pH
Contents:
- re: pH probes: CARE
by Marque Crozman <marquec-at-gastech.com.au> (Mon, 11 Nov 1996)
- pH Probe End-of-Service
by George Booth <booth/hpmtlgb1.lvld.hp.com> (Wed, 21 Jan 1998)
- Aquatic Plants Digest V3 #40
by "David Thomas Gauthier" <gauthie9/pilot.msu.edu> (Thu, 22 Jan 1998)
- A quick comparison of some pH test kits
by rclark/alcor.concordia.ca (Richard Clark) (11 Jan 92)
- Turn your PC into a pH meter
by Tom Petersen <peter334/tc.umn.edu> (Wed, 24 Mar 1999)
- pH meter calibration
by Wright Huntley <huntley1/home.com> (Tue, 20 Apr 1999)
- hanna pH checker
by "Matthew Shaffer" <dfive10/hotmail.com> (Wed, 07 Apr 1999)
- pH Probes
by Sherlock Wong <wong/dt.wdc.com> (Wed, 28 Apr 1999)
- pH Meters
by Ken Simolo <Simolo/chem.chem.rochester.edu> (Fri, 19 Nov 1999)
- Pinpoint pH Monitor
by "Monolith Marine Monsters \(m3\)" <puffie/marine-monsters.com> (Fri, 31 Mar 2000)
- Re:Pinpoint Ph monitor
by "Bill Curtin" <wcurtin/mediaone.net> (Fri, 31 Mar 2000)
- Happy New Year
by Ken Simolo <Simolo/chem.chem.rochester.edu> (Tue, 2 Jan 2001)
- pH Meters and Electrodes (long, as usual)
by George Booth <booth/lvld.agilent.com> (Wed, 16 Aug 2000)
- ISFET Ph pens and controllers
by Wright Huntley <huntley1/home.com> (Thu, 17 Aug 2000)
- pH Monitor
by Wright Huntley <huntley1/home.com> (Mon, 22 Jan 2001)
- pH monitor
by "Chuck Gadd" <cgadd/cfxc.com> (Tue, 23 Jan 2001)
by Marque Crozman <marquec-at-gastech.com.au>
Date: Mon, 11 Nov 1996
>>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)
by George Booth <booth/hpmtlgb1.lvld.hp.com>
Date: Wed, 21 Jan 1998
I have recently received a few questions regarding the lifetime of pH probes
and
how you can tell when the end is near. This is important if you use CO2
controllers because, in our experience, a faulty probe returns (higher or
lower?) voltages than it should, causing the controller to display a higher pH
than it should. Thus your controller may read 7.0 when the actual pH may be
6.0!
And the controller will be happy to inject enough CO2 to keep the pH at what it
thinks is 7.0; enough to create CO2 concentrations about 100 ppm. Enough to
severely kill your fish!
So, how do you tell if your probe is pooping out?
We check pH with a LaMotte narrow range pH test kit at water changes. We trust
the test kit much more than the pH controller and probe. When you notice the
two pH values diverging by more than 0.1, it's time to clean and calibrate the
probe.
The calibration routine will use two standard solutions -- pH 7.0 and either pH
4.0 or 10.0. Never stick the probe in the bottle of standard solutions - pour a
little of each into small containers and dispose of it when you are done! Note
that when you are calibrating, even a bad probe will probably be able to read
the right pH almost instantly. The cal solutions are "high ionic strength" --
lots of ions cause the measuring junction to react quickly.
However, when you put the newly calibrated probe into the low ionic strength
aquarium water, you may notice that it slowly drifts to the correct reading.
This slow drifting is a sign the probe should be replaced soon. It's hard to
say exactly what "slow" would be but if it takes more than 30 seconds to return
the right pH, we think it's time to replace it.
BTW, we like the Broadley-James "Silver" probes sold by Pet Warehouse for
around
$35. We have used more expensive probes but these seem to work the best in low
ionic strength freshwater. If you are keeping marine plants, saltwater is high
ionic strength and probe selection is much less critical.
