ASTM D664; Rationale?

[Bazz54]

Hi,
I'm a chemist with 30 years industrial experience, but new to lubes and to this forum! I have been using D664 to measure TAN on some jet engine oil. Can anyone explain what the rationale is for using aqueous buffer solutions of pH4 and 11 to set the emf's for the start and end point of the titration of the oil samples? I find that with samples with a low TAN, they don't produce an emf from the pH electrode as high as the pH4 start point. However, I find that I get fairly reasonable correlation between the results of D664 and the results I get from the old colour-change method.

When not working on TAN, my personal interest in lubes arises from my ownership of 2002 a Toyota Celica with its somewhat "flawed" 1ZZF engine, many of which die young with lube based problems

Thanks in anticipation.

 

[chubbs1]

WOW! I would PM "MOLEKULE" for an answer on this. The SIMPLEST way to deal with that engine is to do 3-5K oil changes.

As I said if you want a more in-depth answer go to "Forum List' and search "MoleKule" he is a smart guy and Tribologist.

 

[modularv8]

Originally Posted By: chubbs1
WOW! I would PM "MOLEKULE" for an answer on this. The SIMPLEST way to deal with that engine is to do 3-5K oil changes.

As I said if you want a more in-depth answer go to "Forum List' and search "MoleKule" he is a smart guy and Tribologist.


Molekule only purports to know things, he's not a chemist. Impresses the less knowing.

The answer is that it increases the sensitivity in a potentiometric titration. The inflection point will be more precise (repeatable) when standardizing the electrodes to the corresponding pH values listed.

Using the old color change method is not precise because determination of the exact inflection point is not based on instrumentation (human detection).

 

[Bazz54]

Originally Posted By: modularv8
Originally Posted By: chubbs1
WOW! I would PM "MOLEKULE" for an answer on this. The SIMPLEST way to deal with that engine is to do 3-5K oil changes.

As I said if you want a more in-depth answer go to "Forum List' and search "MoleKule" he is a smart guy and Tribologist.


Molekule only purports to know things, he's not a chemist. Impresses the less knowing.

The answer is that it increases the sensitivity in a potentiometric titration. The inflection point will be more precise (repeatable) when standardizing the electrodes to the corresponding pH values listed.

Using the old color change method is not precise because determination of the exact inflection point is not based on instrumentation (human detection).


Well, I can see some point to it, but surely the two pH values should be equally spaced either side of pH7, e.g. pH4 and pH10?
As happens sometimes with old test methods, all the guys who devised it are probably now retired...or dead, and the rationale is hard to find.

 

[modularv8]

The turning point (inflection point on a titration chart) is dependendent on the concentrations (normality/equivalents, etc) of the reagents in the old "color change method". In order for it to be accurate, you standardize the solutions. In a potentiometric titration, the determining factor is the emf of the electrodes. So you are standardizing the electrodes with the pH buffer solutions (which themselves are standardized).

 

[Bazz54]

Originally Posted By: modularv8
The turning point (inflection point on a titration chart) is dependendent on the concentrations (normality/equivalents, etc) of the reagents in the old "color change method". In order for it to be accurate, you standardize the solutions. In a potentiometric titration, the determining factor is the emf of the electrodes. So you are standardizing the electrodes with the pH buffer solutions (which themselves are standardized).


Sure, agree with all of that. But that does not rationalise the selection of pH4 & 11; these appear to be arbitary values, which may enable the procedure to work and be repeatable in different labs, but I'm just trying to find out if there is some reason/logic there that I'm failing to see. When I work with oil samples that have a low TAN (~1), the emf produced on a pH electrode is often lower than that obtained in the pH4 aqueous standard. The colour change method may not be easy to use on dark oil samples, but at least you know exactly why the method works

 

[modularv8]

Originally Posted By: Bazz54


Sure, agree with all of that. But that does not rationalise the selection of pH4 & 11; these appear to be arbitary values, which may enable the procedure to work and be repeatable in different labs, but I'm just trying to find out if there is some reason/logic there that I'm failing to see. When I work with oil samples that have a low TAN (~1), the emf produced on a pH electrode is often lower than that obtained in the pH4 aqueous standard. The colour change method may not be easy to use on dark oil samples, but at least you know exactly why the method works

The pH values of the buffer solutions are arbitrary, they are only reference points; with each falling at the midpoint of the basic/acid side of the full pH scale.