slow response time of Ph circuit

[aman]

I have a simple PH circuit following is the description

A pH probe involves measurement of the DC Voltage created between one
Silver/Silver Chloride reference electrode - that has connection to the
conductive water via a low impedence salt bridge (a porous plug filled
with highly conductivie potassium chloride) - that creates a known and
very constant voltage.

It has an instrumentation amplifier as it is not possible to measure
the voltage directly as it is a high impedance circuit.

It was working fine but lately it takes like 15 minutes to settle down.
So it has a long long response time of 15 minutes now.

What can this be due to ? Any ideas ?

 

[Aubrey McIntosh, Ph.D.]

aman wrote:

I have a simple PH circuit following is the description

A pH probe involves measurement of the DC Voltage created between one
Silver/Silver Chloride reference electrode - that has connection to the
conductive water via a low impedence salt bridge (a porous plug filled
with highly conductivie potassium chloride) - that creates a known and
very constant voltage.

It has an instrumentation amplifier as it is not possible to measure
the voltage directly as it is a high impedance circuit.

It was working fine but lately it takes like 15 minutes to settle down.
So it has a long long response time of 15 minutes now.

What can this be due to ? Any ideas ?

What event separated the working and non-working epochs?
Is this commercial or one-off?

This symptom frequently follows when a commercial probe has been left in
the air overnight or has otherwise been abused. The glass in the bulb
has a lot of nuance in it. Perhaps someone else can post their
rehabilitation process. If the (ab)use involves strong alkalai (50%
KOH) or profoundly strong first group elements (oh, to guess, 15M LiNO3)
I would go read the literature.

If you can get a sequence of voltage measurements each second or so, I'd
love to try a digital filter on the data to see how soon a reliable
reading could be estimated, but that is just for my own curiosity.

 

[Rich Grise]

On Wed, 09 Mar 2005 20:04:27 -0800, aman wrote:

I have a simple PH circuit following is the description

A pH probe involves measurement of the DC Voltage created between one
Silver/Silver Chloride reference electrode - that has connection to the
conductive water via a low impedence salt bridge (a porous plug filled
with highly conductivie potassium chloride) - that creates a known and
very constant voltage.

It has an instrumentation amplifier as it is not possible to measure
the voltage directly as it is a high impedance circuit.

It was working fine but lately it takes like 15 minutes to settle down.
So it has a long long response time of 15 minutes now.

What can this be due to ? Any ideas ?

It sounds like your Silver/Silver Chloride reference is running down.
Sensors like that _do_ wear out, and this is what happens when they do.

So, you'll probably need to replace the expensive part of the sensor.

Good Luck!
Rich

 

[Yukio YANO]

aman wrote:

I have a simple PH circuit following is the description

A pH probe involves measurement of the DC Voltage created between one
Silver/Silver Chloride reference electrode - that has connection to the
conductive water via a low impedence salt bridge (a porous plug filled
with highly conductivie potassium chloride) - that creates a known and
very constant voltage.

It has an instrumentation amplifier as it is not possible to measure
the voltage directly as it is a high impedance circuit.

It was working fine but lately it takes like 15 minutes to settle down.
So it has a long long response time of 15 minutes now.

What can this be due to ? Any ideas ?

Bad Reference Electrode !, or bad Combination Electrode

Plugged Reference Electrode Junction

Ultra pure water, Very high Resistance ~10/15 Megohm !
What are you measuring ! Standard Buffers are ~0.05/0.10 Mol.
If it's slow response to Standard Buffers then likely a plugged Junction
Fast Test, Hold beaker in one hand while measuring pH and shuffle foot
across floor, with Bad Junction, readings will swing wildly due to
static electric build-up.

Yukio Yano