One may be tempted to tweak the controller to match the pH test kit reading.
This is OK as a short term correction but you will not be getting any
indications of how bad the probe may be getting. There is evidence that a probe
at the end of its useful life can, in a matter of hours, go from being merely
off a little bit to completely off. In a CO2 controller situation, this means
the controller will inject beaucoup CO2 to maintain what it thinks is the right
pH.
As always, when using CO2, it pays to be careful and diligent.
George
by "David Thomas Gauthier" <gauthie9/pilot.msu.edu>
Date: Thu, 22 Jan 1998
> Date: Wed, 21 Jan 1998 15:46:59 -0700 (MST)
> From: George Booth <booth-at-hpmtlgb1.lvld.hp.com>
> Subject: pH Probe End-of-Service
Just an addendum to George's post on pH probe death and how to detect it. If
your pH monitor happens to have a "mV" or millivolts setting, the quality of
your probe is easy to check. A pH probe is basically a sensitive voltometer
and converts the reading in mV that it gets to pH, depending on the calibration
that the user sets based on temperature and standardization to solutions of
known pH. A solution of neutral pH should have a mV reading of 0.0. So, if
your probe monitor has an mV display option, check it out in a neutral
buffered standard solution. In general, pH probes in a neutral solution should
read between -30 mV and +30 mV if they are clean and accurate. If the probe
does not read in this range, there are ways to clean it most of the time.
Many probes have refill ports for the internal reference solution (usually KCl
saturated with AgCl). First, try emptying this solution from your probe and
refilling. Repeat a few times if there are a lot of crystals in the body of
the probe (these are KCl and are harmless, but are an indication that your
internal solution is old and needs changing). If this does not do the trick,
you probably have a clogged up reference port (the small pore-like thing on
the side of the probe). There are a number of ways to clean this. I prefer a
30 min. soak in 0.1M EDTA to remove inorganics. Ammonia (white) is supposed to
work, but I have not tried it. 0.1N HCl also works
well for both chemical deposits and bacterial goo, and if you have some access
to pepsin, 1/4 tsp (sorry for the non-metric)/100 ml 0.1N HCl will get rid of
protein deposits. If you have access
to a laboratory, a hot solution of Ammonium hydroxide is the real industrial
strength way to go about cleaning the probe (email me if you want the
specifics). DONT use this on plastic bodied probes, though. Other things to
try, but NOT on plastic probes, are acetone and methanol (both should be pure,
no fingernail polish remover or shoe polish <G>). These do a good job on lipid
based deposits. If you do
use any of these methods to clean your reference port, make sure to empty the
internal reference solution and refill afterwards. You should also
pressure fill the body of the probe by firmly placing a squeeze bottle of
KCl/AgCl internal solution firmly against the fill port and squeezing until
a bead of the stuff appears on the reference port. Now, I mostly
have experience with laboratory grade probes and am assuming that all probes,
including PetWarehouse specials, have all of the components that I described,
including a fill port and a reference port. If this is not the case, sorry,
but maybe we can figure something else out. Bottom line is, if you only need
accuracy to within .1 or so pH points (I haven't found the need for more), you
can keep cleaning and reusing probes for a fairly long time. Sorry for the
long post, and please contact me if I can clear up any vagueness.
Oh, and I just remembered. Sometimes a one hour stir in warm (60C) pH 4.0
standard solution works wonders for mildly crudded up probes.
