View source for Chemistry:Acid value

{{also|Total acid number}}
{{Short description|Milligrams of a base needed to neutralize 1 gram of a given acid}}

In [[Chemistry:Chemistry|chemistry]], '''acid value''' ('''AV''', '''acid number''', '''neutralization number''' or '''acidity''') is a number used to quantify the acidity of a given [[Chemistry:Chemical substance|chemical substance]]. It is the quantity of [[Chemistry:Base|base]] (usually [[Chemistry:Potassium hydroxide|potassium hydroxide]] (KOH)), expressed as milligrams of KOH required to [[Chemistry:Neutralization|neutralize]] the acidic constituents in 1&nbsp;gram of a sample.<ref name="libretext">{{Cite web |date=2016-05-26 |title=14.10.1: Foods- Acid Value and the Quality of Fats and Oils |url=https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_ChemPRIME_(Moore_et_al.)/14%3A_Ionic_Equilibria_in_Aqueous_Solutions/14.10%3A_Titration_Curves/14.10.01%3A_Foods-_Acid_Value_and_the_Quality_of_Fats_and_Oils |df=mdy-all |access-date=2022-10-28 |website=Chemistry LibreTexts}}</ref><ref name="Ahuja2015">{{Cite book |last=Ahuja |first=Satinder |url=https://books.google.com/books?id=DHKDBAAAQBAJ&dq=acid+number&pg=PA301 |page=301 |oclc=900781294 |title=Food, Energy, and Water: The Chemistry Connection |df=mdy-all |date=2015-01-25 |publisher=Elsevier |isbn=9780128003749}}</ref><ref name="Nielsen2010">{{Cite book |last=Nielsen |first=S. Suzanne |oclc=663096771 |url=https://books.google.com/books?id=i5TdyXBiwRsC&dq=acid+value&pg=PA108 |pages=108–109 |title=Food Analysis Laboratory Manual, 2nd Edition |df=mdy-all |date=2010-03-20 |publisher=Springer Science & Business Media |isbn=9781441914637}}</ref><ref>{{Cite book |last=O'Brien |first=Richard D. |pages=220–221 |url=https://books.google.com/books?id=3wpHj3mvra8C&dq=acid%20value&pg=PA220 |title=Fats and Oils: Formulating and Processing for Applications, 3rd Edition |oclc=367589246 |df=mdy-all |date=2008-12-05 |publisher=CRC Press |isbn=9781420061673}}</ref> The acid value measures the acidity of water-insoluble substances like oils, fats, waxes and resins, which do not have a pH value.

The acid number is a measure of the number of [[Chemistry:Carboxylic acid|carboxylic acid]] groups ({{chem2|\sC(\dO)OH}}) in a chemical compound, such as a [[Chemistry:Fatty acid|fatty acid]], or in a [[Physics:Mixture|mixture]] of compounds.<ref name="Ahuja2015" /> In other words, it is a measure of free fatty acids (FFAs) present in a substance. In a typical procedure, a known amount of sample dissolved in an organic solvent (often [[Chemistry:Isopropyl alcohol|isopropanol]]) and [[Chemistry:Titration|titrated]] with a solution of alcoholic potassium hydroxide (KOH) of known [[Chemistry:Concentration|concentration]] using [[Chemistry:Phenolphthalein|phenolphthalein]] as a colour indicator.<ref name="Ahuja2015" /> The acid number for an oil sample is indicative of the age of the oil and can be used to determine when the oil must be changed.<ref name="engineer" />

A liquid fat sample combined with neutralized 95% ethanol is titrated with standardized sodium hydroxide of 0.1 eq/L normality to a phenolphthalein endpoint. The volume and normality of the sodium hydroxide are used, along with the weight of the sample, to calculate the free fatty acid value.<ref name="Nielsen2010" />

Acid value is usually measured as milligrams of KOH per gram of sample (mg KOH/g fat/oil), or grams of KOH per gram of sample (g KOH/g fat/oil).<ref name="engineer" />

==Calculations==
For example, for analysis of crude oil:<ref>{{Cite patent |number=CN103776825A |title=Determining method of acid value of deep-color grease |gdate=2014-05-07 |inventor1-first=Ji |inventor1-last=Junmin |inventor2-first=Wang |inventor2-last=Dongmin |inventor3-first=Liu |inventor3-last=Huamin |url=https://patents.google.com/patent/CN103776825A/en}}</ref>

:<math>AV=(V_{eq}-b_{eq})N\frac{\text{56.1 g/mol}}{W_\text{oil}}</math>

:Where KOH is the titrant, wherease crude oil is the titrand.
:{{mvar|V{{sub|eq}}}} is the volume of titrant (ml) consumed by the crude oil sample and 1&nbsp;ml of spiking solution at the equivalent point,
:{{mvar|b{{sub|eq}}}} is the volume of titrant (ml) consumed by 1&nbsp;ml of spiking solution at the equivalent point,
:56.1&nbsp;g/mol is the [[Physics:Molecular weight|molecular weight]] of KOH,
:{{math|''W''{{sub|oil}}}} is the mass of the sample in grams.

