acids and how they are detected in
natural waters, I would refer you to the Hach water analysis handbook,
which you can get from Hach, and to Standard Methods, which is published
by the APHA, and should be in most libraries.
But there is no such simple quantity as mg/L humic acids derivable from
your test kit results. It is perfectly valid to report them as "mg/L
X by LaMotte test Y" as long as everyone understands what they really means.