Web based translation of text from Yellow Book 69 relating to acid value

Yellow Book No. 69 - Accompanying research for the standardisation of rapeseed oil as fuel for vegetable oil suited diesel engines in vehicles and combined heat and power plants

Acid number The acid number and/or acid number is a measure for the content of free fatty acids in the vegetable oil or FAME and describes the quantity of caustic potash solution, which is necessary for the neutralization of the free fatty acids. The acid number depends strongly on the refining degree and the aging degree of an oil. By water in the oil as well as micro organisms and enzymes, a hydrolytic splitting of the Triglyceride can occur and lead to a rise of the acid number. Sour connections in the fuel lead to corrosion, wear and accumulation of arrears in the engine. In addition free fatty acids can react with basic components of engine oil.

Saponification number

The saponification number describes the quantity of caustic potash solution, which is necessary for the neutralization of the free acids and for the hydrolysis of the esters in the oil. Since by the fatty acids a molecule KOH is bound in each case, the saponification number is indirectly a measure for the average molecular weight of the Triglyceride and thus a characteristic characteristic. With oils with a high portion of short fatty acids as ester components high saponification numbers arise in reverse and. The saponification number is thus the larger, the smaller the average molecular weight of the fatty acids is. In the engine technology is the saponification number of small interest, however for the description of the shelf-life is consulted. Better for it however oxidation stability and acid number are suitable.

Acid number

limit value: max.: 2.0 mg KOH/g testing method: DIN EN ISO 660 [ 16 ] the acid number or acid number indicates the quantity of potassium hydroxide, which is necessary for the neutralization of the free fatty acids occurring in 1 g sample. For the determination of the acid number the oil sample is loosened in a solvent and titrated with a solution from sodium or potassium hydroxide using an indicator. Will indicated the repeatability as 3 %, the comparability with 15 %. the characteristic value acid number and the testing method specified above are also to the company ME standard to be taken up. The acid number of rapeseed oil fuel is a variable characteristic. It affected for example by the storage conditions of the seed and the oil. The acid number is a measure for the portion of free fatty acids in the oil and permits conclusions on the continuation of oil ageing, for example by hydrolytic splitting.

Hydrolytic splitting begins with presence of micro organisms or enzymes and water and can already occur in the oil seed. Short fatty acids become more easily removed than long fatty acids. Illustration 19 shows the acid number of rapeseed oil fuels of different origin. As the median of 0,96 mg KOH/g shows, the limit value of 2,0 mg is good KOH/g to keep. The high values for the acid number are remarkable with the rapeseed oil samples from the oil mills G and D. The erucasaeurehaltige oil sample D ** exhibits the highest, the rapeseed oil full raffinate R the lowest acid number from the examined samples. Both rapeseed oil samples from the combined heat and power unit D cross the limit value. These samples were taken to an accelerated aging out of the heated daily tank of the combined heat and power and were subjected probably by high storage temperatures or the rapeseed oil fuel exhibited already starting from oil mill D a increased acid number. By interest also the values for the oxidation stability of these two oil samples, represented in illustration 34, S 107.

In illustration 20 the influence of sort and cultivation on the characteristic value acid number is represented. With the winter RWS sorts of the treatment stages with and without additional plant protection and sheet fertilization (m.B. and o.B.) obviously the tendency exists that an additional sheet fertilization positively affects developing and thus on the content of free fatty acids. Also the smaller seed density with the cultivation attempt with the sorts Artus and rapidly shows a favorable effect on the acid number. rapeseed oils from the summer RWS sorts star and Hyola exhibit altogether a lower acid number than the winter RWS sorts. Illustration 21 shows that the limit value is not kept by the rapeseed oil samples from the interlaboratory test. It is to be noted that the values were intended for the acid number of these samples according to DIN 51558-1 [ 30 ] and are possibly not directly with the analysis values according to DIN EN ISO 660 comparable. A cause for the high acid numbers of these rapeseed oil samples is supposed that the oil sent for analysis was drawn from uncleaned oil into the sample containers and thus the oil as well as the sediment was stored. The regulation took place at an oil sample taken out of the projection. In the sediment existing enzymes from the seed components accelerate oil aging and increase the portion of free fatty acids in the oil.

correlation between acid number and oxidation stability

acid number and oxidation stability is characteristic values for the ageing of rapeseed oil fuel. They describe however different procedures in the oil, which however partially by the same parameters, as temperature, light and water are affected. A high portion of free fatty acids in the oil possibly leads to a short induction period with the measurement of oxidation stability. The graphic evaluation of the analysis values in illustration 43 does not show obvious connection between acid number and oxidation stability.