Regards
Dave Gauthier
by rclark/alcor.concordia.ca (Richard Clark)
Date: 11 Jan 92
Newsgroup: rec.aquaria
A quick comparison of
Aquarium Pharmaceuticals Master Deluxe pH test kit
Kordon hi pH test kit
Lamotte pH test kit (6.0-7.4)
Tetra freshwater pH test kit
I don't pretend to be comprehensive here, but I thought this info
might be of some use. I did not compare the kits to a known standard,
which would have been nice if available, I just compared them against
each other. I believe these partial results are better than knowing
nothing about the kits. I am basically just comparing their ease of
use, stating the colours that are used (colour blind people might find
that useful - I'm not), and how easy it is to judge the pH of the
sample when compared to some colour standard.
Disclaimer : I am not a chemist. I am not associated either directly
or indirectly with any of the companies mentioned. Any opinions
expressed are exclusively my own.
Impressions :
Disregarding costs, the order I liked the kits was :
Best - Lamotte
Kordon
Aquarium Pharmaceuticals
Worst - Tetra
My impression is that one kit did not stand head and shoulders above the
next lower rated one, I think there is a steady progression from the
worst to the best. If I had to rate them on a scale of ten,
arbitrarily assigning the "best" kit a rating of 10, they
would be 10, 8, 6, 4. (you'll note that only the tetra kit received a
failing grade).
I took a sample of the water from my aquarium for each test. My
mother and I each gave an estimate of which number most closely
corresponded to the colour observed. The samples should give the same
reading unless the pH of my 80 litre tank varied within 15 minutes
(lights off for 2 hours).
Note : The Aquarium Pharmaceuticals kit is a bit old. I've had it for
7 months, and I think the store from which I purchased it had it for a
while too. This puts the AP kit at a disadvantage over the other kits
which were new (from TFP).
---------------------------------------------------------------------
---------------------------------------------------------------------
Aquarium Pharmaceuticals Master Deluxe pH test kit
Procedure :
5 ml of water in a test tube, 3 drops of a liquid reagent, shake
briefly, compare to a colour reference printed on a card. Range is
6.0 - 7.6, colour standards are 0.2 pH units apart. Colours range from
yellow - green - blue
Results :
pH read was between 7.2 and 7.4.
Remarks :
I frequently have trouble determining the exact colour for this kit.
The colours standards are 0.2 pH units apart, but I am seldom
confident of my reading because it's rather hard to accurately compare
the colours.
The master deluxe kit comes with pH UP and pH DOWN solutions to alter
the aquarium pH.
-------------------------------------------------------
Kordon hi pH test kit
Procedure :
A small, hard plastic package containing a powder must be cut with
scissors and placed into a 5 ml sample of water. The colour
comparitor is integrated with the sample holder, encased in the same
plastic as the sample. I found the colour comparison VERY easy,
HOWEVER it should be noted that the different colours standards are in
0.5 pH increments. Range is 6.5-8.5. The colours range from yellow -
orange - red.
Results :
colour indicated pH was just a bit above 7.0, well below 7.5,
guesstimate is 7.1-7.2.
Remarks :
Although the colour samples are widely spread and don't offer the same
resolution as some of the other kits, it was nice to look at the
sample and say with confidence and within 1 second - "it matches that
one". There was no difference in the intensity of the colour between
the colour standard and the water sample, and the colours were quite
"deep".
-----------------------------------------------------
Lamotte pH test kit (6.0-7.4)
Procedure :
8 drops of liquid reagent (bromthymol blue) are placed into a 5 ml
sample of water. The sample test tube is placed into a little device
which simply holds 8 differently coloured standards, each also in a
test tube, with a light diffuser behind them. The colour samples are
in 0.2 pH increments. Range is 6.0 - 7.4. Colours range from yellow -
blue.
Results :
pH read was somewhere between 7.0 and 7.2, guesstimate 7.1
Remarks :
The colour comparison wasn't as easy as with the Kordon kit because
the colours aren't as deep, but as long as the comparator was held up
to a light source (not sun) it didn't take more than a couple of
seconds to compare the colours.
