Re: [Histonet] pH meter

From:John Kiernan

Dear Glen,

Are you sure it's the meter that's moribund, not the 
electrode? There's not a lot that can go wrong with
an old fashioned pH meter; it's basically just a
simple amplifier connected to a microammeter. The
vacuum tube in the amplifier - it's called an 
electrometer tube - ought to be replaceable, though 
they last for years if switched on all the time. 

The d'Arsonval movement of the meter is very rugged
despite its being able to move the needle in response
to tiny currents. The only way to kill such a meter
is by electrocution - burning out the wire of the
moving coil. That doesn't happen with even the 
amplified output of a pH electrode recording pH 14.

Glass and similar pH electrodes, on the other hand,
are very sensitive to physical and chemical insults.
They can even deteriorate badly from not being used
for a few weeks. Solutions that can deposit thin
layers of a metal are bad news for pH electrodes.
That includes some formulations of silver and gold
compounds. Some cationic dyes stain glass, as we
have all seen. A dyed pH electrode may not be
entirely trustworthy. 

The wick on the side of a pH electrode, above the 
delicate glass bulb, or on the side of the protective 
plastic sleeve, can become encrusted with crystals 
from the solution inside the electrode (KCl with a 
tiny amount of AgCl) or it can be stained by dyes or 
other chemicals into which it gets dipped. 

Contamination is as inevitable as taxation, and it 
changes the way the pH electrode (which is really a 
cell) varies its electrical output in response to the 
hydrogen ion concentration of the surrounding liquid.

If a pH meter is behaving strangely, try to revive
the electrode. (The manufacturer's bumph should have
instructions; it usually means various washes, 
replacing the internal solution, and then soaking for 
a few days in saturated aqueous KCl. MUCH cheaper than
buying a new electrode.) 
If that fails, buy a new electrode.

Modern pH meters use solid-state amplifiers and 
provide "digital" display. If precision to 0.1 pH
units is all you need, a little hand-held meter with
an electrode costs ?$200. A new electrode for any
kind of pH meter costs the same or more. 

Expensive modern pH meters make calibration against 
standard solutions easy. The on-board computer replaces
turning a screw and twiddling a knob, and it can also
take care of some temperature effects. 

Checking pH has always been a horrid chore, because 
standard solutions must be checked every time the meter 
is used. That's the law even with 21st century smart
pH meters, because errors come from the dipstick (pH
electrode), not from the meter.  End of sermon.

John Kiernan
Anatomy, UWO
London, Canada
"Dawson, Glen" wrote:
> All,
> My prehistoric pH meter is in the process of biting the dust so I'm in the
> market for a new one.  I'm interested in a pH meter that is very easy to use
> and affordable.  Please sound off on your favorites histo-land.  Vendors are
> more than welcome to respond.
> Thank-you In Advance,
> Glen Dawson BS, HT & QIHC (ASCP)
> IHC Manager
> Milwaukee, WI
> _______________________________________________
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