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re: pH probes: CARE
>>Michael Eckardt <mike at odg_com> wrote:
>This procedure agrees with my (non-Sandpoint) instructions, except
perhaps,
>that I am using a 4.01 standard instead of the 10.0.
>>So you calibrate at 7, and then insert it in the 4.01 solution and set
the
>>slope? This seems to me to make more sense, if one is targeting a pH
range
>>below 7 instead of above (I assume setting the slope at 10 would be
>>oriented more towards the reef tank crowd). I wonder if mine could be
set
>>accurately with a 4.01 solution, or whether they are specificly designed
to
>>have the slope set in one direction or another?
No, there is no difference in calibration as to whether the probe is
calibrated in 4 or 10,
both are 3 pH points away from the zero point. The only difference is that
in the case of
10pH, the voltage is negative and 4pH, its positive.
There might be a slight difference in the case of old probes, where there
has been
leaching of the silver chlorlide across the glass membrane, that the
response might
be slower in one direction than the other, but for our use, this is not
normally measurable.
Older probes that have a leaching problem or a blockage problem across the
prorous
glass membrane, when placed into a buffer solution, will most likely not
get to the value,
continually read near 7pH, or only really move in pH when vigourously
stirred.
Basically probes fall into three categories, double junction gel filled,
double junction liquid
filled, and single junction/separate reference cells.
Liquid filled cells are mainly used where access to the cell is easily
reached, the
maintainence on these cells is reasonable, but the liquid needs to be
replaced on a
regular basis, due to leaching.
Gel filled cells fall into two categories, standard and stacked. The
standard ones use
a small gel filled tip that with standard lab use, has a reasonable life
span. Stacked
gel filled probes, contain extra silver/silver chloride tablets that are
stored in the barrel
of the probe that slowly disolve to replace the ions that are leached into
solution. For
aquarium I would highly recommend these types as they are much cheaper than
their
liquid filled counterparts, and last much longer than standard gel probes.
Older probes can also get silted up as far as the wick is concerned-between
the reference junction and the test cell, this is a case for ditching the
probe and starting afresh.
The lifespan of pH electrodes is a question that no-one can definatively
answer. It
depends upon so many variables, but for aquarium use many of these can be
eliminated.
*The probe must be kept in the dark to prevent the growth of algae on the
delicate glass
membrane, as it blocks the pores.
*The probe must be kept on the filtered side of filter, preferably after a
fine screen filter,
or in a place where sediment has already settled out.
*Calibration, and cleaning carried out on a regular basis, cleaning can be
done with
a neutral hair conditioner solution and a lint-free cloth, followed by KOH
or AgCl solution,
or distilled water. By regular I refer to monthly, cleaning may not have to
be done this
regularly, but after getting a feel for how the probe responds with
calibration will dictate
whether it needs cleaning or not.
*Always keep the tip of the probe wet, to prevent crystalisation and
silting of the wick.
On the other hand, remember that the output of the electrode is in the
order of 10 to
the power of 12 Ohms, and that water has a lower resistivity, and will
short the probe out,
so never get the cable of the probe wet. Leaving the cable of the probe in
water, (if it is
a PVC covered cable)(which most of them are) will swell after a time and
slightly absorb
some of the water. This can cause problems with calibration, and could lead
to
erroneous readings ie: readings that read closer to 7 than they actually
are, as an
example, a pH of 5.8 might read as 6.5, or a pH of 8.6 might read 7.7.
Another problem that occurs with pH probes and leads to correct readings
when the
probe is placed in a buffer solution, but incorrect readings when in the
tank, and that is
in the presents of a body of water that has a ground connection somewhere
(and the controller has a ground connection). This may be through a pump,
heater, a steal framed
tank, or any other connection that may inadvertantly cause a grounding of
the water
body. An easy way to test for this condition, is to take a glass of tank
water and place
the probe in it and see if the reading changes.
Battery operated probes that are remote don't usually suffer from this
condition, although
some problems have been seen where there has been a remote display from a
battery
operated controller, and the display cable came into contact with the water
in one place
and caused this problem to raise its head, so don't assume that because
your controller
is battery driven, that this problem cannot occur.
Don't extend the length of the probe's cable, using a BNC adapter and
extension cable,
this is asking for problems, again due to the high resistance of the
output, any
extra connectors in the line are a potential source of problems. Getting a
split in the
cable is best solved by getting the manufacturer to replace the entire
cable.
Ensure that the connectors are clean and free of debrie and grime, cleaning
with
isopropanol. Moving the controller to a place where water cannot fall on it
will also help
to keep problems to a minimum.
Most of these precautions are probably pretty blatent, but most have been
attributed
to common problems encountered.
>Also, I read somewhere that the life of a probe is 6 months.
>What are your experiences?
With good probe upkeep in an aquarium that fulfills the criterea outlined
above, a
stacked gel filled electrode should last between 18 months and 3 years.
Remember
that the lifespan that manufacturers give are for worst case conditions.
The same
probe placed in a surage(?) treatment plant would probably last 2-3 months.
Measuring a solvent that was highly acidic would have the same sort of
effect.
We are using probes in very weak solutions, in very clean water, with low
particulate matter, and low total disolved solids, so our probes (with
maintainence)
should last us quite a long time. The main problem I have seen is leaching:
that
can be solved with correct selection of probe: clogging of the membrane,
solved
with regular cleaning, and cable problems, solved with careful setting up
of the
probe and fixing the cable in position with cable clamps ( making sure that
water
cannot get in contact with it).
Hope that this is of help,
Marque APD - ANGFA(NSW)
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