-----------------------------------------------------
Tetra freshwater pH test kit (from the laborette kit)
Procedure :
A handy little syringe is provided to help fill the test tube to the
5ml mark. 7 drops of liquid reagent are added to the sample, which is
compared to a colour reference card. Colour comparisons are in 0.5 pH
increments, range is 5.0 - 10.0. Colours range from yellow - green -
blue - purple.
Results :
pH was somewhere between 7.0 and 7.5. This is the only test my mother
and I disagreed on. I thought the colour was closer to 7.0, she
thought it was closer to 7.5. This results in a disagreement of about
0.3 pH units between us.
Remarks :
the colour comparison for this kit was a bit difficult because the
sample colour is much lighter than the colour standards on the card.
This kit's big advantage is range. Graph paper is provided to
chart the pH (and other tests included in the kit).
--
---------
rick clark (rclark-at-alcor.concordia.ca)
(CIS 70272,3270)
by Tom Petersen <peter334/tc.umn.edu>
Date: Wed, 24 Mar 1999
I have tried the pH Turtle which is a similar instrument from Hanna
instruments (see www.hannainst.com) and am not completely satisfied with
it. It seems they are very affected by all the 'minor' stray voltages
that typically are in the aquarium--powerheads, filters, etc. I was able
to get good readings from a ten gallon, but not from a 125 gallon tank. I
am re-attempting the 125 measurements with a stray voltage probe in the
tank, but I have to relocate my PC--so until then, it sits. My goal is to
setup a program that emails me when my pH gets out of range ;-)
Tom Petersen
peter334@tc.umn.edu
by Wright Huntley <huntley1/home.com>
Date: Tue, 20 Apr 1999
> Can you use distilled water to calibrate a pH meter at 7?
>
> Thanks
> Sherlock Wong
Sorry, Sherlock.
Not a chance. Distilled water does not have a pH of 7 (atmospheric CO2
drives it lower) and it has no buffering. [Measuring distilled, DI or
even RO water pH is generally not worth the bother, as results can vary
wildly, BTW.]
You need a solution that has been accurately buffered to pH=7, or the
tiniest contamination from your probe will make the pH read almost
anything.
Frequent calibration checks *are* a vital process, particularly with the
inexpensive probes we use, and our aquatic environment. Buy buffered
solutions, tho.
Wright
- --
Wright Huntley, Fremont CA, USA, 510 494-8679 huntleyone at home dot
com
Stop passing new laws!
Repeal some dysfunctional ones. It will do far more good.*
http://www.self-gov.org/libertarianism.html
________________________________________________
*The 9000+ pages in the IRS code is a neat place to start trimming.
by "Matthew Shaffer" <dfive10/hotmail.com>
Date: Wed, 07 Apr 1999
>Date: Tue, 06 Apr 1999 14:06:13 -0700
>From: Sherlock Wong <wong@dt.wdc.com>
>Subject: PH Meter
>
>Does anybody have any user comments on the
>Checker ph pen? It is around $50 from That Fish
>Place, and it is $35 direct from the manufacturer,
>
>http://www.hannainst.com/products/prodline/tester.htm#pH
>
>They also have a pHep3 pen for $50, which has alot more
>features.
>
>Thanks
>Sherlock Wong
>
Sherlock,
I bought this pH pen from Hanna about two years ago. It works fine,
and I got about a year of good service out of the probe. I do
recomend that you build a "soaker bottle" to store the probe tip in so
that it stays wet ALL THE TIME. I ruined mine by letting it dry out.
Hanna provides a little "boot" that you are supposed to fill with
storage solution and place over the probe tip. This is suficient if
you use the probe every day and replace the storage solution every
day, but if you only test weekly or so, the solution will dry up and
leave the probe dry (which damages it). When my probe quit working
properly (from being dried out), it began to drift slowly and no
longer gave a decisive reading. It also failed to callibrate at the
same spot repeatedly (ie, it would read 7.0 in the 7 buffer, then
after checking the tank pH it would read 6.4 in the same buffer).
