Testing pH in oil; possible?

[Mad_Hatter]

I thought only aqueous solutions could be tested for pH?🤔..is the following possible? (dilute the oil w/H20)
++++++++++++++++++++++++

"I took a small quantity of used oil and put it into a small container. I added water and shook the two liquids vigorously. Acidic compounds would have a tendency to migrate out of the oil and into the water layer. Then I took a piece of common pH measuring paper and dipped it into the water layer. The presence of acidic compounds would be indicated by a color change on the paper. I've never gotten a positive test, either because the test method is flawed or because there were no unneutralized acids in the oil. Is anyone familiar with the proper method of pH measurement in oil? Does this sound like a valid test method?"

 

[kschachn]

In a former job I worked with the pH of non-aqueous solutions, mostly alcohols and diols. There are specialized non-aqueous pH probes but they have long stabilization times and the results are dubious at best, especially considering the definition of pH is related to the ionization of water. However, with certain compounds it is possible to have a hydrogen ion concentration with no water present.

Having said that however, that does not apply to a hydrocarbon like oil. Here, in order to even have a pH there must be water present, and in most every sample there is always some water. In college I ran oil analysis on samples and there was almost never a sample that didn't test out with some water via Karl Fischer. So when you test an oil for pH either with a probe or with test paper it is testing the acidity of the water present in the sample. It is impossible for a hydrocarbon like oil to be acidic without water present as there are no free hydrogen ions to give a positive test result.

There can be potentially acidic actors in an oil sample that will not present themselves as acidic until water is added. We used to sometimes test an oil for pH both before and after water is added. Kind of like an acid digestion for metals, it gives an indication of potentially harmful substances in the sample that are dormant until it comes into contact with water. This relates to your example above but as you point out, there are buffers in oil that skew the pH reading.

Measuring pH is never trivial although many people think all you need is a probe or indicator paper. Solutions can be very complex especially when buffers (known or unknown) are involved.

 

[WyrTwister]

You take oil & add a little water & shake it up ? Even if you did get indication of high or low ph , the data would not be creditable because you had just diluted it , when you added water .

If you used a " standard / consistent " amount of oil and of water , you might do some kind of comparison , between different samples ?

 

[kschachn]

Originally Posted by WyrTwister
You take oil & add a little water & shake it up ? Even if you did get indication of high or low ph , the data would not be creditable because you had just diluted it , when you added water .

No that's not how it works, you would be adding neutral water to the oil. But what you will do is possibly dissolve unionized acidic compounds that wouldn't have been a problem unless more water somehow got into the oil during operation. It is good to know both values however since the water in oil varies during the OCI. The acidity of the oil can vary with a fluctuating water concentration.

 

[gfh77665]

I wonder what the UOA methodology for testing TBN is? I have to believe there is a way to reliably test pH and / or acid content in oil samples. To get a rough "ball park" value for pH in non aqueous samples, we mix 50/50 every time, and vigirously agitate with a magnetic stir plate. Disclaimer: This is not an ASTM method.

 

[kschachn]

Originally Posted by gfh77665
I wonder what the UOA methodology for testing TBN is? I have to believe there is a way to reliably test pH and / or acid content in oil samples. To get a rough "ball park" value for pH in non aqueous samples, we mix 50/50 every time, and vigirously agitate with a magnetic stir plate. Disclaimer: This is not an ASTM method.

This looks like a very good overview from Wikipedia. Note they are using alcohols and diols as we did in our non-aqueous solutions, but also add some water. It really is impossible without water present.

https://en.wikipedia.org/wiki/Total_base_number

 

[Mad_Hatter]

Originally Posted by kschachn
In a former job I worked with the pH of non-aqueous solutions, mostly alcohols and diols. There are specialized non-aqueous pH probes but they have long stabilization times and the results are dubious at best, especially considering the definition of pH is related to the ionization of water. However, with certain compounds it is possible to have a hydrogen ion concentration with no water present.

Having said that however, that does not apply to a hydrocarbon like oil. Here, in order to even have a pH there must be water present, and in most every sample there is always some water. In college I ran oil analysis on samples and there was almost never a sample that didn't test out with some water via Karl Fischer. So when you test an oil for pH either with a probe or with test paper it is testing the acidity of the water present in the sample. It is impossible for a hydrocarbon like oil to be acidic without water present as there are no free hydrogen ions to give a positive test result.

There can be potentially acidic actors in an oil sample that will not present themselves as acidic until water is added. We used to sometimes test an oil for pH both before and after water is added. Kind of like an acid digestion for metals, it gives an indication of potentially harmful substances in the sample that are dormant until it comes into contact with water. This relates to your example above but as you point out, there are buffers in oil that skew the pH reading.

Measuring pH is never trivial although many people think all you need is a probe or indicator paper. Solutions can be very complex especially when buffers (known or unknown) are involved.

Ty.. that was helpful.

 

[sloinker]

Maybe they could use a differential analysis using two pH probes utilizing the same water dilution sample and calculating the difference to arrive at a pH number.

 

[Deontologist]

Originally Posted by kschachn

Having said that however, that does not apply to a hydrocarbon like oil. Here, in order to even have a pH there must be water present, and in most every sample there is always some water. In college I ran oil analysis on samples and there was almost never a sample that didn't test out with some water via Karl Fischer. So when you test an oil for pH either with a probe or with test paper it is testing the acidity of the water present in the sample. It is impossible for a hydrocarbon like oil to be acidic without water present as there are no free hydrogen ions to give a positive test result.


