Author Topic: pH probe calibration  (Read 877 times)

Offline neddles

  • Brewer
  • ****
  • Posts: 280
    • View Profile
pH probe calibration
« on: October 06, 2015, 12:54:12 PM »
Been a while since I calibrated and I was overdue. Went to calibrate and the good news was that calibration was dead on at 4.00 and only .02 off at 7.00. The bad news is that it took 3-4 minutes before it settled in at those values. Is this a sign that my probe is at/near its end of life? (it has been well cared for but it is a little over 2 yrs old.)

Offline Wort-H.O.G.

  • Official Poobah of No Life.
  • *
  • Posts: 4439
  • Play Nice
    • View Profile
    • Harvey's Brewhaus
Re: pH probe calibration
« Reply #1 on: October 06, 2015, 10:23:03 PM »
Been a while since I calibrated and I was overdue. Went to calibrate and the good news was that calibration was dead on at 4.00 and only .02 off at 7.00. The bad news is that it took 3-4 minutes before it settled in at those values. Is this a sign that my probe is at/near its end of life? (it has been well cared for but it is a little over 2 yrs old.)

IME yes, that's an/one indication. you can try cleaning solution and see if that helps.
Ken- Chagrin Falls, OH
CPT, U.S.Army
https://www.facebook.com/pages/Harveys-Brewhaus/405092862905115

http://braukaiser.com/wiki/index.php?title=The_Science_of_Mashing

Serving:        In Process:
Vienna IPA          O'Fest
Dort
Mead                 
Cider                         
Ger'merican Blonde
Amber Ale
Next:
Ger Pils
O'Fest

Offline neddles

  • Brewer
  • ****
  • Posts: 280
    • View Profile
Re: pH probe calibration
« Reply #2 on: October 07, 2015, 12:39:27 PM »
Been a while since I calibrated and I was overdue. Went to calibrate and the good news was that calibration was dead on at 4.00 and only .02 off at 7.00. The bad news is that it took 3-4 minutes before it settled in at those values. Is this a sign that my probe is at/near its end of life? (it has been well cared for but it is a little over 2 yrs old.)

IME yes, that's an/one indication. you can try cleaning solution and see if that helps.
Thanks. I got word that a isopropanol cleaning should help. Im going to give that a try. Unless... you know of a better cleaning solution?

Offline Wort-H.O.G.

  • Official Poobah of No Life.
  • *
  • Posts: 4439
  • Play Nice
    • View Profile
    • Harvey's Brewhaus
Re: pH probe calibration
« Reply #3 on: October 07, 2015, 12:43:33 PM »

Been a while since I calibrated and I was overdue. Went to calibrate and the good news was that calibration was dead on at 4.00 and only .02 off at 7.00. The bad news is that it took 3-4 minutes before it settled in at those values. Is this a sign that my probe is at/near its end of life? (it has been well cared for but it is a little over 2 yrs old.)

IME yes, that's an/one indication. you can try cleaning solution and see if that helps.
Thanks. I got word that a isopropanol cleaning should help. Im going to give that a try. Unless... you know of a better cleaning solution?

I use cleaning solution from scientific- acidic is all I know about it.


Sent from my iPhone using Tapatalk
Ken- Chagrin Falls, OH
CPT, U.S.Army
https://www.facebook.com/pages/Harveys-Brewhaus/405092862905115

http://braukaiser.com/wiki/index.php?title=The_Science_of_Mashing

Serving:        In Process:
Vienna IPA          O'Fest
Dort
Mead                 
Cider                         
Ger'merican Blonde
Amber Ale
Next:
Ger Pils
O'Fest

Offline pfabsits

  • Cellarman
  • **
  • Posts: 41
    • View Profile
    • Hanna Instruments
Re: pH probe calibration
« Reply #4 on: October 07, 2015, 06:45:27 PM »
Instability usually occurs for a couple of reasons. Every combination glass pH electrode has two wires. One that goes to to the tip of the bulb (indicating pH probe) and the other that is inside the electrode body (reference probe). The reference wire is separated from the sample solution through a porous frit known as a junction.

A circuit is formed with a meter from the wire that is inside the indicating pH probe, through the glass bulb, into the solution, then through a junction and then to the internal reference. If there is build up on the glass pH indicating bulb then it will impede the flow of current from the internal wire and the sample solution. The same with the junction. If the porous frit gets clogged then it will insulate the internal reference wire and prevents the circuit from being complete.

Since the meter is reading correctly in the buffer then it is safe to say that the probe is not dirty unless the previous calibration was done with a dirty probe then the build up would automatically be compensated for by the calibration process. BTW when I say dirty I am referring to a build on the glass that can be hard to see with the naked eye. That is organic growth (i.e. algae, etc. ) can be on the bulb. The surface of the bulb would be slightly hazy in appearance. 