Hope it helps,
Matthew Shaffer
NCSU Raleigh
_______________________________________________________________
Get Free Email and Do More On The Web. Visit http://www.msn.com
by Sherlock Wong <wong/dt.wdc.com>
Date: Wed, 28 Apr 1999
Sources of pH probes:
http://www.hannainst.com/
http://www.markson.com/
You can request a Markson catalog, and they give
you a $25 coupon good off a $50 purchase.
They have all sorts of lab supply goodies.
Good Luck
Sherlock Wong
by Ken Simolo <Simolo/chem.chem.rochester.edu>
Date: Fri, 19 Nov 1999
To: apisto/majordomo.pobox.com
Tim, If you are going to spend that kind of money ($125 - $150), I
would recommend the silicon chip sensor pH meters. They can be stored
dry and the electrodes are good for 10,000 - 20,000 readings. Most
of the standard gel pH electrodes (the kind found in most of the
smallhand held pH meters) last for 6 months to 1 year and then you
have to replace the whole meter. The best price/performance that I
have seen are at www.phmeters.com - I just ordered one that I will be
testing for use in our chemistry labs. They came highly recommended
by a salesman who only carries standard hand held pH meters so he was
actually losing business by recommending them! The extra $25 to get
the model 125 (if I remember correctly) is worth it because you get 3
point calibration capability among other things.
I have had good luck with the "inexpensive" ($60) hand held
conductivity meters as well as a new $25 tds meter I got for $25 on
ebay.
Ken
>I have been looking for a really good ph/ec/tds meter. Hanna Instruments
>has two for 150.00 each. Has anyone had any luck with these multi
>meters, or any suggestions on brand names to look for, or are they a
>waste of time. I have heard the Pinpoint PH meter is pretty good. What
>do yall think?
>
>Tim
>
>
>
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by "Monolith Marine Monsters \(m3\)" <puffie/marine-monsters.com>
Date: Fri, 31 Mar 2000
>Date: Thu, 30 Mar 2000 19:03:35 -0500
>From: Andrew Lester <atlester@home.com>
>Subject: Pinpoint Ph Monitor
>
>Recently as part of my CO2 injection system I purchased a Pinpoint Ph
>monitor. The first one I purchased would wildly put out nonsensical
>readings. After contacting the manufacture Iwas told that "the glass in
>the probe had probably shattered and to return it for a new one". The
>second one arrived today and after going through a lengthy calibration
>process my monitor was calibrated. Now I have the monitor
>hooked up to the
When you calibrate, forget about using pH 10.01 fluid, it is rarely stable /
accurate and expires prematurely quick. Use 4.01 and 7.00. You HAVE TO
ground the sump or aquarium where the electrode is being submerged in. If
your electrode is in the aquarium but you ground your sump, that is still
NOT good. Both the pH electrode and the grounding probe have to be in the
same compartment. It is much easier to manufacturer a good monitor than to
make a good electrode. When worse comes to worse, return the probe to the
manufacturer, keep the monitor and get a good probe that is stable and long
lasting. Many of the skinny probes are for intermittent monitoring. You
should look for one that is thick and is engineered for continuous
monitoring. They cost more but definitely save you more money in the long
run. Some DI water should be kept handy to rinse the probe after
calibration.
There is an article written by Adam Whitlock which is posted onto our site.
Please read that.
http://www.marine-monsters.com/front/products/electrodevs.html
Edward
Monolith Marine Monsters (m3)
>aquarium testing the CO2 injection system and after five hours
>it can not
>lock onto one or even two readings. For example it "hunts"
>between 7.07
>to 7.10 to 7.09 to 7.12. It even does this with none CO2 injected
>water. Not the kind of accuracy I expected or need.
>
>Does anyone else on the list use this product? Can you offer
>some insight
>on what might be going on? Is this "normal"?