I agree mostly.

pH in an aqueous solution (aqueous = majority water), like swimming pool water, the water in your washing machine, your coolant, etc. actually refers to the "activity" of the "H+ ion." In most cases, we approximate the activity of the H+ ion as being equivalent to its concentration. "Activity" is a measure of how much of an "effect" particles have on other particles. If a particle has an activity of 0, then it doesn't really affect anything else in the solution. If a particle has an activity of "0.4," then it has a limited effect on other things in the solution. Some solutions of odd substances will have substantially unexpected pH values because of the solutes' relatively low activities; the solutes can't quite perhaps fully dissolve and affect the concentration of [H+] or [OH-].

 

[kschachn]

Originally Posted by Deontologist
I agree mostly.

Interesting, what parts don't you agree with? If you tried to measure the pH of a dehydrated C30 hydrocarbon what would you measure?

 

[Student]

So, does it mean you can't measure the pH of oils, or it just doesn't exist?
I keep seeing info on pH of certain oils, like for example, Sweet almond oil is mildly acidic (4.5-5).
Also, some PhD's claim that oils have no pH value, others that they do. And if you Google it, it will show up. What does it mean ? How is it measured and after all, do oils have oh?

Thanks in advance for the reply!

 

[kschachn]

Originally Posted by Student
So, does it mean you can't measure the pH of oils, or it just doesn't exist?
I keep seeing info on pH of certain oils, like for example, Sweet almond oil is mildly acidic (4.5-5).
Also, some PhD's claim that oils have no pH value, others that they do. And if you Google it, it will show up. What does it mean ? How is it measured and after all, do oils have oh?

Thanks in advance for the reply!

Well look at it this way. "pH" is a measurement for hydrogen ion concentration, so if you have a dry hydrocarbon sample with no hydrogen dissociation then by definition it is neutral. It doesn't take a PhD to determine that.

On the other hand most substances have at least some water in them. As I mentioned above, when I used to test oils in college from our fluid power and automobile engineering departments we used Karl Fischer titration. What you were measuring there was the pH of the small amount of water in the oils. If you have a truly desiccated sample you will not be able to measure pH and the probe will drift endlessly.

Be careful when you read about "non-aqueous pH probes", those are not a magic bullet to measure the elusive pH of a truly non-aqueous material, what it means is that they are probes optimized for use in predominately non-aqueous materials.

Often when you wish to measure the pH of a nonpolar substance such as oil you add neutral water prior to measurement. However, most automobile oils after use have more than sufficient water in them to permit a measurement.

https://assets.fishersci.com/TFS-Assets/LSG/Application-Notes/AN-PHNONAQS-E 1014-RevA-WEB.pdf

Again as I mentioned before the determination of pH in a non-aqueous media is not trivial. Oh and your "sweet almond oil" is nothing like a hydrocarbon nor is it monolithic. It is full of organic acids and always has some water present or else they add some prior to pH measurement.

What's your course of study, "Student"?

 

[saab92x]

So a tester like this: Lubricheck, wouldn’t be an accurate way to test an oil sample when trying to ascertain whether or not it’s time to change the oil (early perhaps, if too many shorter drive cycles have occurred between normally recommended OCIs)?

I ask because with shorter drive durations modern engines don’t get a chance to get up to intended operating temps, possibly ruining synthetic oils and causing buildup on internal engine surfaces. I’d love to test out my driving style to see if my oil’s lubricity is still good or not. Maybe a diy dream; mailing in a sample is such a hassle.

 

[kschachn]

So a tester like this: Lubricheck, wouldn’t be an accurate way to test an oil sample when trying to ascertain whether or not it’s time to change the oil (early perhaps, if too many shorter drive cycles have occurred between normally recommended OCIs)?

I ask because with shorter drive durations modern engines don’t get a chance to get up to intended operating temps, possibly ruining synthetic oils and causing buildup on internal engine surfaces. I’d love to test out my driving style to see if my oil’s lubricity is still good or not. Maybe a diy dream; mailing in a sample is such a hassle.

The tester you link checks lubricity?

 

[Davejam]

Not really sure what you're trying to figure out.
But, Total Base Number/Total Acid Number are basically titrations that give a more accurate representation of pH and resistance to pH change.
They are already standard tests.

 

[saab92x]

The tester you link checks lubricity?

“The Lubricheck combines the measurement data for acidity, metal particulate level, carbon level and foreign liquids (except fuel) into one, easy to understand rating of your oil condition.”
Apparently it does both and quantifies the oil holistically, supposedly... just wanted to know if this tester is nonsense or plausible. If it’s not possible to accomplish a handheld test like this that would be able to tell you “it’s time to change the oil now”, particularly if someone drives too few miles in a car that uses synthetic oil. I’ve heard some engine builders state that short trips in vehicles that use synthetic (albeit awesome oil) it needs time for the engine to get to operating temps to last as long as possible (as in the newer cars going 10,000+ mile intervals), otherwise the oil can cause buildup on the cooler engine internals when they don’t have a chance to fully heat up.

 

[saab92x]

Not really sure what you're trying to figure out.
But, Total Base Number/Total Acid Number are basically titrations that give a more accurate representation of pH and resistance to pH change.
They are already standard tests.

I’d love to be able to do this at home as the oil ages to get a better handle on when an oil change is necessary. It would be much faster than getting a full oil analysis from the lab.

 

[Davejam]

Interesting thought.
Its not that straightforward, so unless you have access to lab chemicals/lab equipment the best I could find are field test kits for TBN and TAN.

TitraLube® TAN - Dexsil

Field Test Kit for Quantifying Total Acid Number in Industrial/Lubricating Oils CONDITION MONITORING of LUBRICATING OILS

www.dexsil.com

 

[saab92x]

Maybe I’ll get the chemical kit and compare the results to the reusable capacitive/resistive tester.