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 
the same.  

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.