Most likely the junction is not clogged as well. Without asking my guess is that any storage solution that was used from the last time dried out. When a pH probe is not stored in storage solution then two things will occur. The first is the layer of water on the glass bulb (known as the hydrated layer) dries up. The hydrated layer is part of the measuring circuit. A probe will read one pH when the probe is dry and a different pH when it is hydrated. It takes about 4 hours for the hydration layer to completely form.

The second issue when a probe is stored dry is the effect on the porous junction. The junction is a barrier from the inside of the probe and the outside solution. This junction is also known as a liquid junction. Basically it is wet and solution/ions migrate from inside the probe through the junction to the outside. If the junction is dry the diffusion of ions is impeded. A dry junction will also show instability in readings.

So what to do. Before your next calibration let the probe sit inside storage solution or buffer for 3-4 hours. This will make sure that the bulb is hydrated and the junction is free flowing. This will help increase the response time.

If you use IPA (Isopropyl Alcohol, not beer) to clean then it will be important to store in solution for 3-4 hours. The IPA will dehydrate the glass bulb. It might be better to use an enzymatic cleaner. Since the wort has a lot of protein an enzymatic cleaner will be the best to break them down. There are protein cleaning solutions for pH probes. Some contact cleaning solutions are enzymatic cleaners but it would be hard to say how effective they would be.  Ideally solutions for pH probes are made for pH probes so they are always the first choice.

If you provide the make and model of the pH meter there can be some additional information that can be provided.

If your meter read within 0.02 pH in the buffer then that is very good. That means there has not been much change to the probe since the last calibration. If the probe was stored dry then I would expect that number to drift over time as the hydration layer forms.

Lastly, compare the pH value on the meter to the value on the buffer packet. A pH meter can be calibrated to a buffer but show a different value after calibration. Some manufactures program the meter to display the solution pH (value on the bottle/packet) that is dependent on temperature. This is not the same as temperature compensation provided by a temperature sensor.  That is for a different phenomenon related to membrane potentials, which is what a pH electrode is based on. 

One of the avid home brewers that work for Hanna Instruments

Offline neddles

  • Brewer
  • ****
  • Posts: 280
    • View Profile
Re: pH probe calibration
« Reply #5 on: October 08, 2015, 01:54:04 AM »
Instability usually occurs for a couple of reasons. Every combination glass pH electrode has two wires. One that goes to to the tip of the bulb (indicating pH probe) and the other that is inside the electrode body (reference probe). The reference wire is separated from the sample solution through a porous frit known as a junction.

A circuit is formed with a meter from the wire that is inside the indicating pH probe, through the glass bulb, into the solution, then through a junction and then to the internal reference. If there is build up on the glass pH indicating bulb then it will impede the flow of current from the internal wire and the sample solution. The same with the junction. If the porous frit gets clogged then it will insulate the internal reference wire and prevents the circuit from being complete.

Since the meter is reading correctly in the buffer then it is safe to say that the probe is not dirty unless the previous calibration was done with a dirty probe then the build up would automatically be compensated for by the calibration process. BTW when I say dirty I am referring to a build on the glass that can be hard to see with the naked eye. That is organic growth (i.e. algae, etc. ) can be on the bulb. The surface of the bulb would be slightly hazy in appearance. 

Most likely the junction is not clogged as well. Without asking my guess is that any storage solution that was used from the last time dried out. When a pH probe is not stored in storage solution then two things will occur. The first is the layer of water on the glass bulb (known as the hydrated layer) dries up. The hydrated layer is part of the measuring circuit. A probe will read one pH when the probe is dry and a different pH when it is hydrated. It takes about 4 hours for the hydration layer to completely form.

The second issue when a probe is stored dry is the effect on the porous junction. The junction is a barrier from the inside of the probe and the outside solution. This junction is also known as a liquid junction. Basically it is wet and solution/ions migrate from inside the probe through the junction to the outside. If the junction is dry the diffusion of ions is impeded. A dry junction will also show instability in readings.

So what to do. Before your next calibration let the probe sit inside storage solution or buffer for 3-4 hours. This will make sure that the bulb is hydrated and the junction is free flowing. This will help increase the response time.

If you use IPA (Isopropyl Alcohol, not beer) to clean then it will be important to store in solution for 3-4 hours. The IPA will dehydrate the glass bulb. It might be better to use an enzymatic cleaner. Since the wort has a lot of protein an enzymatic cleaner will be the best to break them down. There are protein cleaning solutions for pH probes. Some contact cleaning solutions are enzymatic cleaners but it would be hard to say how effective they would be.  Ideally solutions for pH probes are made for pH probes so they are always the first choice.

