PH Meters

Barry Meyers-Rice (miacoden!
Thu Feb 17 13:40:44 1994

My el-cheapo PH meter has a bimetallic electrode with what appears to be
zinc and aluminum components. I recollect that the ionization potential
for both metals varies with PH and that aluminum dissolves in both acidic
and alkaline environments while zinc dissolves only in acid.

As a result, the electrochemical couple moves electrons one way in an
acid environment and the other way in an alkaline one. Problem is, the
aluminum electrode quickly forms a non-conductive coating which makes the
needle crawl back to neutral PH unless the probe is moved in the soil.
All such meters will be accurate within a specified range of moisture
levels and will read accurately only at a specified time delay from the
time when the probe is inserted.

They are very touchy regarding following the instructions exactly. When
that is done, they agree with the results that I get from indicator
solutions and cost < $10.

Bob Cruder -