Sci Tech
2010-11-15 11:23:31 UTC
pH measured by pH meters (actually called potentiometers) is a converted
property from mV of the solution through the Nernst equation. A kind of
an operational amplifier within the meter is taking care of this.
pH scale in aqueous solutions from 0-14 is limited by the pKa of water
at a given temperature (usually taken at 25oC). Most meters have a
temperature compensation probe to correct the reading for temperature
variation.
You can use pH in any aqueous/non-aqueous solution, however, if you do
not have the correct pKa' for that mixture, you have to run a
calibration through which you measure the correct pKa' of that mixture
and manually convert mV reading to pH through the Nernst equation.
It is true that in many pure non-aqueous solvents or others, people use
a pH electrode to measure the pH to check the acidity/alkalinity of that
solvent as a degree of checking its quality. This is not a correct
practice, because the pH meter is calibrated using aqueous standards and
the measurement is done in a totally different medium. Again a question
of pKa', junction potential and mobility of ions that differ strongly
from the calibrated and measured media. So far, you cannot use a pH
meter without a calibration. Luckily, there are some efforts as to
supply ready made pH standards for calibration in media that differ
slightly from aqueous solution such as blood, waste water, sea water,
etc... As to my knowledge, no pH standards available for non-aqueous
solvents or oil. For oil, I agree that it is the H+ considered as
impurities within the oil is taking care of the reading you get because
for the simple reason, pH electrodes are only responsive for H+ ions.
Regards,
SK
On 10.11.2010 01:48 PM, Andrew Usher wrote:
> We're taught to sue the pH scale to measure acidity, and that this
> only works in aqueous solutions - the extension called 'pKa' is
> supposed to work for non-aqueous substances, but I've only ever seen
> it for single molecules, not solutions. Yet all solutions do have a
> fixed ability to donate a proton, though the indicator (or inert
> surface) may be affected by the solution also.
>
> The greater problem is that the pH scale is fixed to the temperature
> 25 C, and does not generalise properly. Because water's self-
> ionisation constant increases with temperature, the strict definition
> of pH requires a neutral solution to change with temperature, and a
> basic one even more, which is hardly desirable, and makes the analysis
> of corrosion at different temperatures unnecessarily difficult.
>
> What we're really meaning to talk about is the energy of proton
> transfer, right? So acidity ought to be measured in volts, just like
> redox equilibria, and for the same reason. This would be nearly
> independent of temperature, as well as applying to all solvents. If
> this were taught, and also energy measured in electron-volts, then it
> would be obvious that both acid-base and redox reactions are merely
> special cases (though the most important) of chemical change in
> general.
>
> Indeed, it would also make obvious why certain redox equilibria change
> with pH, and by how much - namely, by an amount equal to the acidity
> multiplied by the number of protons transferred per electron. I
> propose that pure water be defined as 0 V, at all temperatures - since
> water is certainly the most important solvent.
>
> Andrew Usher
property from mV of the solution through the Nernst equation. A kind of
an operational amplifier within the meter is taking care of this.
pH scale in aqueous solutions from 0-14 is limited by the pKa of water
at a given temperature (usually taken at 25oC). Most meters have a
temperature compensation probe to correct the reading for temperature
variation.
You can use pH in any aqueous/non-aqueous solution, however, if you do
not have the correct pKa' for that mixture, you have to run a
calibration through which you measure the correct pKa' of that mixture
and manually convert mV reading to pH through the Nernst equation.
It is true that in many pure non-aqueous solvents or others, people use
a pH electrode to measure the pH to check the acidity/alkalinity of that
solvent as a degree of checking its quality. This is not a correct
practice, because the pH meter is calibrated using aqueous standards and
the measurement is done in a totally different medium. Again a question
of pKa', junction potential and mobility of ions that differ strongly
from the calibrated and measured media. So far, you cannot use a pH
meter without a calibration. Luckily, there are some efforts as to
supply ready made pH standards for calibration in media that differ
slightly from aqueous solution such as blood, waste water, sea water,
etc... As to my knowledge, no pH standards available for non-aqueous
solvents or oil. For oil, I agree that it is the H+ considered as
impurities within the oil is taking care of the reading you get because
for the simple reason, pH electrodes are only responsive for H+ ions.
Regards,
SK
On 10.11.2010 01:48 PM, Andrew Usher wrote:
> We're taught to sue the pH scale to measure acidity, and that this
> only works in aqueous solutions - the extension called 'pKa' is
> supposed to work for non-aqueous substances, but I've only ever seen
> it for single molecules, not solutions. Yet all solutions do have a
> fixed ability to donate a proton, though the indicator (or inert
> surface) may be affected by the solution also.
>
> The greater problem is that the pH scale is fixed to the temperature
> 25 C, and does not generalise properly. Because water's self-
> ionisation constant increases with temperature, the strict definition
> of pH requires a neutral solution to change with temperature, and a
> basic one even more, which is hardly desirable, and makes the analysis
> of corrosion at different temperatures unnecessarily difficult.
>
> What we're really meaning to talk about is the energy of proton
> transfer, right? So acidity ought to be measured in volts, just like
> redox equilibria, and for the same reason. This would be nearly
> independent of temperature, as well as applying to all solvents. If
> this were taught, and also energy measured in electron-volts, then it
> would be obvious that both acid-base and redox reactions are merely
> special cases (though the most important) of chemical change in
> general.
>
> Indeed, it would also make obvious why certain redox equilibria change
> with pH, and by how much - namely, by an amount equal to the acidity
> multiplied by the number of protons transferred per electron. I
> propose that pure water be defined as 0 V, at all temperatures - since
> water is certainly the most important solvent.
>
> Andrew Usher