If you provide the make and model of the pH meter there can be some additional information that can be provided.

If your meter read within 0.02 pH in the buffer then that is very good. That means there has not been much change to the probe since the last calibration. If the probe was stored dry then I would expect that number to drift over time as the hydration layer forms.

Lastly, compare the pH value on the meter to the value on the buffer packet. A pH meter can be calibrated to a buffer but show a different value after calibration. Some manufactures program the meter to display the solution pH (value on the bottle/packet) that is dependent on temperature. This is not the same as temperature compensation provided by a temperature sensor.  That is for a different phenomenon related to membrane potentials, which is what a pH electrode is based on.
Tons of good info here. Thank you for putting all that together. My probe is always stored in the storage solution and it had a sealed cap that prevents it from drying out. I change out the storage solution every few months.  The most abusive thing I have done to the probe is dip it in 68F/20C wort every 2-3 weeks. Where it spends usually 5 and no longer than 10 minutes before a distilled water rinse and is immediately back in the storage solution. I am 100% confident it has never dried out. I will however look into an enzymatic cleaner. Do you have one you recommend?

Offline pfabsits

  • Cellarman
  • **
  • Posts: 41
    • View Profile
    • Hanna Instruments
Re: pH probe calibration
« Reply #6 on: October 08, 2015, 02:05:48 PM »

http://shop.hannainst.com/hi7073l.html

The solution is two component so a pepsin enzyme is added to the bottle to activate. The cleaner will breakdown over time. If fractions of each component are used then it should be possible to use multiple times.

If you do not go the enzymatic route then a general purpose cleaning solution would serve you well.

The 68 oF wort is not too harsh on a probe. It sounds like you are doing everything right. I am still concerned with the drift. If the drift or instability is a change like 5.43 changing to 5.42 then to 5.41 over a period time then I would not get caught up on the 0ne hundredth of a pH unit. There could be drift just from thermo-equilibrium of the internal of the pH meter with the solution. If the pH meter is at the same temp as a solution then this would not pose a temp problem.

Again, if you provide make and model I might be able to provide some useful tricks of the trade.

One of the avid home brewers that work for Hanna Instruments

Offline neddles

  • Brewer
  • ****
  • Posts: 280
    • View Profile
Re: pH probe calibration
« Reply #7 on: October 08, 2015, 04:36:45 PM »

http://shop.hannainst.com/hi7073l.html

The solution is two component so a pepsin enzyme is added to the bottle to activate. The cleaner will breakdown over time. If fractions of each component are used then it should be possible to use multiple times.

If you do not go the enzymatic route then a general purpose cleaning solution would serve you well.

The 68 oF wort is not too harsh on a probe. It sounds like you are doing everything right. I am still concerned with the drift. If the drift or instability is a change like 5.43 changing to 5.42 then to 5.41 over a period time then I would not get caught up on the 0ne hundredth of a pH unit. There could be drift just from thermo-equilibrium of the internal of the pH meter with the solution. If the pH meter is at the same temp as a solution then this would not pose a temp problem.

Again, if you provide make and model I might be able to provide some useful tricks of the trade.
Thank you. It is a Milwaukee 101.

Offline pfabsits

  • Cellarman
  • **
  • Posts: 41
    • View Profile
    • Hanna Instruments
Re: pH probe calibration
« Reply #8 on: October 08, 2015, 06:48:28 PM »
The Milwaukee meter uses a BNC connector for the pH electrode. It is possible to reset the pH 7 (offset) by shorting the connector on the meter with a paper clip. This will basically simulate 0 mV which is the theoretical voltage for pH probe in pH 7 solution.

1) Short pH probe input
2) adjust pH 7 trimmer so that pH 7 is displayed
3) Reconnect the pH electrode to the meter
4) place the probe in fresh pH 7 buffer
5) Read the pH value
6) calculate the offset
7) offset outside +/-30 mV is indicating a build up

How to calculate the offset
1) Based on a theoretical response a pH electrode will generate 59.16 mV per pH unit at 25 oC
2) 59.16 mV = 5.9 mV per 0.1 pH
3) Take pH value from above and determine the deviation

For example: My meter was reset as above and after placing the electrode in pH buffer the meter displays pH 7.25. Since the reading is 0.25 pH on the alkaline side that means that means the voltage being generated is around - 15 mV. A -15 mV offset is a very good offset. A pH probe from a mfg will be +/-10 mV. 

The reading is negative since alkaline solutions have less hydrogen ions (H+) than the pH 7 solution in the glass bulb. If the reading was 6.75 then the offset would be about +15 mV. As the pH decreases less than 7 the amount of hydrogen ions (plus charges) are increasing relative to the inside pH 7. There is a increasing positive voltage with an increase in hydrogen ions.