Correlation between total contamination and acid number

Enzymes from seed particles can affect the hydrolysis in the rapeseed oil. The analyzed total contamination in the rapeseed oil samples correlated however with the acid number, the measure for the portion of free fatty acids in the oil illustration 43 does not show the dependence of the acid number on the total contamination with the rapeseed oil samples from oelgewinnungsanlagen and combined heat and power plants.

Correlation between water content and acid number

Water in the oil is the condition for hydrolytic splitting of triglycerides and thus the occurrence of free fatty acids. The water content would therefore have with the acid number which measure for the portion of free fatty acids, to correlate. Illustration 46 does not show for the examined samples that in the concentrations in those water usually arises in rapeseed oil, connection with the acid number exists.

Correlation between acid number and flash point

Free fatty acids possibly develops with the heating faster easy-inflammatory decomposition products than from Triglyceriden. The acid number as measure for the portion of free fatty acids in the oil could correlate therefore with the flash point. Illustration 49 shows that with the examined rapeseed oil samples between the acid number and the flash point no obvious connection exists.

6,6 Quick tests

which quick tests are suitable for current quality control. They do not replace however laboratory analysis under the specified test conditions.

For the characteristic values water content, acid number and total contamination could be developed quick tests, which it makes a rough estimation possible whether the limit values from the quality standard can be kept.

For the characteristic value phosphor a practicable rapid determination method could not be developed, since the necessary chemicals solutions require extensive safety precautions.

Quick tests for the water content, acid number and phosphorgehalt were worked on by the company ASG, Augsburg. The quick test total contamination was developed by the Bavarian national institute for agricultural engineering. Described quick tests are to be arranged in a suit-case that the company ASG, Augsburg will have similarly to

  1. 6.2 quick test for acid number the execution of the quick tests acid number water content takes place to the quick test. With the quick test acid number is however Headspace Headspace-Glae filled following DIN EN ISO 660 [ 19 ] with sodium hydroxide solution, a solvent mixture and the indicator p-Naphtolbenzein. The volume of this mixture amounts to 10 ml and leads with addition of a rapeseed oil sample of 2 ml with an acid number more largely equivalent 2,0 mg KOH/g to a color change of blue after red.

Regulation for the execution of the quick tests acid number

  1. the sampling from rapeseed oil fuel to the determination of the acid number, must take place into a dry, clean container, which is filled to approximately 2/3.
  2. The rapeseed oil sample is homogenized for example by fuenfminuetiges strong vibrating.
  3. With a einmalspritze without Kanuele exactly 2 ml of bubble-free rapeseed oil sample. Then a Einmalkanuele is put on and the sample slowly into the test glass with the reagent is injected. It is observed whether a color change of blue takes place after red. The color change takes place with rapeseed oil samples with an acid number from 2,0 mg KOH/g accurately after addition from 2 ml. If no color change is observed, the limit value with high probability is not crossed.
  4. If after addition of 2 ml rapeseed oil sample color change is not observed, the one-way syringe with the rapeseed oil sample should be filled again and so long rapeseed oil into the reagent in Testglae to be slowly course-dripped, until the color change occurs. The used up rapeseed oil quantity is read off from the syringe and measured on the basis the following equation the acid number in the rapeseed oil sample:

x: Acid number of the rapeseed oil sample in mg KOH/g y: Volume of the altogether course-dripped rapeseed oil sample in ml

  1. the test glass with the mixture from rapeseed oil and the testloesung must be duly disposed. Unspent testloesung contains free alkali and may therefore only with eye protector and protection equipment be handled. In the suit-case ready for use filled Testglae is made available.

