1、Calibration of Volumetric Equipment 1CALIBRATION OF VOLUMETRIC EQUIPMENTIn any experiment, you must ascertain the limits and range of your measuring instruments. The experiments in this course are at a sufficient level of sophistication to warrant the calibration of the instruments used in order to
2、correct for instrumental errors and to explore the limits of their measurement capability. In many of our experiments, we will need to measure accurately temperature and liquid volumes. The main objective of this experiment is to determine accurately the actual volume of your transfer pipet and sele
3、cted volumes of your buret, and to calibrate your thermometer. Before coming to the laboratory you should read and understand Appendix B. Also read the sections on volume measurements and re-read the reliability of laboratory measurements in the introductory section of this manual (pp. 19-13 and 17-
4、27, 29-41).EXPERIMENTAL PROCEDUREThe de-ionized water (about 500 ml) used in this experiment should be put into a large flask at the very beginning of the lab period and allowed to stand until it comes to room temperature. Cover it with a small watch glass. Before starting the experiment you should
5、make certain that your 50-ml buret, and 25-ml pipet are scrupulously clean; they do not need to be dry. The criterion for clean walls is that on draining water from them they retain a continuous, invisible, unbroken film on the walls; no droplets. If cleaning is required, see pp. 9, 21-22 of the int
6、roductory section of this manual. ThermometerLeave your thermometer immersed for some five minutes or longer in the large beaker of continuously stirred water provided with a thermometric standard. At some convenient time record the temperature reading (TT) of the standard and the temperature readin
7、g (To) of your thermometer, both to the nearest 0.1C. The “correction“ that should be applied to your thermometer reading when it is used at approximately room temperature is TT - To.Calibration of Volumetric Equipment 2Buret1. Calibrating the Buret. Use de-ionized water for all operations. Check yo
8、ur buret for leakage and drainage time as described on p. 22-23. In order to calibrate your buret, fill it and lower the meniscus to just below the “0“ mark. Follow the recommended technique with respect to the buret tip; then read the buret to the nearest 0.01 ml. Withdraw approximately 25 ml of wa
9、ter into a weighed (but not necessarily dry) 50-ml conical flask. This flask should be covered with a small watch glass while it is being weighed and at all times except when having water added to it; the watch glass minimizes evaporation. The flask should be dry on the outside because water will be
10、 removed as you set the flask down changing the mass. Remember to touch the tip of the buret to the inside of the flask wall before covering it with the watch glass again. Read the buret to the nearest 0.01 ml, and reweigh the flask and water on the analytical balance.Now withdraw an additional quan
11、tity of water into the flask so that the meniscus stands at about the 35-ml mark. Again read the buret to the nearest 0.01 ml, and again reweigh the flask on the analytical balance. Repeat the process with the meniscus at the 45-ml mark.Take the temperature of the water used in each run. This can be
12、 done at the end of each series by putting the thermometer in the water in the flask after the final weighing.2. Repeating the Calibration. Empty the 50-ml conical flask and again weigh it empty (but not necessarily dry on the inside). Repeat the entire series of three measurements at the 25-, 35- a
13、nd 45-ml marks, starting at approximately “0“ again.In order to check rapidly whether the corrections at a given volume (35 ml, for example) are in close enough agreement between the two series of measurements merely calculate the difference between the total observed volume and the total mass of wa
14、ter delivered at that volume. When for any volume the differences between the first two series disagree by more than 0.03, a third measurement at that volume should be made. If it is value at the 45-ml mark that needs be repeated, say, it is not necessary to also repeat those at the 25-, and 35-ml m
15、arks. Simply draw water from the “O“ mark to the 45-ml mark, taking the same precautions as before. These differences serve no purpose other than as a quick indicator of whether your experimental work is satisfactory.Calibration of Volumetric Equipment 33. Calculating the Buret Correction. For each
16、of the marks on the buret (25, 35 and 45 ml) to which water was withdrawn, calculate the observed volume (VO) withdrawn determined from the initial and final buret volume readings. Also for each of the marks to which water was withdrawn, calculate the mass of water withdrawn. Correct these masses fo
