Fuels – Paraffins, aromatics, naphthenes, olefins

Acidity in petroleum products as per ASTM D8045

View of an oil refinery

ASTM D8045, released in 2016, describes the determination of the acid number in crude oil and petroleum products with thermometry. This method offers a variety of advantages over potentiometric titration:

  • Analyses yield a well-defined end point, thus improving reproducibility of results
  • Analyses are much faster
  • Sensor maintenance is not required

> Learn more about thermometry and ASTM D8045

In step with international standards

The importance of fossil fuels is reflected by the large number of standards relating to them. Metrohm as a leading manufacturer of analytical instruments offers long-standing application know-how for quality control of fossil fuels.

> Find an overview of relevant standards here

Acid and base numbers – Important quality parameters

Industrial petro plant at dusk

In the same way as the base number (BN) is a sum parameter for alkaline compounds, the acid number (AN) is a measure for acidic compounds. Both parameters are crucial to assure the quality of the oil.

These two parameters can be determined by nonaqueous acid-base titration with potentiometric, photometric, thermometric, or conductometric end point detection.

> Overview of standards for BN and AN analysis in oil


Potentiometric titration

Both parameters are determined by potentiometric titration in nonaqueous solvents or solvent mixtures. Various standards exist for the analysis of both parameters, e.g.:

  • ASTM D664 (AN),
  • EN 12634 (AN),
  • ASTM D4739 (BN),
  • ISO 3771 (BN).

These titrimetric determinations can be completely automated – from the addition of solvents to the reproducible cleaning of the electrode. Oil samples can even be weighed in fully automatically before titration. This guarantees complete traceability.

> Learn more about efficient acid number analysis according to ASTM D664
> Learn more about a fully automated system with integrated weighing

Photometric titration

AN and BN may also be determined by photometric titration with color indication of the equivalence point according to ASTM D974 or ISO 6618. For this application, Metrohm offers the 100% solvent-resistant Optrode with the possibility to automate photometric titration.

Related products

Fast thermometric titration

Acid number determination can be done easily and quickly in accordance with ASTM D8045 using thermometric titration. Electrode maintenance is not an issue when using thermometric titration. Contamination and matrix influences do not impair the sensor. It also works without rehydration. In comparison to potentiometric titration, thermometric titration is approximately three times faster and can be fully automated, as shown in the TAN determination of the titration curve. 
> Learn more about thermometric titration (as per ASTM D8045)
> Learn more about the Titrotherm instrument for thermometric titration

Solvotrode easyClean: a combined pH glass electrode that can be easily cleaned even of strong contamination.

Optrode: Sensor for titration with photometric endpoint recognition.


Water determination in petroleum products

Petrochemical plant
Water occurs as a contaminant in virtually all petroleum products.

It reduces lubricant properties, promotes microbial oil degradation, leads to sludge formation in the tank, and promotes corrosion.

Therefore, it is very important to know the water content of petroleum products. Karl Fischer (KF) titration, owing to its excellent reproducibility and accuracy as well as its ease of use, ranks amongst the most important water determination methods and accordingly figures in numerous international standards. Because of the low water content in petroleum products, KF coulometry is usually applied.


Water in liquid butane as per ASTM D7995

Ship with LPG or LNG tanks

ASTM released a new norm in 2019 regarding the measurement of water in liquid butane with a gas sampler and coulometric Karl Fischer titration.

The 875 KF Gas Analyzer is one of the instruments that fully fulfills the requirements stated in the norm. Water in butane or propane can cause corrosion problems. Other malfunctions can also occur, such as icy valves. These problems can be actively avoided by accurately monitoring the water content of the liquid gas.

> Learn more about the 875 KF Gas Analyzer

Halogen and sulfur in fuels by Combustion Ion Chromatography (CIC)

Diagram of combustion ion chromatography for halogen analysis by pyrohydrolysis
Adverse effects of halogen- and sulfur-containing fuels

The burning of sulfur-containing fuels leads to the emission of air-polluting sulfur oxides into the atmosphere. Additionally, high sulfur concentrations negatively impact engine performance. Similarly, the presence of corrosive halogens must be minimized, in particular with regard to the refining process.

Combustion IC

Combustion IC is a fully automated method for the pyrohydrolytic determination of individual halogen and sulfur in a multitude of petro products, regardless of whether they are in liquid, solid, or gaseous form. CIC excels in terms of sample throughput, measuring range, and precision as well as accuracy.

The image above shows the Combustion IC method. The gaseous combustion products that exit the oven are fed into an oxidizing absorption solution and detected as sulfate and halide by way of subsequent ion chromatography.


NIRS for optimizing blending and refinery processes

Research Octane Number (RON) and Reid vapor pressure control parameters determined in gasoline blending
Straightforward blending process with NIRS

During the blending process, different fractions of the crude oil distillation are mixed together so as to produce the ready-to-sell diesel or gasoline grades. This is most economical when it is carried out in process systems that work online and can be automated. The endpoint of the blending process is reached when the required fuel specifications are achieved. Key characteristics which indicate the progress of the blending process are the cetane number for diesel blends and one of the octane ratings for gasoline blends. NIRS sensors located directly in the process enable the entire process to be controlled and ensure a high-quality end product. Additional parameters can be monitored in parallel.
To refine crude oil and natural gas into diverse petrochemical products, complex processes are required, from crude oil distillation, cracking, and desulfurization to reforming. All these processes are controlled by near-infrared spectroscopy (NIRS) – whether to determine characteristics or the composition of petroleum products.

> Learn more

Process image (image on the left side) of the Research Octane Number (RON) and Reid vapor pressure control parameters determined during the blending process of gasoline using NIRS. Both parameters can be tracked simultaneously via the live view on the control panel (the black/green section between the two trend curves).

Read more Process Analytics applications


Further applications

Electrochemistry cell with potentiostat/galvanostat


In the petrochemical and biofuel industry, corrosion is a common problem that threatens man and machine and that generates significant production costs. It has been reported that in the USA alone pipeline corrosion accounts for over 20 million US dollars per year. Metrohm Autolab offers a complete product range for investigating the corrosion properties of materials, both for pure and applied research.

Application Finder

Testimonial: CEPSA Research Center

"For over 30 years, our laboratories have had Metrohm equipment, such as titrators to measure surfactants in detergents and the bromine number in olefins as well as the oxidation stability of FAMES and diesel oil. They are all very useful and reliable pieces of equipment."

Cepsa is a Spanish energy company active in the oil and gas sector. They use numerous Metrohm instruments for the characterization of oils and fuels.