How KOH units standardize oil condition monitoring across industries

Engine oils are formulated with many additives, and arguably one of the more important is detergents; these not only keep components clean, but also are basic in nature and provide an alkalinity reserve as acid begins to form due to thermal and chemical stress. 

The Total Base Number (TBN) measures oil alkalinity, i.e. the acid-neutralizing capacity of the oil. The lab adds a known acid to the sample until the base content is neutralized. The reading reports how much basic material the oil could supply. That part is straightforward, however the unit choice causes confusion. If we are adding acid, why do we report a value in terms of a base molecule (namely Potassium Hydroxide, or KOH)? 

Why TBN is reported in potassium hydroxide (KOH) equivalents for oil analysis

Oil condition monitoring laboratories use potassium hydroxide equivalence. TBN is reported as milligrams of KOH per gram of oil. Many reports also show mg KOH per cubic centimeter when density is the controlling basis. The KOH reference is a convention across methods and labs. It ties results to a single base with consistent stoichiometry and molecular weight so different oils can be compared without guessing what base species they actually contain. The following is a little background.

KOH serves as the standard because it is stable in preparation, completely dissociates in aqueous-alcoholic media, and its neutralization factor is reliable. The measurement answers a simple question: how many milligrams of KOH would be required to neutralize the same number of acid equivalents found in 1 g of this oil? Different oils carry calcium sulfonates, phenates, salicylates, overbased detergents, and ashless neutralizing agents. The KOH yardstick lets them sit on the same scale.

Acid-base neutralization in the test follows equivalence. The acid titrant consumes the alkaline reserve until the endpoint is reached by potentiometric response or indicator shift. The consumed acid is back-calculated to an equivalent mass of KOH. Reporting as mg KOH per g or per cc keeps the number portable. Density differences between oils explain why some plants ask for mg KOH per cc. They track sump volumes and top-off by volume, not mass, so the per-cc basis ties better to field adds.

ASTM standards and why KOH became the global benchmark for TBN testing

The unit did not appear by accident. Older laboratory practice standardized on KOH for base number work because of solubility and well-known normality factors. Once manufacturers and test houses aligned on it, comparability followed. Engine builders and lubricant suppliers anchored limits and alarms to KOH units. Changing the unit would break long data sets that reliability depends on.

Two families of TBN methods see daily use. ASTM D2896 uses perchloric acid potentiometric titration. ASTM D4739 uses hydrochloric acid potentiometric titration. These choices are not cosmetic. They address different sample chemistries and service states.

New oils contain strong basic materials, including overbased detergents with high metal carbonate reserve, ash-forming species, metal hydroxides in micellar cores. Perchloric acid in glacial acetic solvent is a strong titrant system with high driving force. It neutralizes these strong bases and gives the full neutralization capacity of the package. D2896 reads higher on fresh oils because it counts species that weaker acids would not fully capture. It is effective in clean matrices with minimal oxidation products. It maps the advertised reserve that an oil can bring to an engine at service start.

Used oils change. Oxidation products, nitration species, fuel sulfur byproducts, wear metals, and soot all are degradation products commonly found in used oil. The strong oxidizing character of perchloric acid can interact with degradation products and confound endpoints. Hydrochloric acid in mixed solvent is milder and targets the remaining functional base species without overreacting to the damaged matrix. ASTM D4739 uses HCl to estimate the active alkalinity still available in service. It often reads lower than D2896 on the same used sample. That is useful to maintenance because it tracks the reserve that can still neutralize real acids in the system.

In this method, reactivity matters. Perchloric acid has much higher acid strength and oxidation potential. That property is an advantage for fresh oils with strong bases. It is a liability in dirty oils where side reactions blur the curve. Hydrochloric acid offers better specificity to the operational base reserve in used oil. The endpoint is cleaner in the presence of oxidation byproducts and fuel soot. This is why plants often set condemn limits on D4739 for in-service monitoring, and keep D2896 for new oil incoming QA.

How contaminants in used oil affect TBN measurements and reliability decisions

Contaminants complicate the landscape. Used oils carry metal soaps, weak organic acids, and combustion byproducts. HCl avoids pushing redox reactions that would inflate or distort the reading. New oils lack those interferences and benefit from the stronger system that fully neutralizes the detergent core. The purpose differs. New oil testing establishes the total reserve delivered to the crankcase. Used oil testing estimates how much is left to keep acids under control.

The units – mg KOH per gram – is the default in method math. Converting to mg KOH per cc is straightforward with density. Those that track volume-based makeup prefer per-cc units for easier arithmetic at the tank. Both bases point to the same neutralization capacity. Mixing them in one trend without conversion creates false drift, so keep the unit basis consistent on a given asset.

As for practical notes for reliability, always match the method to the question:

• For a baseline of new oil, D2896 is suitable. For in-service monitoring, D4739 aligns with the remaining basic reserve.
• Do not compare D2896 values to D4739 values one to one. They are not the same measurement.
• Confirm unit basis on reports and convert if needed.
• Check density if the lab reports mg KOH per cc and your history is mg KOH per g.
• Also, it's a good idea to establish alarm bands per method to avoid cross-method noise.

Failure modes tied to TBN loss include corrosive wear of bearings, increased oxidation rate, and sludge formation. Once TBN trends downward alongside rising TAN and rising oxidation FTIR peaks, drain intervals must be reevaluated. Fuel sulfur content and blow-by rate drive consumption. Overbased detergent chemistry and treat rate set the starting line. Soot handling and dispersancy hold the middle.

Key takeaways for tracking TBN in oil condition monitoring programs

Some summary points the crew can use. TBN is neutralization capacity expressed as KOH equivalent so different base chemistries can be compared. D2896 with perchloric acid fits new oils with strong bases. D4739 with HCl fits used oils with contaminants. Keep units consistent and look to tie limits to the method you use. Verify density when converting to per-cc and always trend with context, not single data points.

References and standards for the bench include ASTM D2896 Potentiometric TBN by perchloric acid, ASTM D4739 Potentiometric TBN by hydrochloric acid, ASTM D974 color-indicator acid and base numbers for some applications, less precise for modern additized oils but useful in certain applications.