The normality (N) of titrant is calculated as:

:<math>N = \frac{1000 \times W_\text{KHP}}{\text{204.23 g/mol } \times V_{eq}}</math>

:Where {{math|''W''{{sub|KHP}}}} is the mass (g) of [[Chemistry:Potassium hydrogen phthalate|potassium hydrogen phthalate]] (KHP) in 50&nbsp;ml of KHP standard solution, 
:{{mvar|V{{sub|eq}}}} is the volume of titrant (ml) consumed by 50&nbsp;ml KHP standard solution at the equivalent point, 
:204.23&nbsp;g/mol is the molecular weight of KHP.

==Applications==
An increment in the amount of FFAs in a [[Chemistry:Fat|fat]] or [[Chemistry:Oil|oil]] sample indicates hydrolysis of triglycerides. Such reaction occurs by the action of [[Biology:Lipase|lipase]] [[Biology:Enzyme|enzyme]] and it is an indicator of inadequate processing and storage conditions. The source of the enzyme can be the tissue from which the oil or fat was extracted or it can be a contaminant from other cells including microorganisms.<ref name="libretext" />

For determining the acid value of [[Chemistry:Mineral oil|mineral oil]]s and [[Chemistry:Biodiesel|biodiesel]], there are standard methods such as ASTM D 974 and DIN 51558, and especially for biodiesel the European Standard EN 14104 and ASTM D664 are both widely used worldwide.<ref name="Ahuja2015" /> Acid value of biodiesel should be lower than 0.50 mg KOH/g in both EN 14214 and ASTM D6751 standard fuels. This is because the FFAs produced can corrode automotive parts, hence these limits protect vehicle engines and fuel tanks.<ref name="engineer" />

Low acid value indicates good cleansing by soap.<ref name="SciDir">{{cite web |title=Acid Value - an overview |website=[[ScienceDirect]] |url=https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/acid-value#:~:text=Lower%20acid,detergency |archive-url=https://web.archive.org/web/20220131133321/https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/acid-value#:~:text=Lower%20acid,detergency |archive-date=2022-01-31 |access-date=2022-10-28 |df=mdy-all }} [https://www.sciencedirect.com/science/article/pii/B9781630670658500128 '''link to original article''']</ref>

When oils and fats become rancid, [[Chemistry:Triglyceride|triglyceride]]s are converted into [[Chemistry:Fatty acid|fatty acid]]s and [[Chemistry:Glycerol|glycerol]], causing an increase in acid value.<ref name="Fernando2007">{{Cite journal |last1=Fernando |first1=Sandun |last2=Karra |first2=Prashanth |last3=Hernandez |first3=Rafael |last4=Jha |first4=Saroj Kumar |date=2007-05-01 |df=mdy-all |title=Effect of incompletely converted soybean oil on biodiesel quality |url=https://www.sciencedirect.com/science/article/pii/S0360544206001496 |journal=Energy |quote=The acid number can become a serious issue when feedstocks with high free fatty acids...  |volume=32 |issue=5 |pages=844–851 |doi=10.1016/j.energy.2006.06.019 |issn=0360-5442}}</ref> A similar situation is observed during aging of biodiesel through analogous oxidation and when subjected to prolonged high temperatures ([[Chemistry:Ester|ester]] [[Physics:Thermal decomposition|thermolysis]]) or through exposure to acids or bases (acid/base ester [[Chemistry:Hydrolysis|hydrolysis]]).<ref name="engineer">{{Cite web |title=Acid Value Number or Neutralization Number of Oil |url=https://www.engineersedge.com/lubrication/acid_number_neutralization_number.htm |access-date=2022-10-28 |df=mdy-all |website=www.engineersedge.com}}</ref>

