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Old 09-14-2012, 04:57 PM   #11
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Is there anything wrong with STORING the probe in distilled water... of do you have to use a special solution made up of K ?

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Old 09-14-2012, 05:04 PM   #12
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Originally Posted by HbgBill View Post
Is there anything wrong with STORING the probe in distilled water... of do you have to use a special solution made up of K ?
I give Google credit for this one:

http://www.all-about-ph.com/ph-electrode-storage.htm:
Never store your electrode in water

DO NOT store your pH electrode in distilled or deionised water as this will cause ions to leach out of the glass membrane and reference electrolyte, causing a slow and sluggish response.

Kai
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Old 09-14-2012, 06:02 PM   #13
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Originally Posted by Kaiser View Post
I give Google credit for this one:

http://www.all-about-ph.com/ph-electrode-storage.htm:
Never store your electrode in water

DO NOT store your pH electrode in distilled or deionised water as this will cause ions to leach out of the glass membrane and reference electrolyte, causing a slow and sluggish response.

Kai
I get: no such url at this domain.. altho there is a link on the page to take me to a lot of info.

I did, however, call Thermoworks and the guy spoke with the lady in the lab who said either the buffered storage solution, or distilled water or the pH 7 solution would work. Now I'm more confused..
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Old 09-14-2012, 07:08 PM   #14
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http://www.all-about-ph.com/ph-electrode-storage.html

Personally I wouldn't mess around. The solution is a lot cheaper than the electrode.
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Old 09-14-2012, 08:53 PM   #15
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Wynne got it. The colon at the end of the link messes it up

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Old 09-14-2012, 10:19 PM   #16
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Originally Posted by craigevo View Post
lastly, one thing it wont measure very easily is water. eg sparge water - it goes from 5.5 to 6.0+ over a minute or two, stir the water and it goes down again then creeps up again. I have to take an average and leave it at that. I normally do another reading once I have collected the entire kettle volume and assume my sparge water ph was fine as long as the ph of the kettle wort is not much different to the mash ph eg 5.4 to 5.5. I am not sure if this derivation is reasonable or not.
Erratic readings in water of low ion content is perfectly normal. Though it is extremely feeble, an electric current must flow in order for the pH meter to measure the voltage produced by the electrode. Pure water has very high resistivity i.e. it is actually a good insulator. Thus electric charges can accumulate and dissipate in unpredictable fashion causing the erratic reading you see. Where pH measurements o 'low ionic strength' solutions are to be made one uses special buffers (with not only known pH but known ionic strength) and then 'swamps' the sample with an ISA (ionic strength adjuster) which sets the sample to the same ionic strength as the buffers. pH readings will now be accurate and stable. Now even if the water has a little acid or salt dissolved in it it will be capable of conducting electricity to the extent necessary for stable readings. If your water is not truly of low ionic strength (i.e. distilled water) you should not see these irregularities.

You probably don't want to mess with ISA's but you can easily boost ionic strength by adding a little salt to the samples. This should stabilize your readings. The salt should be the salt of a strong acid and strong base so the salt itself is neutral. Divalent ions give you more bang for your buck in this regard than monovalent ones. I think zinc sulfate is a popular ISA but you need to be sure that a distilled water solution is neutral before using it. You do not want a stable but incorrect reading.

Even in normal ionic strength situations a stable reading depends on an equilibrium between the solution and junction on one side of the circuit and solution and membrane on the other. If you disturb that equilibrium by 'rinsing away' the equilibrium layers you will change the pH reading downward, as you are seeing. This is, imaginatively enough, called 'stirring error'. If you need to make measurements when the solution is being stirred (as in when measuring alkalinity) you should really calibrate with the buffers being stirred in exactly the same way as the sample.
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Old 09-14-2012, 10:20 PM   #17
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Thanks guys.. found it

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Old 09-14-2012, 10:41 PM   #18
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Thanks A.J, I did not know that. To what extend does the probe itself act as a weak acid? You mentioned the formation of an equilibrium layer.

I usually move the probe in the sample and calibration solution to avoid the formation of such an equilibrium layer.

Kai

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Old 09-14-2012, 10:56 PM   #19
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Thanks A.J, I did not know that. To what extend does the probe itself act as a weak acid? You mentioned the formation of an equilibrium layer.
To fairly answer (i.e. give an answer that has a chance of being more or less correct) that I need to consult Galster and he is on the bookshelf back home in Va to which I won't be returning for perhaps 3 weeks. Relying on memory, a dangerous thing, pH membrane glass is doped with lithium and it is exchange between hydrogen in the solution and lithium in the glass that sets up the potential. I don't remember any mention of the probe itself acting as an acid but if anything it would be a base (Bronsted) as it absorbs protons. OTOH once the equilibrium is established it would absorb no more protons so that after the first 'rinse' (move, stir) one wouldn't expect to see further pH change from that effect. Of course it is not only what is going on at the glass that is important. The measured pH also depends on the voltage developed at the reference junction where different, but equally complex and hard to model effects take place. It is a very complicated subject.
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Old 09-15-2012, 12:11 AM   #20
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Originally Posted by Kaiser View Post
This is because your water is weakly buffered. The pH probe actually consumes a tiny amount H+ and thus works like a base which drives up the pH of the liquid close to the membrane if it is weakly buffered. When you move it around the measured pH drops b/c you even out the H+ concentration.

The good thing is that if your water is weakly buffered it can’t have a strong effect on mash or sparge pH and its actual pH also matters very little.

Kai
Quote:
Originally Posted by ajdelange View Post
Erratic readings in water of low ion content is perfectly normal. Though it is extremely feeble, an electric current must flow in order for the pH meter to measure the voltage produced by the electrode. Pure water has very high resistivity i.e. it is actually a good insulator. Thus electric charges can accumulate and dissipate in unpredictable fashion causing the erratic reading you see. Where pH measurements o 'low ionic strength' solutions are to be made one uses special buffers (with not only known pH but known ionic strength) and then 'swamps' the sample with an ISA (ionic strength adjuster) which sets the sample to the same ionic strength as the buffers. pH readings will now be accurate and stable. Now even if the water has a little acid or salt dissolved in it it will be capable of conducting electricity to the extent necessary for stable readings. If your water is not truly of low ionic strength (i.e. distilled water) you should not see these irregularities.

You probably don't want to mess with ISA's but you can easily boost ionic strength by adding a little salt to the samples. This should stabilize your readings. The salt should be the salt of a strong acid and strong base so the salt itself is neutral. Divalent ions give you more bang for your buck in this regard than monovalent ones. I think zinc sulfate is a popular ISA but you need to be sure that a distilled water solution is neutral before using it. You do not want a stable but incorrect reading.

Even in normal ionic strength situations a stable reading depends on an equilibrium between the solution and junction on one side of the circuit and solution and membrane on the other. If you disturb that equilibrium by 'rinsing away' the equilibrium layers you will change the pH reading downward, as you are seeing. This is, imaginatively enough, called 'stirring error'. If you need to make measurements when the solution is being stirred (as in when measuring alkalinity) you should really calibrate with the buffers being stirred in exactly the same way as the sample.
Thanks to both of you.

I have one of those meters, and I have soft water. Trying to measure the pH of my tap water, the meter never achieves a stable reading. OK, replace "never" with "within 30 minutes" to be more accurate.

If I add some salts to the water, it does achieve a stable reading fairly quickly (but I cannot remember a minute value for fairly quickly.)

Perhaps it's not totally useless.

-a.
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