pH Changes due to stirring

Thomas Merchant

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Apr 25, 1997, 3:00:00 AM4/25/97

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OK, time for another stupid pH question.

Why does the pH reading change from a stirred to an unstirred state?

In a still solution I will take a reading, start the stirrer, and the
reading will change, by as much as 0.15. When the stirrer is turned
off, the reading will usually eventually return to its original
unstirred value.

I think that this is happening too fast to be a CO2 dissolution
affect, and occurs in buffers as well as test solutions. Is this a
function of the pH probe, filling solution, and/or an indication of a
probe nearing the end of its useful life? Or is this something I will
simply have to live with?

Thanks in advance.

t. mcniel

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Apr 25, 1997, 3:00:00 AM4/25/97

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My guess would be a local build-up of ions near the measuring end of the
probe in an ustirred soln.

In article <3360ae92...@news.gatech.edu>, tm...@acme.gatech.edu (Thomas
Merchant) wrote:

>OK, time for another stupid pH question.
>
>Why does the pH reading change from a stirred to an unstirred state?
>

--
"It is by caffeine alone that i set my mind in motion."

Uncle Al Schwartz

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Apr 25, 1997, 3:00:00 AM4/25/97

to Thomas Merchant

Thomas Merchant wrote:
>
> OK, time for another stupid pH question.
>
> Why does the pH reading change from a stirred to an unstirred state?
>

> In a still solution I will take a reading, start the stirrer, and the
> reading will change, by as much as 0.15. When the stirrer is turned
> off, the reading will usually eventually return to its original
> unstirred value.
>
> I think that this is happening too fast to be a CO2 dissolution
> affect, and occurs in buffers as well as test solutions. Is this a
> function of the pH probe, filling solution, and/or an indication of a
> probe nearing the end of its useful life? Or is this something I will
> simply have to live with?
>
> Thanks in advance.

It might be slow hydration of dissolving CO2. It might be
concentration polarization at the electrode. It might be
something else. You neatly omitted mentioning WHAT pH you are
nominally at.

--
Alan "Uncle Al" Schwartz
Uncl...@ix.netcom.com ("zero" before @)
unc...@uvic.ca (summer only, cAsE-sensitive!)
http://www.ultra.net.au/~wisby/uncleal.htm
(Toxic URL! Unsafe for children, Democrats, and most mammals)
"Quis custodiet ipsos custodes?" The Net!

Carlos Rodriguez

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Apr 25, 1997, 3:00:00 AM4/25/97

to

>Why does the pH reading change from a stirred to an unstirred state?

>In a still solution I will take a reading, start the stirrer, and the
>reading will change, by as much as 0.15. When the stirrer is turned
>off, the reading will usually eventually return to its original
>unstirred value.

My guess would be the pH diff being due to ionic strength near the junction
being different than the bulk. Dissolved CO2 is always a problem, but
typically, I only experienced troubles at high pH values (and solutions that
'sat' for long periods of time). I'm not sure, but I think the absorption of
CO2 is not particularly fast.

hope this helps,
carlos

Sheila D. Cox

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Apr 26, 1997, 3:00:00 AM4/26/97

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tm...@acme.gatech.edu (Thomas Merchant) wrote:

>OK, time for another stupid pH question.

>Why does the pH reading change from a stirred to an unstirred state?

>In a still solution I will take a reading, start the stirrer, and the
>reading will change, by as much as 0.15. When the stirrer is turned
>off, the reading will usually eventually return to its original
>unstirred value.

>I think that this is happening too fast to be a CO2 dissolution

>affect, and occurs in buffers as well as test solutions. Is this a
>function of the pH probe, filling solution, and/or an indication of a
>probe nearing the end of its useful life? Or is this something I will
>simply have to live with?

>Thanks in advance.

If your stirrer is electrical, you could be introducing an electrical
ground loop through the reference side of your pH electrode. To test
for a potential ground loop, place a copper wire into the sample and
the other end to a known earth ground (preferable not the pH meter
ground). If the problem goes away, you got a ground loop. Sensorex
makes an electrode which will correct for the ground loop. Visit
their site at http://www.sensorex.com/

William R. Penrose

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Apr 27, 1997, 3:00:00 AM4/27/97

to

>Thomas Merchant wrote:
>>
>> OK, time for another stupid pH question.
>>
>> Why does the pH reading change from a stirred to an unstirred state?
>>
>> In a still solution I will take a reading, start the stirrer, and the
>> reading will change, by as much as 0.15. When the stirrer is turned
>> off, the reading will usually eventually return to its original
>> unstirred value.
>>
>> I think that this is happening too fast to be a CO2 dissolution
>> affect, and occurs in buffers as well as test solutions.

There might be an effect from the stirrer motor, especially if the electrode
is sensitive to waving your hand near it. Try using an air-driven stirrer.
Does the effect vary with speed?