17、r buoyancy. (The density of the weights used for calibrating the analytical balances is 8.4 g/ml.) Knowing the temperature of the water and its density (Appendix G), calculate from these corrected masses the true volume (VT) of water withdrawn. With the “correction“ defined as VT - VO, calculate the
18、 correction that must be added at each of the three marks where calibration was carried out; for each of these marks, average the two or more values you obtained. The following table shows a sample calibration.CALIBRATION OF A BURET AT 25C(only one series of measurements is shown)Nominal Vo, Observe
19、d Actual VT, Actual VT - VOVol. Interval Vol. Delivered Mass Delivered Vol. Delivered Correction(ml) (ml) (g) (ml) (ml)0-25 25.04 24.97 25.07 +0.030-35 34.76 34.61 34.75 -0.010-45 45.06 44.90 45.08 +0.024. Graphing the Corrections. Construct a simple graph in your notebook, plotting “correction“ as
20、ordinate and “buret reading“ as abscissa; this is the calibration curve for your buret. In this graph the “buret readings“ plotted should be the nominal values of 25, 35, and 45 ml. Unlike all the other graphs to be constructed from experimental data in this course, this one should have the points c
21、onnected consecutively by straight line segments, not by a smooth curve. The resulting graph should not be interpreted as meaning that your buret suddenly changes diameter at certain points along the tube. Instead, you should realize that you lack specific information about the exact manner in which
22、 the buret diameter varies along the tube, that you will assume a gradual linear change from one calibration point to the next, and that in any case you will round off any correction taken from the graph to the nearest 0.01 ml.Calibration of Volumetric Equipment 4Pipet1. Calibrating the Pipet. Just
23、as in the case of the buret, check your pipet for proper drainage time. Fill it to the mark with de-ionized water and observe the time for it to empty while it is held vertically. Consult your instructor if the time of efflux differs greatly from that recommended on page 22.Using the necessary care
24、for the proper use of the pipet (pp. 20-21), pipet 25 ml of water into a weighed 50-ml flask. Dont forget to wait for proper drainage or to touch the tip to the inside wall of the flask. Cover the flask and reweigh it on the analytical balance. Note that you should measure the masses to 0.1 mg becau
25、se you may be able to measure to a precision of 0.001 ml. Repeat this operation so as to give five successive additions of water from the same pipet. After the final weighing take the temperature of the water.2. Calculations. Calculate the mass of water obtained from each addition to the flask. Corr
26、ect these masses for buoyancy. Knowing the temperature of the water and its density (Appendix G), calculate the true volume (VT) of water delivered in each of the five additions to the flask from these corrected masses. Calculate the average value of VT. Defining “correction“ as VT -25.000, calculat
27、e the correction which must be applied to your pipet.For your five values of VT, calculate the standard deviation and the 95% confidence interval for a single measurement.Calibration of Volumetric Equipment 5Name_ Lab Section_Date Report Submitted_CALIBRATION OF VOLUMETRIC EQUIPMENT1. Thermometer Th
28、ermometric Standard, true reading (TT) _CDesk Thermometer, observed reading (To) _CThermometer Correction (TT - To) _C2. Buret Series A: Observed Mass of Corrected True Vol. Correction T(H2O), C Volume, Vo Water Mass Volume, VT VT Vo _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Series B: T(H2O), C_ _ _ _ _ _ _ _ _
29、 _ _ _ _ _ _ Series C: (if needed)T(H2O), C_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Calibration of Volumetric Equipment 6Name_Nominal buret reading 0 - 25 0 - 35 0 - 45Average volume correction _ _ _3. Pipet T(H2O), C _Mass of Water Corrected Mass True Volume, VT_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Average _Standard
30、deviation _95% Confidence interval of a single measurement _Volume correction, VT - 25.0000 _4. Show sample calculation for pipet calibration on the reverse side.5. Solve the following problem. Show the calculations on the reverse side.What would be the calculated volume of your pipet if the buoyancy correction was not applied? What is the relative error in parts per thousand of the volume of your pipet when the buoyancy correction is not applied?Volume of pipet _ Relative error, ppt _Calibration of Volumetric Equipment 7
