“Acid value”的意思、由来-开放百科全书

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Acid value

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In chemistry, acid value (or neutralization number or acid number or acidity) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance.^[1] The acid number is a measure of the number of carboxylic acid groups in a chemical compound, such as a fatty acid, or in a mixture of compounds. In a typical procedure, a known amount of sample dissolved in an organic solvent (often isopropanol) and titrated with a solution of potassium hydroxide (KOH) of known concentration using phenolphthalein as a color indicator.

The acid number is used to quantify the acidity of a substance e.g. biodiesel. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.

V_eq is the volume of titrant (ml) consumed by the crude oil sample and 1 ml of spiking solution at the equivalent point, b_eq is the volume of titrant (ml) consumed by 1 ml of spiking solution at the equivalent point, and 56.1 g/mol is the molecular weight of KOH. W_Oil is the mass of the sample in grams.

The molar concentration of titrant (N) is calculated as such:

In which W_KHP is the mass (g) of KHP in 50 ml of KHP standard solution, V_eq is the volume of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point, and 204.23 g/mol is the molecular weight of KHP.

There are standard methods for determining the acid number, such as ASTM D 974 and DIN 51558 (for mineral oils, biodiesel), or specifically for biodiesel using the European Standard EN 14104 and ASTM D664 are both widely used worldwide.

Acid number (mg KOH/g oil) for biodiesel should to be lower than 0.50 mg KOH/g in both EN 14214 and ASTM D6751 standard fuels. This is since the FFA produced may corrode automotive parts and these limits protect vehicle engines and fuel tanks.

As oil-fats rancidify, triglycerides are converted into fatty acids and glycerol, causing an increase in acid number. A similar observation is observed with biodiesel aging through analogous oxidation processes and when subjected to prolonged high temperatures (ester thermolysis) or through exposure to acids or bases (acid/base ester hydrolysis). Low Acid value indicates good cleansing by soap.

References

1. ^{{cite web | url=http://www.thefreedictionary.com/acid+value | title=Acid value | work=The Free Dictionary | accessdate=July 8, 2016}}

{{cite web | url=http://www.cmd.gov.hk/hkcmms/vol6/eng/E_Appendices_EXIV.pdf | title=Appendix XIV: Determination of Acid Value | work=cmd.gov.hk | accessdate=July 8, 2016}}

2 : Analytical chemistry|Lipids

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