Activity help

David Hershey dh321 at excite.com
Tue Aug 31 01:12:02 EST 1999


Other pH activities include the following:

Grow plants in solution culture ahead of time and then place them into
nutrient solutions with either all nitrate or all ammonium as the
nitrogen source and measure the pH change either colorimetrically or
with a pH meter. Start with a nutrient solution pH of about 5. I have
never tried to see if you can see a difference within 50 minutes but if
the plants are nitrogen starved beforehand, you may be able to
especially if you kept the solution volume small and the solution
unbuffered. Researchers have also measured pH changes along root
surfaces by placing them on agar plates containing pH indicators and
inorganic salts. Have students try to figure out why the roots change
the external pH differently when given ammonium or nitrate as the
nitrogen source.

Soil pH testing is an easy and quick activity. Students can sample soil
they bring in from home, get from the schoolyard, or even use potting
soils. 

A nice activity with a digital pH meter with a large readout is to
demonstrate liming of soils using sphagnum peat moss, which starts out
about pH 4. Tell students a little about the importance of liming
(raising soil pH) in plant agriculture and that liming materials usually
contain calcium. Next adding powdered gypsum (CaS04.2H20) to the peat
moss explaining that it is what is in wallboard. Let the students tell
you that the pH is dropping not rising. Explain about cation exchange
capacity and that the calcium ions in gypsum displace H ions from
exchange sites on the peat moss and create sulfuric acid in the soil
solution. Next, use powdered calcium carbonate (what blackboard chalk is
made of) and show them that the peat moss pH will rise because the
carbonate combines with H ions to make carbon dioxide and water. That is
easily demonstrated by dropping acid on calcium carbonate and watching
the bubbles of carbon dioxide.


David Hershey
dh321 at excite.com


 


Janice M. Glime wrote:
> 
> One fun activity is to use red cabbage juice as an indicator.  I may have
> read it first on this discussion list.  It has various shades of color
> from pink to green to blue as one subjects it to pH from acid to alkaline.
> One could use it like litmus paper to test various buffers and household
> items.  It really takes on unusual colors when the household item adds its
> own color, but then it cannot be used to get a "litmus" reading.
> Grind the cabbage and make a juice extract in water.  Amounts aren't
> critical, but not much water is needed.  A set of buffers makes a nice
> comparison set.
> One could then examine a variety of flowers in pink, red, and blue and
> subject them to acid and base to see if their colors change, indicating
> that they are anthocyanin pigments.  These pigments are easily extracted
> by grinding the petals in water.  Geraniums work well, and whatever else
> is available to you.
> Alternatively, one could see what Sphagnum does to the color of the water
> if it starts out in a buffer.  (I haven't tried that.)
> 
> Janice
> ***********************************
>  Janice M. Glime, Professor
>  Department of Biological Sciences
>  Michigan Technological University
>  Houghton, MI 49931-1295
>  jmglime at mtu.edu
>  906-487-2546
>  FAX 906-487-3167
> ***********************************



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