>______________________________________________
> From Andrew Lester atlester@home.com
>
by "Bill Curtin" <wcurtin/mediaone.net>
Date: Fri, 31 Mar 2000
I have a Pinpoint Ph monitor and also have Perfecto SHO lights. I found out
that you cannot take a reading with the lights on. There is some type of
interference from the ballast in the SHO light that causes the monitor
readings to jump around. When I shut the SHO lights off the reading
stabilizes. That might be your problem. AS far as your monitor varying .03+
or - that is not bad for a $100 monitor.
> From: Andrew Lester <atlester@home.com>
> Subject: Pinpoint Ph Monitor
>
> Recently as part of my CO2 injection system I purchased a Pinpoint Ph
> monitor. The first one I purchased would wildly put out nonsensical
> readings. After contacting the manufacture Iwas told that "the glass in
> the probe had probably shattered and to return it for a new one". The
> second one arrived today and after going through a lengthy calibration
> process my monitor was calibrated. Now I have the monitor hooked up to
the
> aquarium testing the CO2 injection system and after five hours it can not
> lock onto one or even two readings. For example it "hunts" between 7.07
> to 7.10 to 7.09 to 7.12. It even does this with none CO2 injected
> water. Not the kind of accuracy I expected or need.
.
by Ken Simolo <Simolo/chem.chem.rochester.edu>
Date: Tue, 2 Jan 2001
To: apisto/listbox.com
>
>I`m looking for a good PH meter
>
>Thanks
>
>George
I would recommend the silicon chip sensor pH meters. They can be
stored dry and the electrodes are good for 10,000 - 20,000 readings.
Most of the standard gel pH electrodes (the kind found in most of the
small hand-held pH meters) last for 6 months to 1 year and then you
have to replace the whole meter. The best price/performance that I
have seen are at www.phmeters.com - I have now used one of their's
for 18 months with no problems. They came highly recommended by a
salesman who only carries standard hand held pH meters so he was
actually losing business by recommending them! The electrode is
stored dry which is a very nice feature. For measurements, all you
need is 1 - 2 drops of liquid. The extra $25 to get the model 125
(if I remember correctly) is worth it because you get 3 point
calibration capability among other things. During the last 18
months, my meter has drifted by only 0.1 pH units. The only negative
is their cost but they pay for themselves in just 10 - 15 months as
compared to gel electrodes.
Ken
by George Booth <booth/lvld.agilent.com>
Date: Wed, 16 Aug 2000
We've just replaced some aging pH electrodes in our three pH controllers and had
the usual annoying experience. Has anybody else done this or is it just us?
It is "common knowledge" that pH electrodes drift over time and eventually go
bad. It is therefore prudent to periodically calibrate the electrodes and
replace them every year or two years, depending on how paranoid you are.
Calibrating an old or new electrode involves using two strong buffer solutions,
pH 7.0 and either pH 4.0 or pH 10.0. The probe is alternately placed in each
solution (thoroughly rinsing in between) and the meter's "standard" and "slope"
adjustments are tweaked until the meter reads 7.0 and 4.0/10.0. There is some
interaction between the adjustments, so you go back and forth until they are
stable.
The instructions that came with our pH controllers suggested that the probes be
calibrated weekly. "Calibrating weekly" lasted about a month. They didn't seem
to drift that fast and it was a pain. We now check the water with a good pH test
kit every other week and recalibrate when needed. Even then, the recalibration
might be simply adjusting the meter to read what the test kit indicates the pH
should be. Why have we gotten so sloppy?
We got three new probes last week, one for each tank. We buy Broadley-James
"Silver" electrodes from Pet Warehouse ($34, www.petwhse.com). We have tried
more expensive electrodes but have found these to be about the best for
relatively soft water. This must be the first time in history that "cheapest"
was "best".
We carefully calibrated all three electrodes and put them in the tanks. All
three read properly compared to a narrow range pH test kit. The next day we
rechecked pH and all three were reading from 0.2 to 0.3 too high (what the meter
read as 7.0 was actually 6.8 or 6.7, thus we were injecting more CO2 than we
wanted). We groaned, "Here we go again".
