(2.4)--1.4直接电位法测溶液pH值.ppt

《(2.4)--1.4直接电位法测溶液pH值.ppt》由会员分享，可在线阅读，更多相关《(2.4)--1.4直接电位法测溶液pH值.ppt（12页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、Chapter Potentiometric AnalysisChapter Potentiometric Analysis.1.4 Direct Potential Determination of Solution pH1.4 Direct Potential Determination of Solution pHSchematic diagram of direct potential methodReference electrodesIon meter or pH meterIndicator electrodeAgitatorWe have learned some method

2、s of measuring pH value,such as indicator liquid method,pH test paper method,the accuracy of both methods is not very high,then how to improve the accuracy of measuring pH value?Today we are going to learn a new method for measuring pH-direct potential.Principle of Determination:pH is a negative log

3、arithm of the activity of hydrogen ions,i.e.pH=-lga(H+).The pH glass electrode is used as indicator electrode(negative electrode),and the saturated calomel electrode is used as reference electrode(positive electrode).The indicator electrode and reference electrode are inserted into the cell to be te

4、sted.Working battery:Glass electrode|Liquid to be tested|glycury electrodeAt 25,the battery electromotive force is:E=(+)(-)E=SCE(K Glass 0.0592pH)=SCEK Glass+0.0592pH E=E+0.0592 pH When pH experimental conditions such as temperature are constant,the electrode potential of the pH glass electrode is l

5、inearly related to the pH of the test solution.Saturation calomel electrodeAcid-junction meterGlass film electrode.The pH scaling method It is used to determine the electromotive force of standard buffer solutions with known pH value and test fluid x with known pH value.The same reference electrode

6、and indicator electrode are used to form a working cell under the same conditions.Electromotive force of standard solutions is determined as follows:Es=Ks+2.303RTpHs/F Electromotive force of unknown solution:Ex=Kx+2.303RTpHx/FThe two equations are subtracted to obtain:Ex-Es=0.0592(pHx-pHs).Standard

7、buffer solution pH standard buffer solution must be used to determine the pH of the solution.It is the standard buffer solution.There are usually seven types of standard buffers,of which the standard buffers with pH of 6.86,9.18 and 4.00 are commonly used.(1)Application of PH standard buffer solutio

8、n:Calibration of acid meter before pH measurement.To verify the accuracy of the acid meter,for example,after calibrating the acid meter with pH6.86 and pH4.00,the pH electrode pH inserted into the solution of pH9.18 to check whether the value of the instrument is consistent with that of the standard

9、 solution.Test the performance of pH electrode.(2)The preservation of standard solutions:The standard solution should be sealed in glass or polyethylene bottles in airtight storage.Under room temperature,the standard solution should be kept for 23 months.When it is found to be cloudy,moldy or precip

10、itated,it cant be continued.The used standard solution is not allowed to be turned back.The pH value of standard buffer solution varies with temperature.Three methods of preparation of commonly used standard solutions:Potassium hydrogen phthalate standard buffer solutionc is 0.05mol/L,weigh 10.21g o

11、f potassium hydrogen phthalate(KHC8H4O4)dried at(115.05.0)for 23h,dissolve in distilled water without CO2 and dilute to 1000ml.Mixed phosphate standard buffer solutionSodium dihydrogen phosphate(Na2HPO4)and potassium dihydrogen phosphate(KH2PO4)Sodium dihydrogen phosphate Na2HPO4(3.53 0.01)g and pot

12、assium dihydrogen phosphate(KH2PO4)(3.39 0.01)g were obtained at(115.0 5.0)for 2 to 3 hours respectively.They were dissolved in distilled water which KH been boiled for 15 to 30 minutes and cooled rapidly,and diluted to 1000ml.Sodium tetraborate standard buffer solutionNa2B4O710H2O boraxC(Na2B4O710H

13、2O)is called Borax(Na2B4O710H2O)(3.80 0.01)g(note:cant be dried!)Soluble min distilled water boiled in advance for 15 to 30 minutes and cooled rapidly and diluted to 1000ml.Keep sealed in polyethylene plastic bottle.Care should be taken to prevent the entry of CO2 into the air when storing it(note:m

14、ay be used for acid meter calibration).Meter At At present,more pH gauges are used for digital display present,more pH gauges are used for digital display of the pHS-3F series acid meter.of the pHS-3F series acid meter.Method Method of use for pHS-3F acid meter:of use for pHS-3F acid meter:When meas