Note: 59.16 mV per pH is based at 25 oC. the number is different at different temps. But performing this test at room temp will provide a good estimate.

pH meters allow calibration to around 1 pH mV response. Meaning a probe can generate around +/- 60 mV and still be calibrated. As a mfg we actually would not recommend using a probe outside +/-30 mV even though the meters can be calibrated outside that range. So with this test if your meter shows < pH 6.5 or greater than pH 7.5 then you are in an area that something should be done, like cleaning.

One thing to remember that when performing this test that it is assumed the the pH buffer is fresh. If the buffer is not then test should not be performed. It is critical (even for calibration) that pH 7 is pH 7. Buffers do change over time. Once a bottle of buffer is opened then it is safe to say that it is good for around six months as long as no contamination is occurring (i.e. pouring buffer back in the bottle).

Since your meter is manual calibration, I do not think it is possible to check slope. If it had automatic calibration and the ability to go to a default cal then it would be possible to calculate the slope by seeing the pH value in a pH 4 buffer. But since the concern is cleaning then the above will help you see the offset voltage. Typically in pH measurement a build up on the electrode causes an offset error and no change to slope since the build up on a sensor would affect the mV response of both buffers.



One of the avid home brewers that work for Hanna Instruments

Offline Wort-H.O.G.

  • Official Poobah of No Life.
  • *
  • Posts: 4439
  • Play Nice
    • View Profile
    • Harvey's Brewhaus
Re: pH probe calibration
« Reply #9 on: October 08, 2015, 11:11:56 PM »

The Milwaukee meter uses a BNC connector for the pH electrode. It is possible to reset the pH 7 (offset) by shorting the connector on the meter with a paper clip. This will basically simulate 0 mV which is the theoretical voltage for pH probe in pH 7 solution.

1) Short pH probe input
2) adjust pH 7 trimmer so that pH 7 is displayed
3) Reconnect the pH electrode to the meter
4) place the probe in fresh pH 7 buffer
5) Read the pH value
6) calculate the offset
7) offset outside +/-30 mV is indicating a build up

How to calculate the offset
1) Based on a theoretical response a pH electrode will generate 59.16 mV per pH unit at 25 oC
2) 59.16 mV = 5.9 mV per 0.1 pH
3) Take pH value from above and determine the deviation

For example: My meter was reset as above and after placing the electrode in pH buffer the meter displays pH 7.25. Since the reading is 0.25 pH on the alkaline side that means that means the voltage being generated is around - 15 mV. A -15 mV offset is a very good offset. A pH probe from a mfg will be +/-10 mV. 

The reading is negative since alkaline solutions have less hydrogen ions (H+) than the pH 7 solution in the glass bulb. If the reading was 6.75 then the offset would be about +15 mV. As the pH decreases less than 7 the amount of hydrogen ions (plus charges) are increasing relative to the inside pH 7. There is a increasing positive voltage with an increase in hydrogen ions.

Note: 59.16 mV per pH is based at 25 oC. the number is different at different temps. But performing this test at room temp will provide a good estimate.

pH meters allow calibration to around 1 pH mV response. Meaning a probe can generate around +/- 60 mV and still be calibrated. As a mfg we actually would not recommend using a probe outside +/-30 mV even though the meters can be calibrated outside that range. So with this test if your meter shows < pH 6.5 or greater than pH 7.5 then you are in an area that something should be done, like cleaning.

One thing to remember that when performing this test that it is assumed the the pH buffer is fresh. If the buffer is not then test should not be performed. It is critical (even for calibration) that pH 7 is pH 7. Buffers do change over time. Once a bottle of buffer is opened then it is safe to say that it is good for around six months as long as no contamination is occurring (i.e. pouring buffer back in the bottle).

Since your meter is manual calibration, I do not think it is possible to check slope. If it had automatic calibration and the ability to go to a default cal then it would be possible to calculate the slope by seeing the pH value in a pH 4 buffer. But since the concern is cleaning then the above will help you see the offset voltage. Typically in pH measurement a build up on the electrode causes an offset error and no change to slope since the build up on a sensor would affect the mV response of both buffers.

Yet another example of why I love this forum....SME on just about every topic!


Sent from my iPad using Tapatalk
Ken- Chagrin Falls, OH
CPT, U.S.Army
https://www.facebook.com/pages/Harveys-Brewhaus/405092862905115

http://braukaiser.com/wiki/index.php?title=The_Science_of_Mashing

Serving:        In Process:
Vienna IPA          O'Fest
Dort
Mead                 
Cider                         
Ger'merican Blonde
Amber Ale
Next:
Ger Pils
O'Fest