[[Chemistry:Transesterification|Transesterification]] of waste cooking oil, having high acid value and high water content, can be performed using heteropolyacids such as dodecatungstophosphoric acid (PW12) as a catalyst.<ref name="Cao2008">{{Cite journal |last1=Cao |first1=Fenghua |last2=Chen |first2=Yang |last3=Zhai |first3=Fengying |last4=Li |first4=Jing |last5=Wang |first5=Jianghua |last6=Wang |first6=Xiaohong |last7=Wang |first7=Shengtian |last8=Zhu |first8=Weimin |df=mdy-all |date=2008-09-01 |title=Biodiesel production from high acid value waste frying oil catalyzed by superacid heteropolyacid |url=https://pubmed.ncbi.nlm.nih.gov/18646228/ |journal=[[Engineering:Biotechnology and Bioengineering|Biotechnology and Bioengineering]] |volume=101 |issue=1 |pages=93–100 |doi=10.1002/bit.21879 |issn=1097-0290 |pmid=18646228|s2cid=205497850 }}</ref><ref>{{Cite patent|number=US 8962873 B2|title=Systems And Methods For Making Bioproducts|gdate=2015-02-24|invent1=A|invent2=Rebecca|invent3=Danny|invent4=Barrie|inventor1-first=Summers William|inventor2-first=Williams|inventor3-first=Gulledge|inventor4-first=Tripp Robert |url=https://lens.org/128-931-785-027-93X}}</ref>

In 2007, Sahoo ''et al''. made biodiesel consisting of mono-esters of polanga oil extract of the plant [[Biology:Calophyllum inophyllum|Calophyllum inophyllum]] produced by triple stage transesterification and blended with high speed diesel, which was then tested for its use as a diesel substitute in a single cylinder diesel engine.<ref name="Sahoo2007">{{Cite journal |last1=Sahoo |first1=P. K. |last2=Das |first2=L. M. |last3=Babu |first3=M. K. G. |last4=Naik |first4=S. N. |date=2007-02-01 |df=mdy-all |title=Biodiesel development from high acid value polanga seed oil and performance evaluation in a CI engine |url=https://www.sciencedirect.com/science/article/pii/S0016236106003024 |journal=Fuel |volume=86 |issue=3 |pages=448–454 |doi=10.1016/j.fuel.2006.07.025 |issn=0016-2361}}</ref>

==Testing==
Total acidity, fatty acid profiles, and free fatty acids (FFAs) can be determined for oils such as [[Chemistry:Sunflower oil|sunflower]] and soybean oils obtained by green processes involving [[Physics:Supercritical carbon dioxide|supercritical carbon dioxide]] (scCO<sub>2</sub>) and pressurized liquid extraction (PLE). The identification and separation of the primary fatty acids responsible for acidity can ensure higher quality of fat and oil products.<ref>{{Cite journal |last1=Medeiros Vicentini-Polette |first1=Carolina |last2=Rodolfo Ramos |first2=Paulo |last3=Bernardo Gonçalves |first3=Cintia |last4=Lopes De Oliveira |first4=Alessandra |date=2021-12-30 |title=Determination of free fatty acids in crude vegetable oil samples obtained by high-pressure processes |df=mdy-all |journal=Food Chemistry: X |volume=12 |pages=100166 |doi=10.1016/j.fochx.2021.100166 |pmid=34825173 |pmc=8604745 |issn=2590-1575}}</ref>

In 2020, Dallas Group of America (DGA)<ref>{{Cite web |title=Products & Applications |url=https://dallasgrp.com/products-applications/#:~:text=DALSORB |access-date=2022-10-28 |website=The Dallas Group of America |language=en-US}}</ref> and American Oil Chemists' Society (AOCS) devised a standard method (5a-40) for testing free fatty acid in cooking oils.<ref>{{Cite journal |date=2022-11-26 |df=mdy-all |title=Validation of the Free Fatty Acid Test Kit for the Measurement of the Free Fatty Acid Content of Vegetable Oils, Fish Oils, Animal Fats (Tallows), Meat and Fish Meals, and Potato Chips and Grain-Based Snack Products: AOAC Performance Tested Method 052004 |journal=[[Chemistry:Journal of AOAC International|Journal of AOAC International]]|url=https://academic.oup.com/jaoac/article/104/2/300/5869795 |access-date=2022-10-28 |publisher=[[Organization:Oxford University Press|Oxford University Press]] (OUP); [[Organization:AOAC International|AOAC International]]}}</ref><ref name="testing" /> The DGA FFAs hand-held test kit was produced from the AOCS test method, but without the burets, flasks, and laboratory hardware. Its portable nature is convenient for both small and large frying operations. Testing next to the fryer or in the comfort of a laboratory setting is simple with the DGA FFAs test kit. It gives accurate results for cooking oil used in potato chips, corn dogs, meat browning, bread products, roasted peanuts, and more.<ref name="testing">{{Cite web |date=2021-03-02 |title=How to Test for Free Fatty Acid in Cooking Oil |archive-url=https://web.archive.org/web/20221012031948/https://dalsorb.com/how-to-test-free-fatty-acid/ |archive-date=2022-10-12 |url=https://dalsorb.com/how-to-test-free-fatty-acid/ |df=mdy-all |access-date=2022-10-28 |website=DALSORB}}</ref>

