Chemistry:Acid value

From HandWiki
See also: Total acid number
Short description: Milligrams of a base needed to neutralize 1 gram of a given acid


In chemistry, acid value (AV, acid number, neutralization number or acidity) is a number used to quantify the acidity of a given chemical substance. It is the quantity of base (usually potassium hydroxide (KOH)), expressed as milligrams of KOH required to neutralize the acidic constituents in 1 gram of a sample.[1][2][3][4] 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 carboxylic acid groups (–C(=O)OH) in a chemical compound, such as a fatty acid, or in a mixture of compounds.[2] 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 isopropanol) and titrated with a solution of alcoholic potassium hydroxide (KOH) of known concentration using phenolphthalein as a colour indicator.[2] 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.[5]

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.[3]

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).[5]

Calculations

For example, for analysis of crude oil:[6]

[math]\displaystyle{ 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.
Veq is the volume of titrant (ml) consumed by the crude oil sample and 1 ml of spiking solution at the equivalent point,
beq is the volume of titrant (ml) consumed by 1 ml of spiking solution at the equivalent point,
56.1 g/mol is the molecular weight of KOH,
Woil is the mass of the sample in grams.

The normality (N) of titrant is calculated as:

[math]\displaystyle{ N = \frac{1000 \times W_\text{KHP}}{\text{204.23 g/mol } \times V_{eq}} }[/math]
Where WKHP is the mass (g) of potassium hydrogen phthalate (KHP) in 50 ml of KHP standard solution,
Veq is the volume of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point,
204.23 g/mol is the molecular weight of KHP.

Applications

An increment in the amount of FFAs in a fat or oil sample indicates hydrolysis of triglycerides. Such reaction occurs by the action of lipase 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.[1]

For determining the acid value of mineral oils and 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.[2] 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.[5]

Low acid value indicates good cleansing by soap.[7]

When oils and fats become rancid, triglycerides are converted into fatty acids and glycerol, causing an increase in acid value.[8] A similar situation is observed during aging of biodiesel through analogous oxidation and when subjected to prolonged high temperatures (ester thermolysis) or through exposure to acids or bases (acid/base ester hydrolysis).[5]

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.[9][10]

In 2007, Sahoo et al. made biodiesel consisting of mono-esters of polanga oil extract of the plant 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.[11]

Testing

Total acidity, fatty acid profiles, and free fatty acids (FFAs) can be determined for oils such as sunflower and soybean oils obtained by green processes involving supercritical carbon dioxide (scCO2) 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.[12]

In 2020, Dallas Group of America (DGA)[13] and American Oil Chemists' Society (AOCS) devised a standard method (5a-40) for testing free fatty acid in cooking oils.[14][15] 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.[15]

Acid values of various fats and oils

Fat / oil Acid value (mg KOH per g sample)
Beeswax 17 – 36[16]
Canola oil 0.071 – 0.073[17]
Maize oil 0.223 – 0.224[17]
Soyabean oil 0.60 – 0.61[17]
Virgin olive oil 0.8 – 2[18]
Used frying oil 0.1 – 2.5[19][20]