We recalibrated the probe in one tank. It read 7.2 in the 7.0 cal solution (as
expected). We adjusted the "standard" to 7.0 and completed the cal procedure.
When we put the probe in the tank (still pH 6.8), the controller now read "6.3".
Arrrggh! We tried soaking the electrode in storage solution ("to rehydrate it").
It still read 7.0 in the cal solution and still read 6.3 in the tank. We
monitored it for two hours and it slowly drifted up to the correct reading. It
still reads correctly 2 days later. We did NOT recal the other two electrodes;
we simply changed the controller "set point" to make sure we are injecting the
right amount of CO2. We will deal with those later.
We seem to go through this same experience every time we recal the electrodes or
get new ones. Thus we tend to not recal very much.
We discussed this with Broadley-James some time ago. It appears the problem is
the "ionic strength" of the solution being tested. Most pH electrodes are used
to check buffers for proper pH. Buffers (and cal solutions) are strong ionic
strength solutions. Soft freshwater is a weak ionic strength solution.
Electrodes do not do well in weak ionic strength solutions. Bummer.
This is one reason we don't trust the pH pens. Electrodes are fussy things.
Labs keep electrodes in a storage solution between uses and calibrate before
every use. I suspect pH pens don't get this loving care. They might work well
out of the box but how about long term? How long do the electrodes last under
typical conditions? Are they checked periodically to see if they are still
accurate? How quickly do they respond to soft water?
Ain't high tech great?
Comments? Other horror stories?
George Booth in Ft. Collins, Colorado (booth@frii.com)
http://www.frii.com/~booth/AquaticConcepts
by Wright Huntley <huntley1/home.com>
Date: Thu, 17 Aug 2000
>
> Date: Thu, 17 Aug 2000 08:26:34 -0600 (MDT)
> From: George Booth <booth@lvld.agilent.com>
> Subject: Re: ISFET Ph pens and controllers
snip...
>
> > How many ways of measuring Ph electronically are there anyway?
>
> I'm clueless. "6"?
Naah. There's really only one, AFAIK.
Professor Arnold O. Beckman, of Cal Tech, invented it in the mid 30s and
registered it as
"Apparatus for Testing Acidity," U.S. Patent No. 2,058,761.
Basically it was based on the fact that dissimilar metals in an electrolytic
solution will always act like a battery. The voltage, positive or negative
can be linearly related to the log of the Hydrogen ion concentration -- pH.
Good instruments are based on selecting extremely stable conductors that
take acid and alkali well, and amplifying and scaling their voltages to a
usable meter. Thermocouple effects, etc. must also be eliminated in the
design, of course.
The various types of probes are different just to make them more stable, to
read quicker, or be more resistant to corrosive damage. [Temperature
compensation,etc., are bells and whistles we can usually ignore, as George
has indicated.]
When George had trouble with the really soft snow run-off in CO, it was
because the few ions present failed to give enough current to accurately
read the voltage (FETs -- Field-Effect Transistors -- are a huge help,
there, BTW). Higher tds water is easier to read and probably gives better
repeatability. If he had some really pure NaCl to add to the water, I bet it
would have given better (more repeatable) readings.
The metal electrodes seemingly need to be imbedded in a wettable matrix,
like a gel or porous glass. IDK why. That slows readings down by varying
amounts. That factor is where the problems usually arise, also. I've never
needed to replace a probe because of a trick I learned so long ago I can't
even cite the source.
I use three solutions to calibrate. They are buffered to pH 4, 7, and 10.
When calibration seems to get squirrely, or readings stabilize too slowly,
it usually is a buildup of contamination in the porous material of the
probe. That's easy in my less-than-pristine tanks.
