This question will answer the following questions:
Are new oils acidic?
Why do new oils have an acid number (TAN)?
How can an oil be both acidic and alkali?
Why doesn’t a TBN neutralise the Acid Number?
Should I test the acid number of EP gear oils?
I got an interesting question from another lab the other day posted into the blog. You may think this strange, but I get technical questions from labs all over the world fairly regularly on technical issues regarding fluid analysis. Many are rather detailed method based topics such as tips on various lab methods to help produce better data and calibrations, but this one I thought was worthy of an article as it is relevant to end users of oil analysis too. The question was as follows:
“Hi, I have been following your blog for over a year now. I am a diagnostician at an OCM lab for 3 years and have found your posts very helpful in improving my knowledge. I have a question. We often test TAN and one of our customers has some new oils that have very high values up to 3 or 4mgKOH/g. Why is this? Surely this level of acid would start causing corrosion in machinery and we should be highlighting this to our customers.”
And here is the response I sent:
Thank you for message and indeed it may seem strange at first a new oil having an Acid Number. Indeed I have also been asked in the past by a lab technician at another lab [see question below] if an engine oil has an Acid and Base number does that mean it is both acidic and alkali and why doesn’t one neutralise the other?
“Hi, I know about Acid and Base Numbers and regularly perform these test in the lab using a [brand name removed] titration system, but there is something I just don’t get. If a TBN is designed to neutralise the acids in engine oil, how then if I still have a normal TBN can the oil still have a TAN value. Doesn’t the TBN neutralise the acid? Look forward to your reply.”
So I think you are not alone in having this type of question even from a lab background and I will post a blog article about this too as I think end customers may find this useful too.
So to allay your fears, new oils can have high acid numbers and be perfectly normal. Engine, hydraulic and gear oils are not compounded with additives which are acidic, like vinegar and apple juice etc., although when oils oxidise they form these types of carboxylic acids – hence we can detect oxidation by measuring the acid number.
The initial pH is the pH of the oil when diluted in the TAN solvent and while this pH is not truly the pH of the solution by definition of pH (rather an apparent pH of the solution), it is what everyone accepts as the starting point of the acid/base chemical reaction of the oil additives and the titration solution (0.1m KOH [potassium hydroxide].)
Now to answer your question – why does a new engine oil produce a value of TAN in the first place? This is because the oil additives, including anti wear additive ZDDP and Extreme Pressure additives contain compounds (when mixed with 0.1m KOH) that react chemically as though they were acidic. That is, if KOH reacts with a substance – so uses up part of the KOH solution that is being added (the Titre) – it shows as acidic in the TAN calculation (Titre x Molarity x 56.1)/weight.
This is why it is necessary to determine the initial TAN value of a lubricant, which should then be subtracted from the used oil TAN value to determine the TAN increase, which will be down to fluid oxidation and increasing acidity.
TAN values can be difficult to detect in some used oils and especially when there are polymers and soot particles present – even producing several apparent “end-points”. So determining the correct one requires analytical skills by an experienced laboratory technician. This problem is more complicated when the apparent “pH” reaches pH 4. At which time the chemical reaction starts to measure strong acids like sulphuric acid – if present that are truly corrosive, as opposed to weak acids – like those of oxidised oils (similar to vinegar or apple juice). Note, any strong acids present in an oil is a critical situation and should be reported immediately to your customer.
Although you didn’t ask this second question directly, I have mentioned it already as having being sent in the past from another lab and I think it helps explain the wider concepts more fully and dives a little more into the chemistry of the process.
So yes TAN and TBN can appear in the same new oil at the same time and the reason one has not neutralised the other, is because TAN at this stage is not real – there are no acids present in new engine and hydraulic oils. New Hydraulic oils often show a TAN value, which if monitored closely will decrease with use, as the additive becomes spent, but then starts to increase as the TAN from the oil oxidation overwhelms the loss in TAN from the oil additive use. The TAN value from there on starts to increase as the TAN value from the true acids of oil oxidation dominates the situation.
In used oils it is possible to have both (TAN & TBN). This is because the oxidised oil weak acids, which have been neutralised by the strong Base (alkali TBN additive, which is equally neutralised with the weak acids). So they do in fact neutralise each other and this situation remains so – until the strong mineral acid in the TBN acid Titre solution displaces the weak acids that were originally neutralising the strong base. At which stage the strong acid Titre Solution is then neutralised by the strong base additive that was once neutralising the weak acids – so producing a TBN value because the titre solution has been neutralised – again affecting the formula: Titre x Molarity x 56.1/weight. This means the absence of neutralising each other only occurs during the test itself and can be likened to the schrodinger’s cats thought experiment, where you cannot know about the subject under test without interfering with it in some way.
When the same sample is subjected to the TAN test, the strong alkali solution (0.1M KOH) displaces any weak base (alkali) components in the oil, e.g. amines currently neutralised by weak acids present in the lubricant plus the effects of other additives such as anti-oxidants, dispersants and the Zinc component of the ZDDP antiwear additive reacting with the strong alkali Titre Solution Together this uses up some of the alkali Titre Solution, which in turn affects the formula: Titre x Molarity x 56.1/weight to indicate the concentration of TAN present. This creates an artificial TAN value in addition to any real TAN. Hence the importance of baselines and trending.
Hopefully this explains how two opposing chemicals exist together when common sense suggests they should neutralise each other.
As a side note, today there is a tendency to favour trending the Oxidation level of the lubricant by FTIR alone – as opposed to measuring TAN. Simply because this value is simpler to measure and has less interferences. It is proportional to the deterioration of the lubricant and is far less susceptible to ambiguity. What’s more it technically occurs earlier in the acid formation process as it detects the aldehydes precursors as well as the carboxylic acids so it can be an earlier predictor of degradation. It also simultaneously measures nitration and sulphation, for those who require this extra information on engine oils.
In terms of TAN and Neut Number usefulness there are some key areas where these are essential tests such as on gas engine oils where the industry is very good at monitoring regularly, providing reference oils and the primary mode of failure is oxidation. Equally, applications where the oil contains very little to no additives that interfere with the test such as gas/steam turbine/Transformer oils colourmetric TAN/Neut Number can be useful. However, for most other hydraulic and gear oils I personally find oxidation a much better indicator of lubricant deterioration.
I hope you found this article useful. I also have a video on the topics of Acid and Base Numbers as well as oxidation degradation products such as varnish which you can see below.