Bill

************************************************************
Bill Penrose, Sr. Scientist, Transducer Research, Inc.
600 North Commons Drive, Suite 117
Aurora, IL 60504
630-978-8802, fax -8854, email wpen...@interaccess.com
************************************************************
Purveyors of fine gas sensors and
contract R&D to this and nearby galaxies.
************************************************************

Ed A. Luinstra

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Apr 27, 1997, 3:00:00 AM4/27/97

to

In article <3360ae92...@news.gatech.edu>,

Thomas Merchant <tm...@acme.gatech.edu> wrote:
>OK, time for another stupid pH question.
>
>Why does the pH reading change from a stirred to an unstirred state?
>
>In a still solution I will take a reading, start the stirrer, and the
>reading will change, by as much as 0.15. When the stirrer is turned
>off, the reading will usually eventually return to its original
>unstirred value.

You need two electrodes to do pH measurements. The common pH electrode is
actually a combination of a glass electrode and a reference electrode
such as calomel or Ag/AgCl. You could also measure pH using separate
glass and reference electrodes.

The reference electrode makes contact with your test solution via an
aqueous solution (usually KCl) in the electrode body, through a small
hole on the side near the glass bulb. The hole contains glass fibers or
ceramic stuff to prevent quick loss of the KCl solution.

Anyway, this junction of the KCl solution and the solution you are
testing will have an EMF associated with it. Since the stirring rate
will affect the concentration gradient at the junction, I would guess the
potential will also change, and consequently also the pH reading.

The local H+ concentration at the glass membrane could also be affected by
stirring.

===
| |
/ o \ Ed Luinstra
/ O \ Sulfur Process Consultant
(___o___) luin...@freenet.calgary.ab.ca

rolfyboy

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Apr 28, 1997, 3:00:00 AM4/28/97

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On Fri, 25 Apr 1997 13:23:41 GMT, tm...@acme.gatech.edu (Thomas
Merchant) wrote:

>OK, time for another stupid pH question.
>
>Why does the pH reading change from a stirred to an unstirred state?
>
>In a still solution I will take a reading, start the stirrer, and the
>reading will change, by as much as 0.15. When the stirrer is turned
>off, the reading will usually eventually return to its original
>unstirred value.
>

>I think that this is happening too fast to be a CO2 dissolution

>affect, and occurs in buffers as well as test solutions. Is this a
>function of the pH probe, filling solution, and/or an indication of a
>probe nearing the end of its useful life? Or is this something I will
>simply have to live with?
>
>Thanks in advance.

Tom,

We studied this problem for Hach when we were working on their process
pH analyzers. Here's what we found...

1- Literature search

Ion selective membrane electrodes have a definite instability
at high flow rates (>150 cm/s). the potential responce becomes
erratic, possibly due to distortion of the membrane by the pressure of
the sample. (Analysis with Ion-Selective Electrodes, editor J. J.
Bouma)

Tests between stagnant and flowing samples showed a definite
increase in MV with flowing samples. (various cites)

Flow instabilities can cause streaming potentials at high flow
rates. The sreaming potential produces an oscillation in the MV
readings in direct response to the periodic flow oscillation. The
averaging mechanism of the pH meter damps the oscillation but the
average reading will increase as flow increases. (Bouma, supra)

2- Which reading is correct?

a. Reading pH with no stirring is incorrect, possibly due to
the loss of potential as the reference solution concentration
increases locally next to the reference junction??? (I wouldn't trust
this knee-jerk guess!!!)

b. Reading pH with excessive stirring is also incorrect. It
is doubtful whether you can deactive the averaging of the meter, but
if you could, you would increase the stirring rate until just before
the reading became unstable.

3- What to do about it?

a. Use two electrodes rather than a combination electrode.

b. Increase the conductivity of the solution using KCL or some
other buffer.

c. Maintain a stirring rate which is large, but does NOT
create a vortex or introduce air bublbles into the solution.

d. If your probe is in-line, be sure that the reference
junction is located DOWNSTREAM of the pH sensor.

dmr...@ix.nospam.netcom.com

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Apr 29, 1997, 3:00:00 AM4/29/97

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rolf...@plantaganet.com (rolfyboy) wrote:

>We studied this problem for Hach when we were working on their process
>pH analyzers. Here's what we found...

<snip>

>2- Which reading is correct?

> a. Reading pH with no stirring is incorrect, possibly due to
>the loss of potential as the reference solution concentration
>increases locally next to the reference junction??? (I wouldn't trust
>this knee-jerk guess!!!)

> b. Reading pH with excessive stirring is also incorrect. It
>is doubtful whether you can deactive the averaging of the meter, but
>if you could, you would increase the stirring rate until just before
>the reading became unstable.

>3- What to do about it?

> a. Use two electrodes rather than a combination electrode.

> b. Increase the conductivity of the solution using KCL or some
>other buffer.