I just let the probe soak for up to a few hours in the pH=4 solution. The
acid seems to clear out most of the contamination and restore activity to
near normal. You could probably use vinegar or dilute pool acid to get the
same results. You don't want to etch away the conductors, so do it only
until the desired stable operation is reached.
It works fine for me, but don't complain if you have a different brand and
the probe is destroyed. OK?
Wright
- --
Wright Huntley, Fremont CA, USA, 510 494-8679 huntleyone at home dot com
There are two rules for success in life:
Rule 1: Don't tell people everything you know.
*** http://www.self-gov.org/index.html ***
by Wright Huntley <huntley1/home.com>
Date: Mon, 22 Jan 2001
> Date: Mon, 22 Jan 2001 00:39:14 -0700
> From: "Chuck Gadd" <cgadd@cfxc.com>
> Subject: pH Monitor
>
> Ok, just purchased a used pH monitor. I have no idea how old the probe is, so I
> won't be surprised or dissapointed if I need to replace it. I'm going to get
> some calibration fluids this week. Right now, I've got the probe in my
> sump tank, and the meter reading seems to be jumping thru a range of values
> from 6.6 to 6.8. I know that the values aren't really usable until I
> calibrate, but is the jumping values a sign that the probe is toast? I
> did notice that today when doing a water change, the probe reading climbed, as
> I'd expect since I add new water into the sump. And by tonight, the values
> were back down to the 6.6-6.8 range, so it seems to be functioning somewhat.
>
> What calibration fluids do I need? I see that they sell a 4, 7, and 10.
I have all three. I soak a dirty probe in the 4 to let the acid wash out the
worst of the crud/scale. I use 4 and seven to set the gain and midpoint.
Then it's really nice to see how linear it is by checking at 10. Most are
not exactly on, so you can tweak the two settings to get the best average,
or to get the best accuracy in the range you normally use (just below 7?). I
think anyone can get by with buffers for 4 and 7. 10 is gilding the lily for
an extra fractional point.
> Also,
> do I need to be concerned about adding grounding probe? I seem to recall someone
> mentioning that stray voltage from pumps and/or light ballasts could effect the
> reading.
I have no controllers, but I believe it is true. The pH probe is just a
battery, and you are asking for precision measurement of tiny voltages. It
needs no help to be way off.
>
> And if I do need to replace the probe, any recommendations on where to find
> the best price?
>
Sorry, I've never had to replace one!
Wright
- --
Wright Huntley, Fremont CA, USA, 510 494-8679 huntleyone at home dot com
Quit bashing Microsoft. They do very good things. They
hire the handicapped -- for example, utter morons
to compose all error messages.
*** http://www.libertarian.org/ ***
by "Chuck Gadd" <cgadd/cfxc.com>
Date: Tue, 23 Jan 2001
Just a quick follow-up: I'm still waiting for my calibration solutions
to arrive, but concerning the jumping values I was seeing, I received a
private reply form Andrew Bolling concerning a method to "rejuvenate"
an old pH probe.
As I said, my probe seemed to be generally tracking the pH changes in my
sump when I did a water change, but the reading values would just over
a .3 pH range constantly. 6.61, 6.73, 6.69, 6.83, 6.75, 6,91 etc, etc,
etc. Well, I followed Andrews advice, which was to soak the probe in an
acid, then to soak it in saturated KCl solution. After doing that, I
dropped the probe into my sump. The pH slowly climbed until it reached
6.64, and it's sitting there exactly, no swinging around. I moved the
probe to another tank (without CO2 injection), and it settled quickly at
7.44.
So, thanks to all those who helped me out, especially to Andrew who
might have saved me the cost of a new probe. I'll report how it goes when
I receive the calibration solution. And it's not on a pH controller, so it's
no big deal if it fails in the future. So in addition to the regular
calibration, I'll plan on doing the acid and KCL soak every 6 months or so,
and see how long the probe will last.
- --
Chuck Gadd
The Krib
Chemistry
The Krib