Re:Potassium carbonate and potassium tests

Subject:  Potassium Carbonate

Mark Hack wrote, Feb. 1:

>In an attempt to lower nitrate and let my plants grow I am adding Fe, MgSO4
>and Potassium Carbonate instead of K2SO4 with the following logic which I
>hope is correct.
>1.      My natural CO2 level should be 2-3.5. By carbonate hardening this
>should shove up the CO2 level.
>2.      I need to drop the pH to make the potassium carbonate soluable
>(besides being desirable for the discus)
>3.      Postasium Carbonate needs to be dosed higher than K2SO4.
I don't think that adding any kind of carbonate is a very good way of
adding CO2.  The carbonate will pick up some CO2 and become bicarbonate,
which is a source of CO2 for plants, but so was the CO2 before it was
picked up.  Having some carbonates in the tank increases the capacity of
the water to hold CO2, but there is little benefit in continuing to add
carbonates.   I recommend you add CO2 directly, use calcium carbonate as a
calcium and a carbonate source, and use KCl or K2SO4 to supply potassium.
In biology lab, we demonstrate photosynthesis by putting Elodea in a sodium
bicarbonate solution and shining a bright light on the plant.  We get
photosynthesis, all right, but the pH goes way up, and if the plant is left
in the solution for several hours, it usually is killed.


>Subject: Potassium test
Paul Lesniak wrote, Feb. 1:
>  I had noticed over the past couple of months that it was stated by
>different people that  'we' don't have a way of testing for potassium in
>water. I looked around at our lab at work and I found a couple of
>different methods; one was using an Atomic Absorption unit ( we don;t have
>the right bulb for this test though), the other was using the Hach

>The spectrophotometer test is the Tetraphenylbotrate method and measures
>up to >a concetration of 7ppm. I am wondering  if anyone has heard of this
>type of
>potassium test and if so is it the kind of test that would give the
>information >an aquarist would be looking for. I have not had the
>opportunity to run this >test as we do not test for potassium in our
>process. I can however, get the >necessasry reagents to do the test , but
>it will takea bit of doing.
>      So, before I go to the trouble of getting the reagents, is this test
>likely to give the information we can use or would I be wasting my time??
>                                                        Wondering,
                                                            Paul. Lesniak

This is very interesting.  I had a chemist tell me that only atomic
absorbtion could be used because potassium didn't form any colored
complexes.  I think it would be useful for aquatic plant hobbyists to be
able to test for potassium as long as the test isn't very complex and
doesn't use expensive chemicals.  I, for one, would be interested.  I know
that Orion makes an ion selective electrode for potassium, but it is
expensive and has a working life of only around 6 months.  I wonder why the
vendors who sell environmental water chemistry test kits don't have one for

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In warm, humid Jackson, Mississippi where the plants are beginning to think
that winter is over.