> c. Maintain a stirring rate which is large, but does NOT
>create a vortex or introduce air bublbles into the solution.

> d. If your probe is in-line, be sure that the reference
>junction is located DOWNSTREAM of the pH sensor.

e. Use consistency when taking measurements. Use the same stir rate
for each sample. This will help insure that your data express the
proper correlation between pH value and your variable of interest.

-Dave in AZ

--
To reply by e-mail remove the "nospam" from return address

Sweattman

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Apr 30, 1997, 3:00:00 AM4/30/97

to

I would suggest talking with Peter Boyle the R&D Director at pHoenix
Electrode Co. Spending the last 22 years in the field, I'm sure he would
have some insight. He's also very candid and honest with any solutions he
may have. You can reach him via e-mail at pho...@ix.netcom.com or via
fax at 713-772-4671. Voice is 713-772-6666 ext 217.
Regards,
Sweattman.

Peter Macintosh

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Apr 30, 1997, 3:00:00 AM4/30/97

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This is a multi-part message in MIME format.

------=_NextPart_000_01BC5590.D0226D60
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

Ed A. Luinstra <luin...@freenet.calgary.ab.ca> wrote in article
<5k0hre$g...@ds2.acs.ucalgary.ca>...
>
> In article <3360ae92...@news.gatech.edu>,

> Thomas Merchant <tm...@acme.gatech.edu> wrote:
> >OK, time for another stupid pH question.
> >
> >Why does the pH reading change from a stirred to an unstirred state?
> >
> >In a still solution I will take a reading, start the stirrer, and the
> >reading will change, by as much as 0.15. When the stirrer is turned
> >off, the reading will usually eventually return to its original
> >unstirred value.
>
>

> You need two electrodes to do pH measurements. The common pH electrode is

> actually a combination of a glass electrode and a reference electrode
> such as calomel or Ag/AgCl. You could also measure pH using separate
> glass and reference electrodes.
>
> The reference electrode makes contact with your test solution via an
> aqueous solution (usually KCl) in the electrode body, through a small
> hole on the side near the glass bulb. The hole contains glass fibers or
> ceramic stuff to prevent quick loss of the KCl solution.
>
> Anyway, this junction of the KCl solution and the solution you are
> testing will have an EMF associated with it. Since the stirring rate
> will affect the concentration gradient at the junction, I would guess the

> potential will also change, and consequently also the pH reading.
>
> The local H+ concentration at the glass membrane could also be affected
by
> stirring.
>
> ===
> | |
> / o \ Ed Luinstra
> / O \ Sulfur Process Consultant
> (___o___) luin...@freenet.calgary.ab.ca
>
>

So..... which measurement is correct?
Peter
------=_NextPart_000_01BC5590.D0226D60
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

<html><head></head><BODY bgcolor=3D"#FFFFFF"> Ed A. Luinstra =
<luin...@freenet.calgary.ab.ca> wrote in article <5k0hre$g...@ds2.acs.ucalgary.ca>... > > In article <3360ae92...@news.gatech.edu>, > Thomas Merchant <tm...@acme.gatech.edu> wrote: > >OK, time for another stupid pH =
question. > > > >Why does the pH reading change from a =
stirred to an unstirred state? > > > >In a still =
solution I will take a reading, start the stirrer, and the > =
>reading will change, by as much as 0.15. When the stirrer is =
turned > >off, the reading will usually eventually return to =
its original > >unstirred value. > > > You =
need two electrodes to do pH measurements. The common pH electrode is =
 > actually a combination of a glass electrode and a reference =
electrode > such as calomel or Ag/AgCl. You could also measure pH =
using separate > glass and reference electrodes. > =
 > The reference electrode makes contact with your test solution =
via an > aqueous solution (usually KCl) in the electrode body, =
through a small > hole on the side near the glass bulb. The hole =
contains glass fibers or > ceramic stuff to prevent quick loss of =
the KCl solution. > > Anyway, this junction of the KCl =
solution and the solution you are > testing will have an EMF =
associated with it. Since the stirring rate > will affect the =
concentration gradient at the junction, I would guess the > =
potential will also change, and consequently also the pH reading. =
 > > The local H+ concentration at the glass membrane could =
also be affected by > stirring. > > =
         =3D=3D=3D > =
         | | > =
        / o \ =
    Ed Luinstra > =
       /   O \ =
   Sulfur Process Consultant > =
      (___o___)   luin...@freenet.calgary.ab.ca > > So..... which measurement is =
correct? Peter
</f=
ont></body></html>
------=_NextPart_000_01BC5590.D0226D60--

Sweattman

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May 1, 1997, 3:00:00 AM5/1/97

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If you are finding noise when you wave your hand near the electode, then
you have a bad electrode. Sounds like very poor isolation. You can call
pHoenix or Sensorex for a replacement but chances are they won't sell you
one direct. Talk to ColeParmer or Omega. I'm sure they will help.