Analysis procedures for vegetable oils Dr. Thomas Wilharm, analytics service company, Neusaess the foils used in the lecture are attached in the appendix. Dr. Thomas Wilharm presents first, which physical and chemical characteristics should be taken up by vegetable oils to a requirement standard. There is on the one hand characteristics, which permit the identification of the fuel, e.g. the distinction of rapeseed oil fuel and Diesel fuel. With the characteristic values density and iodine number can be for example clearly from each other differentiated the two fuels mentioned. On the other hand there is characteristics, which exert influence on the burn and the motor behavior and for the certain minimum qualities be specified must. Fuel characteristics, which affect the motor behavior, are for example the ignitability, the koksrueckstand after Conradson and the phosphorgehalt.

Dr. Wilharm points out that certain methods of analysis were compiled particularly for fossil Diesel fuels, however also for vegetable oil validity possesses. It should take place an examination, to what extent the characteristic values obtained thereby are comparable and are as meaningful these. The characteristic values, which are taken up to a requirement standard for rapeseed oil as fuel, must consider the chemistry of vegetable oils.

Thus for example fatty acids with double bonds lower the oxidation stability of vegetable oils. A double bond decreases the oxidation stability of the vegetable oils by the factor 20, two double bonds by the factor 200 and three double bonds by approx. the factor 1000. The ester connections are hydrolytically unstable. If the hydrolysis is favoured by enzymes (the hydrolase), the presence of water, micro organisms, increased temperature and metals, e.g. copper.

Characteristic values of the requirement standard, which serve the characterisation of the material, are to a large extent constantly because natural. These are with rapeseed oil as fuel for example the density, the kinematic viscosity, the heat value, the ignitability (Cetane number) and the iodine number. These characteristic values do not have to be intended therefore for each load, if the supplier of the product is reliable.

Other characteristics become by kind and condition of the seed, which oil production process as well as storage and transport affect. These are for rapeseed oil as fuel for example the total contamination, ash, the water content, the phosphor and the acid number.

The content of ash as test criteria is also a component of the requirement standard for Diesel fuel and detected for example sodium and potassium ions, which can cause hot corrosion. The ash content with Diesel fuel is affected primarily by inappropriate storage.

Factors of influence for the water content of rapeseed oil are the seed dampness, the storage and transport. The water content in the Diesel fuel is more critical of regarding than in the vegetable oil. Since fossil Diesel fuel of nonpolar is as vegetable oil, is formed here already with smaller water admixtures a boundary layer water/Diesel, so that micro organisms can grow. With fossil Diesel fuel the limit value for water was therefore set lower than with rapeseed oil Diesel fuel.

The acid number is a measure for the portion of free fatty acids in the oil. The occurrence of free fatty acids becomes by the fat hydrolysis (s.o.) affected. Free fatty acids lead to corrosion and react with basic components of engine oil. It comes to oxidation and polymerization procedures in engine oil. The acid number of vegetable oil besides by the sampling and conditions of support of the samples is affected (samples are always cold stored).

During revising the requirement standard for rapeseed oil as fuel it is to be considered according to opinion of Dr. Wilharm whether additionally a characteristic value for oxidation stability and the alkali content (sodium, potassium) should be taken up.

With the following characteristics changes must be made according to opinion by Dr. Wilharm:

Can the oil seed before the processing be tested whether already aged you? ?.

An examination of the acid number and oxidation stability is possible new realizations by current research projects at the agricultural engineering Weihenstephan is expected. ?.

Does a connection exist between the portion of free fatty acids, certainly over the acid number and oxidation stability? If, which characteristic is more meaningfully? ?.

The characteristics describe different procedures: The acid number describes the continuation of the hydrolysis, oxidation stability describes the Vorschaedigung of the product by oxidation procedures. Both characteristics give referring to oil aging.


It is suggested, the limit value for the acid number of max. to raise 1,5 to 2,0 mg KOH/g. The engines can bear oils with a substantially higher acid number of up to 5 mg KOH/g. Problems develop with the entry of free fatty acids in engine oil.

