Discussion and Procedure
To carry out this procedure you will require, in addition to a volumetric buret, two clean, dry 125 mL Erlenmeyer flasks and one #5 rubber stopper.
Select a 50 mL buret from the buret cabinet in the north-west corner of the lab. After you have cleaned this buret attach a piece of tape with your name to the open end of the buret. This will serve to identify it. You will be using this buret for this calibration and for the two volumetric analyses. Proceed with the calibration as described below.
1. Disassemble the stopcock, noting carefully how it is assembled, then clean the bore of the buret with a warm solution of Alconox. The teflon stopcock usually does not require much cleaning; however it might be necessary to soak it in warm Alconox solution. Do not use a brush on the stopcock since it will scratch the relatively soft teflon. These scratches are the primary reason for leaking burets. Rinse the buret and stopcock well with water and distilled water. Reassemble the stopcock.
2. Fill the buret with distilled water and check to see that no bubbles of air are entrapped in the tip. Drain water slowly until the meniscus is at the 0.00 mL mark. Touch the tip of the buret to the side of a beaker to remove the drop hanging from the tip. After about a minute, to allow for drainage, make an initial reading of the meniscus, estimating the volume to the nearest 0.01 mL. Record the initial reading. Allow the buret to stand for 5 minutes and recheck the reading. If the stopcock is tight, there should be no noticeable change in the reading. If the reading has changed tighten the blue (or orange) nut on the stopcock and let stand for another 5 minutes. Check the reading again. If the buret continues to leak consult your instructor. Click here for guidance on reading a buret.
3. You will need to prepare a "buret card" to be used every time you read your buret. Obtain a 3"x5" card from your instructor and using a black felt tip pen make a horizontal mark on your card, one inch thick and practically the length of the card. When the top of the black band is held just below the bottom of the meniscus you will see a reflection of the band in the meniscus against the white of the card behind. This offers you a repeatable method of determining the position of the meniscus. You must make sure during your readings that your line of sight is perpendicular to the buret so as to avoid parallax due to the center of the meniscus being a greater distance from your eye than the scale against which you are reading it. If your line of sight is looking downward or looking upward, the meniscus will appear to be higher or lower, respectively, than its true value. It is extremely important that you learn how to read a buret with a repeatable precision of ±0.01 mL so as to eliminate this source of errors in all of the volumetric analyses that you perform. What is the volume of the liquid in the buret shown above? Does that volume agree to within 0.01 mL of the volume proposed by your instructor?
4. While checking the buret, weigh, to the nearest 1 mg, a dry 125 mL Erlenmeyer flask fitted with a #5 rubber stopper. Once the tightness of the buret stopcock has been established, record the level of the meniscus (which should be at 0.00 mL). Run an accurately measured volume of about 10 mL into the weighed flask at a flow rate of approximately 10 mL per minute. Touch the tip to the wall of the flask. Wait 1 minute, record the meniscus level. The difference between the two readings is the "apparent volume". Now stopper the Erlenmeyer flask with the #5 rubber stopper and then weigh it to the nearest 1 mg. The difference between the two weights gives the mass of water equivalent to the apparent volume. Record the temperature of the water in the flask. With the aid of the table below convert this mass of water into the true volume at 20 oC.
Volume occupied by 1.000 g of water weighed in air using stainless steel weights. Corrections for the buoyancy of stainless steel and the thermal expansion of the glass buret have been applied.
|T, in oC||Volume at T||Volume corrected to 20 oC|
This is accomplished by multiplying the value, corresponding to your temperature, in the right-most column of the table, by the mass of water in the flask. The correction that must be applied is obtained by subtracting the apparent volume from the true volume. Notice that this correction may be either positive or negative and is an additive correction term applied to the apparent volume. We use Class A burets in this class. The tolerance allowed by the National Institute of Standards and Technology for Class A 50 mL burets is ±0.05 mL. Your correction ought not to exceed this deviation from 0 to 50 mL. Continuing the delivery of water into the same flask, add water in increments of 10.00 mL to 20, 30, 40 and 50 mL. Weigh the flask after the delivery of each increment. While you are carrying out the procedure, dry a second flask. When you have finished the drainage to 50 mL in the first flask and made your final weighing, repeat the entire procedure using the second flask as your receiving vessel. The deviations you observe between the first and second run will give you important information about the reproducibility of your buret readings.
5. Calculate the correction value for each volume. If any of your readings exceed a 10 mL increment by a few hundredths of a mL, there is salvation. For example, if for the 10 mL reading your meniscus was at 10.02 mL, you should subtract 10.02 from your true volume and then plot the resulting value. Make an accurate plot of correction value vs. apparent volume in your lab notebook. Don't average the values (yet) for the two runs, but superimpose them on the same graph. Place the correction value on the ordinate and the volume on the abscissa. Make this graph as large as possible on one page of the notebook. Use the long side of the page as the volume coordinate and the short side for the correction terms. Keep in mind that the correction values may be either positive or negative, make allowance for this when you lay out your graph. Connect the points on the graph by straight lines. Correction values for intermediate volumes may now be read off the graph. The two plots should "shadow" each other. Any pair of points ought not to differ by more than 0.04 mL. The similarity between the two plots will give you confidence about the trustworthiness of your data. Now, use the average correction value for each pair of readings as your final buret correction.
6. You need not submit a report for this calibration but write in your laboratory notebook the answers to the questions posed below.
Questions on Buret Calibration
- Your weight of water is converted to the true volume using data from Table 27-3 of SHW. What are the three corrections that are embodied in those values?
- Explain why it is not necessary to weigh the water samples on the analytical balance.
- When the glass of a buret expands due to an increase in temperature does the diameter of the bore increase or decrease?
- Most volumetric glassware is calibrated at what temperature?
- What do the letters T.D. and T.C., that are found on various types of volumetric glassware, signify?
- Ulrich de la Camp and Oliver Seely (California State University, Dominguez Hills).