re: pH probes: CARE

>>Michael Eckardt <mike at odg_com> wrote:

>This procedure agrees with my (non-Sandpoint) instructions, except 
>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 
>>slope?  This seems to me to make more sense, if one is targeting a pH 
>>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 
>>accurately with a 4.01 solution, or whether they are specificly designed 
>>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 
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 

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 
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 

*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 
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 
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 

Ensure that the connectors are clean and free of debrie and grime, cleaning 
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 
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. 
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 

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 
should last us quite a long time. The main problem I have seen is leaching: 
can be solved with correct selection of probe: clogging of the membrane, 
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 
cannot get in contact with it).

Hope that this is of help,

Marque   APD - ANGFA(NSW)

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