Precautions for determining the acid value of biodiesel
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The acid value is the most important of all the indicators for biodiesel, as it determines the ‘quality grade’ of the product and also reflects the progress of the reaction process. The vast majority of companies use acid value testing as a means of monitoring production processes; therefore, the accuracy of this testing is extremely crucial. Here, I summarize some of my experiences from over a decade working in various companies for your reference. 1. Use of solvents: In order to cut costs, many enterprises, and analysts for the sake of convenience, use only ethanol rather than ether when determining acid values. This method of use is effective (with minimal error) for crude esters and finished methyl esters that have low acid values and good solubility. However, it is not accurate when applied to raw materials or asphalt. Therefore, it is recommended that enterprises use a mixture of ether and ethanol as the solvent when determining the acid value of raw materials and asphalt. For testing crude esters and finished methyl esters (during in-plant quality control), ethanol can be used; nevertheless, it is advisable to employ the mixed solvent when testing finished products prior to shipment. 2. Sample quantity: Acid value determination is carried out using volumetric analysis, with the color change of the indicator serving as the endpoint; therefore, color change is a crucial factor ； Due to the dark colors of the raw materials, crude esters, and asphalt, it is generally recommended to use a sample weight of around 0.5 g; for the finished methyl ester, a sample weight of around 5 g is suggested. 3. Types and concentrations of standard solutions: Generally, it is recommended to use KOH standard solutions, as potassium soaps have better solubility than sodium soaps, which makes it easier to obtain an accurate acid value. However, NaOH can also be used, with the need to pay attention to the conversion factors ； Next is the solution concentration; it is recommended to prepare standard solutions of two different concentrations – using a high-concentration standard solution for testing raw materials, and a low-concentration one for testing finished products and crude esters. 4. Acid value measurement after washing: Many companies use the method of measuring the acid value after washing when testing raw materials and crude esters. The intention behind this is to determine the true acid value of the sample, but such results actually run counter to that goal ； Since the acid value serves as a guide for production, water washing is often not carried out in actual manufacturing processes, especially before the alkaline catalysis step. If the acid value is measured by means of water washing at that time, the true actual acid value (including the amount of free sulfuric acid) is masked, which can affect the actual production process ； Furthermore, due to the viscosity and emulsifying properties of the material, the water cannot be completely separated after washing; as a result, the actual weight of the sample is \"lower\" due to the presence of water. This makes the measured acid value appear lower on the surface, suggesting that the reaction was thorough, but in reality the true acid value is masked. From my own experiments, I’ve found that the acid value after washing differs by up to twice compared to that without washing! 5. Solvent blank: Some companies do not perform a blank titration when testing the acid value, which affects the actual acid value readings; this is due to the fact that these companies’ laboratory technicians have not received proper training. 6. Solvent recovery: Some companies do not recycle the solvents they use, which results in a lot of waste. In reality, it is quite simple to recover solvents, and these recovered solvents can be used perfectly for control systems. This article is dedicated to my friends who always keep an eye on me! Your encouragement is my greatest motivation! June 26, 2012
This post was last edited by cwmtc on 2012-6-27 08:27. Thank you very much to Teacher Ning for sharing; I have learned a great deal from it! But there is one more thing I would like to ask Teacher Ning: Since the raw materials, crude esters, and asphalt are dark in color, it is difficult to detect color changes at the titration endpoint after dissolution. Are there any other better methods aside from reducing the amount of sample used and adding more solvent? Which indicator is better to choose?
In terms of indicator color, the national standard specifies two methods: the hot solvent method (using ethanol) and the cold solvent method. Depending on the indicator used, you can choose one based on the color of the crude ester obtained through different processes. Of course, increasing the amount of solvent can also be an option (especially during in-process control).
Teacher Ning: Do you have the national standards there? Could you share it? Thank you
Both the raw materials and asphalt are very dark in color; would it be possible to replace the phenolphthalein indicator specified in the standards with bromothymol blue indicator? What is the impact on the acid value?
I am extremely grateful to Teacher Ning for sharing; I have learned so much from it!
Personally, I think using alkali blue 6B as an indicator is quite good. However, 0.5g of sample is a bit too much when testing asphalt, as it makes it difficult to determine the titration endpoint; 0.2–0.3g is sufficient for crude oil. The concentration of KOH should not be too high – around 0.1N is adequate.
Teacher Ning’s comments are very insightful; I’ve learned a lot from them
I wonder if there are any online automatic devices for measuring acid value?
According to the national standards for the acid value of oils, ethanol is used for neutralization first before testing the sample; but is it possible to test the sample directly and then use a blank sample, and subtract the volumes of the two titrations from each other?