From view of the oil mill operators a rise of the acid number is desirable. Oxidation stability and the acid number do not correlate: The acid number gives referring to acceleration to oil aging, oxidation stability gives to reference in which measure the oil by oxygen is pre-aged, usually accompanies however a rise of the acid number and a decrease of oxidation stability with a viscosity rise.

Acid number. variable characteristic. In place of the testing method DIN 51 558 T1 the testing method ISO 660. steps The limit value becomes on max. 2.0 mg KOH/g raised.

Acid number: The acid number describes the content of free fatty acids in the vegetable oil. The demanded limit value of 2,0 mg KOH/g can be kept by most examined oil samples. Only erucasaeurehaltige rapeseed oils and samples, which were taken out of a heated tank, exhibit values of more than 2.0 mg KOH/g.

Oxidation stability: A pre-aging of the fuel can be examined by determination of oxidation stability. The measuring instrument suitable for it is the so-called Ranzimat, with air into the oil sample held on a constant temperature level is introduced. From the oil sample escaping volatile products are led into a second container with entmineralisiertem water, in which a conductivity measuring cell is. The duration of the induktionsperiode is a measure for the stability of the oil samples in relation to oxidation. Problematic fuel with low oxidation stability can behave particularly in the injection system, if it comes by verharzungen to accumulation of arrears into fuel pipes, in the injection pump as well as at nozzles and nozzle holders. With the examined samples oxidation stability was examined with 110°C and 120°C. The results (see appendix) show that with samples, those were taken out of tanks and in particular with the samples, which were exposed to thermal load (e.g. in a heated daily tank) oxidation stability are clearly lower than with fresh oil samples. Also a % admixture of RME or Diesel fuel leads 10 to a reduced oxidation stability. In the last meeting the question emerged whether the ageing of oils was sufficient fixed not already by the acid number. The application of oxidation stability as a function of the acid number does not show a correlation within the examined range. The acid number suggests only the current content in free fatty acids, which rises initially with removing oxidation stability, with strongly advanced pre-aging in addition, again to decrease can. Also a correlation between the iodine number and oxidation stability could not be determined, although with increasing number of double bonds the oxidative decay could be accelerated. A reason for it is that only a close spectrum at iodine number values was regarded here. If one became still further vegetable oils to it-takes, a dependence could presumably be observed.

Acid number: Limit value FFA: Ask discussed fully?

11-acid number

Collection of the statements and suggestions of the participants:

?. With bio Diesel the limit value was determined at 0,5 mg KOH/g, higher values can lead to corrosion. ?. This is confirmed by an engine manufacturer ?. Therefore a good RWS seed quality is necessary with low water contents ?. Further factors of influence on the acid number are the reifegrad of the seed and the portion of break material, which develop with transport and storage. This should be removed ?. Also that humidity content of the RWS seed, as well as the degree of the mikrobiellen decomposition have influence on the acid number ?. An engine manufacturer points out that the engines are adapted by an appropriate choice of materials also to the higher acid number by vegetable oil. Depending upon kind of the used metals, acid numbers of 0,5 to 15 mg KOH/g are possible; whether the limit value is specified for the acid number on 2,0 or 3,0 mg KOH/g, is less crucial thereby ?. A participant expresses itself for the fact that the acid number should not be lowered?. A further participant does not express itself for as low a value as possible, since frequently conventional engines are reequipped, with those an exchange of engine construction units made of unsuitable metals effected ?. It is suggested that further data are to be collected for later examination and the limit value with 2,0 mg KOH/g leave.

Resolution: Acid number ?. Testing method DIN EN ISO 660 ?. The limit value remains with max. 2.0 mg KOH/g ?. Further oil samples are to analyze around the stop bar of the limit value to examine Should a weighting of the characteristic values be made

for the quality assurance from greatest importance are: Total contamination, acid number, oxidation stability, phosphorgehalt and water content. A weighting is however not made.

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Authors Darren, Chris Watkins
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Language English (en)
Translations Russian, Indonesian
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Created January 25, 2008 by Darren
Modified April 1, 2024 by Irene Delgado
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