==Acid values of various fats and oils==
{| class="wikitable sortable"
! Fat / oil
! Acid value (mg KOH per g sample)
|-
|[[Chemistry:Beeswax|Beeswax]]
|{{nts|17}} – {{nts|36}}<ref>{{Cite book |last=Min |first=David B. |url=https://books.google.com/books?id=sPglndmgXU8C&dq=acid%20value%20of%2017%E2%80%9336&pg=PA102 |title=Food Lipids: Chemistry, Nutrition, and Biotechnology, Third Edition |df=mdy-all |date=2008-03-17 |oclc=213371194 |publisher=CRC Press |isbn=9781420046649}}</ref>
|-
|[[Biology:Canola oil|Canola oil]]
|{{nts|0.071}} – {{nts|0.073}}<ref name="Kardash2005" />
|-
|Maize oil
|{{nts|0.223}} – {{nts|0.224}}<ref name="Kardash2005">{{Cite journal |last1=Kardash |first1=Elena |last2=Tur'yan |first2=Yakov I. |date=2005-03-24 |title=Acid Value Determination in Vegetable Oils by Indirect Titration in Aqueous-alcohol Media |url=https://hrcak.srce.hr/file/4665 |archive-url=https://web.archive.org/web/20210226194541/https://hrcak.srce.hr/file/4665 |archive-date=2021-02-26 |df=mdy-all |journal=Croatica Chemica Acta |volume=78 |issue=1 |pages=99–103 |issn=0011-1643}}</ref>
|-
|Soyabean oil
|{{nts|0.60}} – {{nts|0.61}}<ref name="Kardash2005" />
|-
|Virgin olive oil
|{{nts|0.8}} – {{nts|2}}<ref>{{Cite journal |last1=Grossi |first1=Marco |last2=Lecce |first2=Giuseppe Di |last3=Toschi |first3=Tullia Gallina |last4=Riccò |first4=Bruno |date=May 2014 |title=Fast and Accurate Determination of Olive Oil Acidity by Electrochemical Impedance Spectroscopy |url=https://hal.archives-ouvertes.fr/hal-01276347/file/Fast%20and%20Accurate%20Determination%20of%20Olive%20Oil%20Acidity%20by%20Electrochemical%20Impedance%20Spectroscopy.pdf |journal=[[Engineering:IEEE Sensors Journal|IEEE Sensors Journal]] |volume=14 |issue=9 |pages=2947–2954 |doi=10.1109/JSEN.2014.2321323 |bibcode=2014ISenJ..14.2947G |s2cid=10659764 |issn=1558-1748}}</ref>
|-
|Used frying oil
|{{nts|0.1}} – {{nts|2.5}}<ref name="Park2020">{{Cite journal |last1=Park |first1=Jung Min |last2=Koh |first2=Jong Ho |last3=Kim |first3=Jin Man |date=2020-09-01 |df=mdy-all |title=Determining the Reuse of Frying Oil for Fried Sweet and Sour Pork according to Type of Oil and Frying Time |url=https://www.kosfaj.org/archive/view_article?pid=kosfa-40-5-785 |journal=Food Science of Animal Resources |volume=40 |issue=5 |pages=785–794 |doi=10.5851/kosfa.2020.e54 |pmid=32968730 |pmc=7492171 |issn=2636-0780}}</ref><ref name="Park2016">{{Cite journal |last1=Park |first1=Jung-Min |last2=Kim |first2=Jin-Man |date=2016-10-31 |df=mdy-all |title=Monitoring of Used Frying Oils and Frying Times for Frying Chicken Nuggets Using Peroxide Value and Acid Value |journal=Korean Journal for Food Science of Animal Resources |volume=36 |issue=5 |pages=612–616 |doi=10.5851/kosfa.2016.36.5.612 |issn=1225-8563 |pmc=5112423 |pmid=27857536}}</ref>
|}

==See also==
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==References==
{{reflist}}

[[Category:Analytical chemistry]]
[[Category:Lipids]]

{{Sourceattribution|Acid value}}