15、uring the pH of the solution,the pH glass When measuring the pH of the solution,the pH glass electrode and the calomel electrode are inserted into the electrode and the calomel electrode are inserted into the solution to be tested to form a battery,and the lead end of solution to be tested to form a

16、 battery,and the lead end of the electrode is connected with the pH meter.The the electrode is connected with the pH meter.The millivoltmeter is calibrated with 12 standard buffer millivoltmeter is calibrated with 12 standard buffer solution,then the test solution is measured and the pH solution,the

17、n the test solution is measured and the pH value of the test solution is read directly on the pH meter.value of the test solution is read directly on the pH meter.pHS-3F type acidimeterpHS-3F type acidimeter.The determination step (1)(1)The The preparation work preparation work Turns Turns on the in

18、strument power switch,preheating 20 min.on the instrument power switch,preheating 20 min.The The initial value of the instrument is set to zero by means of a zero-initial value of the instrument is set to zero by means of a zero-voltage potentiometer.voltage potentiometer.Put Put the function key in

19、 the pH position.the function key in the pH position.The The two electrodes are mounted on the electrode clamp and the terminal two electrodes are mounted on the electrode clamp and the terminal of the electrode is inserted into the corresponding terminal on the back of of the electrode is inserted

20、into the corresponding terminal on the back of the instrument.the instrument.Wash Wash the two electrodes with distilled water,insert the part into the the two electrodes with distilled water,insert the part into the solution,solution,and and use the filter paper to suck up the water on the outer wa

21、ll of use the filter paper to suck up the water on the outer wall of the electrode.the electrode.Switchmillivolt meterAgitatorSwitch(2)Calibration of(2)Calibration of pH pH A A single point calibration methodsingle point calibration method inserts the two electrodes into a standard buffer solution w

22、ith known pH and inserts the two electrodes into a standard buffer solution with known pH and adjusts the temperature adjusting knob so that the scale line refers to the adjusts the temperature adjusting knob so that the scale line refers to the temperature value of the standard buffer solution.Turn

23、 the slope knob temperature value of the standard buffer solution.Turn the slope knob clockwise to the end(maximum).After the electrode is balanced,adjust the clockwise to the end(maximum).After the electrode is balanced,adjust the positioning adjusting button so that the instrument can read the pH

24、of the buffer positioning adjusting button so that the instrument can read the pH of the buffer solution at the current temperature,that is,complete the single point calibration solution at the current temperature,that is,complete the single point calibration of the instrument.Note:Do not touch this

25、 positioning button in subsequent of the instrument.Note:Do not touch this positioning button in subsequent measurement operations,otherwise this operation will have to be re-carried out.measurement operations,otherwise this operation will have to be re-carried out.pH glass electrode should be immer

26、sed in distilled water for more than 24 hours pH glass electrode should be immersed in distilled water for more than 24 hours before use,so that the glass film can fully swell and balancebefore use,so that the glass film can fully swell and balance.For accuracy requirements of less than 0.1PH,a sing

27、le-point calibration can be used.(2)Calibration of(2)Calibration of PhPh Two-point Two-point calibration methodcalibration method After finishing the single-point calibration,remove the electrode,remove the After finishing the single-point calibration,remove the electrode,remove the standard buffer

28、solution,clean the two electrodes and then insert another standard standard buffer solution,clean the two electrodes and then insert another standard buffer solution close to the pH of the tested solution.Turn the slope adjustment buffer solution close to the pH of the tested solution.Turn the slope

29、 adjustment knob to make the instrument display the pH of the standard buffer solution,that is to knob to make the instrument display the pH of the standard buffer solution,that is to say,the two-point calibration of the instrument pH is completed.say,the two-point calibration of the instrument pH i

30、s completed.Note:Note:The above operation should be repeated 23 times.After adjustment,the The above operation should be repeated 23 times.After adjustment,the adjustment knob of positioning and slope should not be moved again,otherwise it adjustment knob of positioning and slope should not be moved

31、 again,otherwise it needs to be calibrated again.Every time you change a test solution,you must clean needs to be calibrated again.Every time you change a test solution,you must clean the electrode with distilled water.the electrode with distilled water.Two-point calibration must be used for precisi

32、on-grade laboratory PH meters.Two-point calibration must be used for precision-grade laboratory PH meters.(3)Determination of pH value of solution Insert the cleaned two electrodes into the solution to be tested.After the electrode is balanced,read the pH of the solution to be tested and record it.NameTemperature（）PHStandard buffer solution 1Standard buffer solution 2Liquid to be measured-The explanation is over,thank The explanation is over,thank you!you!