CALIBRATION of APPARATUS as an EXERCISE in QUANTITATIVE ANALYSIS* W. B. MELDRUM AND W. E. CADBURY, JR. Haverford College, Haverford. Pennsylvania
The desirability of calibrating volumetric apparatzls even for student w e is discussed and pedagogical advantages of the exercise noted. Data are given indicating the magnitude of possible errors involved and the concordance i n calibration results of various students.
+ + + + + + TI?ONE may. i-u d-~ efrom some of the most widely
1 used laboratory manuals of the present time, thk
cahbrat~onof volumetric apparatus, flasks, pipets, :and burets, is not generally favored as an exercise in the beginning course in quantitative analysis. Most elementary quantitative manuals describe how it is done and explain why it is desirable for "work of high accuracy." They do not direct, however, that the stu.dent should be required to carry out the exercise in the case of the measuring apparatus which he himself will use in volumetric methods of analysis. For ten years or more students a t Haverford College have been required to calibrate their flasks, burets, and pipets as an early exercise in the beginning quantitative course. .Our experience has shown us that the results, both practically and pedagogically, justify the time spent upon this work, and on these bases we recommend the exercise. The exercise seems to be pedagogically sound, for it serves as a suitable vehicle for instruction in the significance of the terms "accuracy" and "precision" and for acquiring in actual practice the idea that there are limits to the accuracy with which a volume can be read. Also, the exercise teaches the student, before he embarks on more time-consuming analyses, how to use his tools in the right way. Our experience indicates that failure to calibrate volumetric apparatus may have a due iduence on .analytical results obtained by the student and may explain the frequent lack of concordance between the student's analyses and the instrnctor's values. Also, they indicate pretty clearly that the faith of those who believe implicitly in the legends etched upon pipets, burets, and flasks may be sadly misplaced. All the volumetric apparatus .used in this laboratory was purchased from reputable supply houses and is by no means representative of the worst obtainable; yet very serious deviations of the actual from the stated
volumes are not unusual. An examination of the results obtained in some cases should convince anyone that such calibration is not only desirable but necessary if analyses are to be even approximately correct. Each student calibrates one flask, one pipet, and two burets each year, and the values given in the illustrative samples that follow were obtained independently over a period of years' CALIBRATION OF FLASKS
The volumetric flask is calibrated by the accepted method. The flask is cleaned and dried, weighed to 0.01 g., filled to the mark with distilled water a t very nearly 20°C., and again weighed. The weight of the water is corrected to vacuo, and from this weight and the known density of water, the true volume of the flask is determined. The flasks used are graduated to contain 250 ml. a t 20°C. I n calibrating all volumetric apparatus attention is paid to the temperature of the water used. In order that a calibration is to be of value, the temperature of calibration must be the same, within the limits of two or three degrees, as the working temperature. When water or aqueous solution is the liquid being measured, the magnitude of the error is about 0.02% per degree; hence a five-degree variation would cause an error of about 0.1%. I n the table that follows, results for three successive years are given for each flask. Flask No. I9 17 250.57 ml. 249.88 250.56 249.82 250.58 249.79
Volume reported:
18
ml.
247.19 ml 247.23 247.25
I n the case of the first two flasks, Nos. 19 and 17, the errors due to lack of calibration would not be serious in ordinary analytical work, about 0.2% for the former, and less than O.lyofor the latter. But the error introduced by neglecting to calibrate No. 16 and to apply the necessary correction would be intolerable-more than l Y 0 a n d might readily account for some student's "poor analytical technic." CALIBRATION OF PIPETS
Pipets are usually calibrated "to deliver." The pipet, carefully cleaned to render it free-flowing, is filled to the mark with distilled water a t 20°C., and the * Presented before the Division of Chemical Education of the water run into a previously weighed weighing bottle and weighed to 0.001 g. The favored procedure of emptyA. C. S., at Washington, D. C., March 29, 1933. 504
ing the pipet is to let it drain for one minute and then to remove as much as possible of the water remaining in the tip by touching the tip to the wall of the weighing bottle several times. The true volume of the pipet is calculated in the same manner as that of the flask. The following are representative of results obtained over a period of years. No. 4 .~~~
Pibd -
Volume reported:
~
25.00 ml. 24.95 24.94 24.98
15 25.05 ml. 25.06 25.04 25.06
23 25.07 ml 25.09 25.07 25.06
An error of 0.05 ml. in 25 ml. represents an error of 0.2%. CALIBRATION OF BURETS
Burets are calibrated by the following method. The buret is filled with distilled water a t 20°C. Approximately 5 ml. are run out into a previously weighed weighing bottle and weighed to 0.001 g. The buret is read to 0.01 ml. Then 5 ml. more are rnn out and weighed, and this is continued for each 5-ml. section of the buret. From the calculated true volume of the water and the readings of the buret the correction to be applied is determined. This method has the disadvantage that any error due to water lost by evaporation or otherwise is repeated in each subsequent measurement on the same buret. Actually we find that results, using this procedure, are not trustworthy, as evaporation from moisture on the stopper may account for a loss of as much as 10 mg. in as short a time as 5 minutes. A better method is to refill the buret each time, the first time to run out 5 ml., the second time 10 ml., and so on, emptying and re-weighing the weighing bottle each time. In this way no additive errors are introduced. The greater amount of time required by this method of calibration is quite justified by the greater accuracy of the results. For a complete calibration of a buret, measurements should be made for a smaller interval, e. g., for each milliliter. However, the usual assumption is made,
as in the calibration of thermometers by the Bureau of Standards, that variations in the bore of glass tubing are not abrupt but gradual, and may therefore be represented as the most probable case by a straight-line curve between adjacent points of calibration. For purposes of ordinary analytical work calibrations a t 5ml. intervals are considered to comply with the permitted limits of accuracy. In the accompanying calibration curve the volumes as read are plotted horizontally, the corrections to be added vertically; corrections for volumes intermediate between calibrated volumes are interpolated directly. f0.12 +O.lO u +0.08 +0.06 +0.04 +0.02 0.0 g -0.02 :: -0.04 -0.06 -0.08 -0.10 -0.12
a
-.-
8
0
5
10
15
20
25
Volume.
Curves are shown for buret No. 24 as determined by three observers. Two sections of the buret are seriously in error: near 10 ml. and above 15 ml. At 10 ml. the error is about 0.60jo, and a t 25 ml. i t is about 0.4%. It is obvious that if this buret were to be used without applying the correction, the error introduced would be sufficient to invalidate results. It is evident from concordance of various experimenters working independently in different years that the calibrations are of real significance. It is evident, too, that errors introduced by employing the volumes read, without correction, might be considerable.
