(7.3)--任务2测定酸度、酸值.ppt
Task 2 Determination of acidity and acid valueI.Determination of acidity,the significance of acid value 1、Acidity,acid value Through the previous study,we know acidity and acid value of petroleum products ares used to measure the amount of acidic substances in oil products.The quality of potassium hydroxide required for neutralizing 100 ml petroleum products is called acidity,expressed as mgKOH/100mL.The quality of potassium hydroxide required for neutralizing 1g acid substances in petroleum products is called acid value,expressed as mgKOH/g.2Sources of acidic substances If conditions of acid pickling and refining process are properly controlled,there is hardly any inorganic acid in oil products.Organic acids in oil products are mainly naphthenic acid and fatty acid.Most of them are inherent in crude oil and have not been completely exhausted during the refining process.Part is produced by oxidation during petroleum refining,oil transportation or storage.3.Meaning of determination(1)Judge the quantity of acidic substances in oil products the higher the acidity(acid value)is,the more acidic substances the oil contains.Diesel jet fuel has specific requirements for acidity or acid value.(2)Judge the corrosiveness of oil to metal materials.An increasing amount of organic acid with water in oil products can seriously corrode metal.(3)Judge the deterioration degree of lubricating oil.If the acid and alkali values exceed the oil exchange index,the oil should be replaced.When the increased acid value of gasoline oil is larger than 2.0 mg KOH/g or that of diesel oil is larger than 2.5 mg KOH/g,the oil must be replaced.II.Overview of Acidity and Acid Value Determination Methods Standards:GB/T258-1977(1988)Determination of Acidity of Gasoline,Kerosene and Diesel Oil,GB/T264-1983(1991)Determination of Acid Value of Petroleum Products,GB/T7304-2000Determination of Acid Value of Petroleum Products and Lubricants(Potentiometric Titration)and SH/T0688-2000Determination of Alkali Value of Petroleum Products and Lubricants Determination of Acidity of Diesel OilFeature:Both are using water or organic solvents to extract the acidic or alkaline substances from the sample and then analyzing corresponding endpoints with chemical titration or potentiometric titration to judge the color changes of indicators.Potentiometer also works.Quality indicators of some oil products acid values are shown in Table 7-2.1.Testing acidity of lightweight oil products Acidity of gasoline,kerosene and diesel oil.Extract acidic substances from the sample with boiling ethanol solution according to basic principles.Titrate with potassium hydroxide and ethanol solution of known concentration.Determine the endpoint with an acid-alkali indicator.Calculate the acidity of the sample via the lost volume of potassium hydroxide and ethanol solution.The reactions are as follows:RCOOH +KOHRCOOK +H2OH+OHH2O The acidity of the specimen is calculated by the following equation:（7-1）T=56.1C 2Testing acid value of petroleum products and lubricants(Potentiometric titration method)According to GB/T7304-2000Determination of Acid Value of Petroleum Products and Lubricants(Potentiometric Titration)standard test method.It is mainly suitable for the determination of acidic substances in petroleum products and lubricants that can be dissolved in mixed solvents of toluene and isopropanol.See Figure 7-1 for the potentiometric titration device.A radical principle:Accurately weigh and take some samples of certain amount in the titration tank.Use toluene isopropyl alcohol mixed with solvent containing a small amount of water,also known as titration solvent,to dissolve the sample.Fix the glass indicator electrode-calomel reference electrode in the titration tank.Adjust the potentiometer to the measurement state.Stir the mixture thoroughly until the acid in the sampleuniformly distributed,to isopropyl alcohol solution of potassium hydroxide directly titrate the specimen with the titration solvent mixture,in the hand-drawn or automatically drawn curve of potential-titrant dosage(E-V diagram)only the obvious sudden jump point as the end point.The chemical reactions are.RCOOH +KOHRCOOK +H2OThe acid value of the specimen is calculated by the following equation:（7-2）For the determination,according to the predicted acid value of the specimen,weigh the specimen accurately according to the requirements specified in the standard and add it to the titration cell.Add a certain amount of titration solvent to the titration cell with the sample,start the magnetic stirring device,and increase the stirring speed as much as possible without causing splashing and bubbling of the mixed test solution,so that the acid in the sample can be released evenly.Install the electrode(which can also be fixed before stirring)and record the initial volume of potassium hydroxide isopropanol solution in the burette and the mixingIII.Main Factors Affecting the determination 1Chemical titration (1)Indicator dosage.Add indicators according to specified requirements to avoid titration errors (2)Control of boiling conditions.Boil and quickly titrate the liquid to be tested at the specified temperature and time to improve the extraction efficiency and reduce the effect of CO2 on the testing result (3)Determination of titration endpoints Titrate with phenolphthalein indicator until the ethanol layer becomes light rosy.Titrate with cresol red indicator until the ethanol layer changes from yellow to purple-red Titrate with bromothymol blue indicator until the ethanol layer changes from yellow to green or blue-green.(4)Color change of extracted solution.When the color of the extracted solution is dark potentiometric titration must be applied instead.In long-term practice,We have cultivated and formed a craftsmanship spirit-dedicated,self-disciplined,precise and aspiring.We move on to a new course and raise the sail before setting out,hoping the craftsmanship spirit to be more deeply rooted in peoples hearts.2Potentiometric titration method(1)Electrode maintenance Insert into the titrating solvent for rinse after use and then wash it with distilled water and isopropanol respectively When there is no need of use,the glass indicator electrode is immersed in distilled water and calomel reference electrode is stored in saturated potassium chloride isopropyl alcohol solution(2)Pre-treatment of used oil samplesIn order that the sediments in the sample can be evenly dispersed,heat the sample to 60 5 and stir it Filter with a screen whose aperture is154m if necessary(3)Treatment of insoluble samples When there are heavy bituminous residues that are difficult to dissolve,The dissolution of the sample can replace toluene with trichloromethane(4)Endpoint determination.If there is no obvious jump point,the potentiometric value of the new non-aqueous acidic or alkaline buffer solution is set as the titration endpoint.
