Several recent items on the HistoNet listserver indicate that
not everyone calibrates her or his pH meter as often as it
ought to be done. It takes about 3 minutes to check against
standard buffers at pH 4, 7 and 10 and make slight adjustments
if necessary. I was brung up to do this every time I went to use
the meter. It's what I still do, and sometimes it's necessary
to make an adjustment in the course of a morning or afternoon.
I can't believe that a pH meter calibrated only once a month
could be accurate to anything better than a single pH unit,
and that's not good enough for many histo-jobs.

pH papers are another matter. These don't need to be
calibrated and with 4 or 5 types of indicator paper you
can cover the range from 3 to 10 with a precision claiming
0.1, which might really mean 0.2 - good enough for most
of us. The difficulty is that often wishy-washy pallid shades
have to be matched, and it's often impossible to decide
what the pH is because the strip has a slightly diffferent
cast of colour from anything on the chart. Also, paper
is no good for strongly coloured liquids. 

There are a few solutions, notably those containing gold,
that must be tested with paper because the metal ions can
permanently damage the glass electrode of a pH meter.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1