See also

References

  1. ↑ 1.0 1.1 "14.10.1: Foods- Acid Value and the Quality of Fats and Oils". 2016-05-26. 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. 
  2. ↑ 2.0 2.1 2.2 2.3 Ahuja, Satinder (2015-01-25). Food, Energy, and Water: The Chemistry Connection. Elsevier. p. 301. ISBN 9780128003749. OCLC 900781294. https://books.google.com/books?id=DHKDBAAAQBAJ&dq=acid+number&pg=PA301. 
  3. ↑ 3.0 3.1 Nielsen, S. Suzanne (2010-03-20). Food Analysis Laboratory Manual, 2nd Edition. Springer Science & Business Media. pp. 108–109. ISBN 9781441914637. OCLC 663096771. https://books.google.com/books?id=i5TdyXBiwRsC&dq=acid+value&pg=PA108. 
  4. ↑ O'Brien, Richard D. (2008-12-05). Fats and Oils: Formulating and Processing for Applications, 3rd Edition. CRC Press. pp. 220–221. ISBN 9781420061673. OCLC 367589246. https://books.google.com/books?id=3wpHj3mvra8C&dq=acid%20value&pg=PA220. 
  5. ↑ 5.0 5.1 5.2 5.3 "Acid Value Number or Neutralization Number of Oil". https://www.engineersedge.com/lubrication/acid_number_neutralization_number.htm. 
  6. ↑ Junmin, Ji; Wang Dongmin & Liu Huamin, "Determining method of acid value of deep-color grease", patent CN103776825A, issued 2014-05-07
  7. ↑ "Acid Value - an overview". https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/acid-value#:~:text=Lower%20acid,detergency.  link to original article
  8. ↑ Fernando, Sandun; Karra, Prashanth; Hernandez, Rafael; Jha, Saroj Kumar (2007-05-01). "Effect of incompletely converted soybean oil on biodiesel quality". Energy 32 (5): 844–851. doi:10.1016/j.energy.2006.06.019. ISSN 0360-5442. https://www.sciencedirect.com/science/article/pii/S0360544206001496. "The acid number can become a serious issue when feedstocks with high free fatty acids...". 
  9. ↑ Cao, Fenghua; Chen, Yang; Zhai, Fengying; Li, Jing; Wang, Jianghua; Wang, Xiaohong; Wang, Shengtian; Zhu, Weimin (2008-09-01). "Biodiesel production from high acid value waste frying oil catalyzed by superacid heteropolyacid". Biotechnology and Bioengineering 101 (1): 93–100. doi:10.1002/bit.21879. ISSN 1097-0290. PMID 18646228. https://pubmed.ncbi.nlm.nih.gov/18646228/. 
  10. ↑ , Summers William; Williams Rebecca & Gulledge Danny et al."Systems And Methods For Making Bioproducts" 8962873 B2 patent US 8962873 B2, issued 2015-02-24
  11. ↑ Sahoo, P. K.; Das, L. M.; Babu, M. K. G.; Naik, S. N. (2007-02-01). "Biodiesel development from high acid value polanga seed oil and performance evaluation in a CI engine". Fuel 86 (3): 448–454. doi:10.1016/j.fuel.2006.07.025. ISSN 0016-2361. https://www.sciencedirect.com/science/article/pii/S0016236106003024. 
  12. ↑ Medeiros Vicentini-Polette, Carolina; Rodolfo Ramos, Paulo; Bernardo Gonçalves, Cintia; Lopes De Oliveira, Alessandra (2021-12-30). "Determination of free fatty acids in crude vegetable oil samples obtained by high-pressure processes". Food Chemistry: X 12: 100166. doi:10.1016/j.fochx.2021.100166. ISSN 2590-1575. PMID 34825173. 
  13. ↑ "Products & Applications" (in en-US). https://dallasgrp.com/products-applications/#:~:text=DALSORB. 
  14. ↑ "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 of AOAC International (Oxford University Press (OUP); AOAC International). 2022-11-26. https://academic.oup.com/jaoac/article/104/2/300/5869795. Retrieved 2022-10-28. 
  15. ↑ 15.0 15.1 "How to Test for Free Fatty Acid in Cooking Oil". 2021-03-02. https://dalsorb.com/how-to-test-free-fatty-acid/. 
  16. ↑ Min, David B. (2008-03-17). Food Lipids: Chemistry, Nutrition, and Biotechnology, Third Edition. CRC Press. ISBN 9781420046649. OCLC 213371194. https://books.google.com/books?id=sPglndmgXU8C&dq=acid%20value%20of%2017%E2%80%9336&pg=PA102. 
  17. ↑ 17.0 17.1 17.2 Kardash, Elena; Tur'yan, Yakov I. (2005-03-24). "Acid Value Determination in Vegetable Oils by Indirect Titration in Aqueous-alcohol Media". Croatica Chemica Acta 78 (1): 99–103. ISSN 0011-1643. https://hrcak.srce.hr/file/4665. 
  18. ↑ Grossi, Marco; Lecce, Giuseppe Di; Toschi, Tullia Gallina; Riccò, Bruno (May 2014). "Fast and Accurate Determination of Olive Oil Acidity by Electrochemical Impedance Spectroscopy". IEEE Sensors Journal 14 (9): 2947–2954. doi:10.1109/JSEN.2014.2321323. ISSN 1558-1748. Bibcode: 2014ISenJ..14.2947G. https://hal.archives-ouvertes.fr/hal-01276347/file/Fast%20and%20Accurate%20Determination%20of%20Olive%20Oil%20Acidity%20by%20Electrochemical%20Impedance%20Spectroscopy.pdf. 
  19. ↑ Park, Jung Min; Koh, Jong Ho; Kim, Jin Man (2020-09-01). "Determining the Reuse of Frying Oil for Fried Sweet and Sour Pork according to Type of Oil and Frying Time". Food Science of Animal Resources 40 (5): 785–794. doi:10.5851/kosfa.2020.e54. ISSN 2636-0780. PMID 32968730. PMC 7492171. https://www.kosfaj.org/archive/view_article?pid=kosfa-40-5-785. 
  20. ↑ Park, Jung-Min; Kim, Jin-Man (2016-10-31). "Monitoring of Used Frying Oils and Frying Times for Frying Chicken Nuggets Using Peroxide Value and Acid Value". Korean Journal for Food Science of Animal Resources 36 (5): 612–616. doi:10.5851/kosfa.2016.36.5.612. ISSN 1225-8563. PMID 27857536. 



Retrieved from "https://handwiki.org/wiki/index.php?title=Chemistry:Acid_value&oldid=3337877"