T H E ACCURATE CALIBRATION O F CAPILLARY TUBES
BY K. J. ISAAC AND IRVINE MASSON
In connexion with work on the compreesibility of gaseous mixtures b j Dolley and one of US,^ it was found that the methods of ascertaining the bore or volume of capillary glass tubes, described in the literature, are generally subject to serious errors. Improved methods were therefore worked out which enabled us to know the volume of any length of a socm. capillaryof about1 mm. bore with an accuracy of one part in several t,housand. The most convenient of these methods is here described, because similar knowledge is required for many purposes, and it is hoped that other workers may thereby be saved considerable labour.
It may be premised that the tubes with which we are concerned are closed a t one end of the capillary, a fact which calls for certain experimental arrangements whose simplification will be obvious to anyone who applies the general method to tubes which can be left open at both ends. Mercury is used as the calibrating fluid; and we have found the very simple method described by Dixon and McKee2 to be entirely adequate. No process of measurement which involves horizonta2 threads of mercury is reliable, since in such a thread the meniscus sags, and its volume cannot be estimated; further, the shape is variable from point to point even in clean tubes of small bore, so that its effect does not cancel out in successive measurements of length. Consequently the tube must be used and calibrated vertically; and in this case, since the diameter of the tube is approximately known, measurement of the height of the meniscus affords a very close estimate of its volume when the data of Schalkwijk3 are applied. The clean, dry, and dust-free tube is mounted, as shown in Fig. I , in a jacket kept at constant temperature, preferably that at which the tube is eventually to be used. In our tubes, direct graduation was not permiseible, since they would have been less able to resist high internal pressures; a light glass scale was therefore attached to each tube, during calibration and use, by means of tight rubber bands. Relative movement of scale and tube is guarded against by taking readings of the external top end of the tube, which point serves as the zero mark in all the measurements. Masson: Proc. Roy. SOC. 103 A, 524 (1923) J. Chem. SOC. 123, 895 (1923) SCommPhys. Lab. Leiden, No. 67 (1901); Verslag. Icon. Akad. Wet. Amst., 1900, p.462; 1901, 512.
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Whatever reading-instrument is used in conjunction with the scale must provide measurements of height-diff erences which are reliable to about 0.01 mm. We use a telescope provided with a spirit-level and with a Hilger micrometer eyepiece, which gives about IOO drum-divisions per millimetre read at a distance of several feet. The foot of the capillary, which is to be dispensed with when the tube is eventually in actual use, passes through the bung of the jacket and carries the side tube with tap B, besides having a m capillary tap C whose stem is drawn out _ _ ___- - eto a very fine jet. This stem also bears a small rubber cork for the attachment, of tjhe weighing-vessel A. The t'ap B communicates with a high- vacuum pump, such as one of the mercury condensation type; doubtless a Toepler pump combined with a cooled charcoal bulb would also serve.
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T h e vessel A, cont,aining pure mercury, is placed in position, and a Fleuss or other oil vacuum pump is temporarily connected with its side arm by a rubber tube. Taps €3 and C are opened and the tube is exhausted, the oil pump serving to prevent the mercury from rising as far as tap C. When exhaustion is complete, the taps are shut and the oil pump is disconnected; tap C is now opened slowly until the mercury has risen just above it, when the exhaustion is continued for a while in Fig. I order to remove any air trapped at the opening. Tap B is then finally closed and the mercury is allowed to rise to a point convenient for the lowest reading of level. The jacket teAperature, the level of the tip of the meniscus, and the height of the meniscus, are noted. The vessel A is removed, weighed, and replaced. The oil pump is re-connected, and a partial vacuum is produced in A PO that when C is opened the mercury falls slightly; this is necessary in order to remove any trapped air from the jet. C is closed, the oil pump is removed,
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I(. J. ISAAC AND IRVINE MASSQN
and the mercury is allowed to rise to a new level about 2cm. above the first; a fresh set of readings of heights and weight is then taken. This process is repeated at intervals of about 2cm. until the tube is completely full of mercury. If the tube be of height greater than the barometric, a cycle pump can replace the oil pump in the last stages. This completes the first calibration; and the mercury is now to be drawn down again, and a second calibration is made in the same way, the intervals overlapping those of the first set. Finally, a single “overall” determination is made as a check, one weighing being made when the tube is completely full and the other when the mercury has been withdrawn and then allowed to rise to near the lowest mark. A “rising” meniscus is better-shaped than a “falling”, hence the need for the latter manoeuvre.
Calculation. All distances along the capillary are measured from the external top of the tube. The observations of temperature, heights, and weight of mercury being recorded tabularly, each set is treated thus:-From the data of Schalkwijk (loc. cit.) a table can readily be drawn up to show the volume, and hence the equivalent cylindrical height, of any mercury meniscus of measured height in a tube of approximately known bore. (For I mm. tubes, the equivalent cylinder has practically one half the height of the meniscus). The observed level of the mercury is, accordingly, corrected to what it would be if its surface were flat. These corrected distances, each about 2cm. apart, correspond with the measured weights of mercury, and hence with volumes, due corrections being of course applied for buoyancy, scale-errors, and the like. The mean cross section or volume per linear centimetre within each of these 2cm. sections is thus found, and the values are plotted on squared paper, using the distances from the external end of the tube as abscissae. The “curve” takes the form of a series of steps, the horizontal lines marking the mean vol./cm. between the points indicated by the vertical lines. A smooth curve is now drawn through the mid-points of the vertical lines of the stepe, and this shows the vols. per cm. along the length of the tube to near the closed end. The results of the second calibration are now drawn in the same way on the same paper. A mean curve is then drawn, by eye, between the two. The usual maximum deviation of either curve from the mean is little more than O.OOOOI cm.z, so that from the mean curve may be read, with great exactness, the cross section at any point. By summation, the aggregate volume of the tube is obtained, and is compared with the experimental “overall” aggregate. Any difference-and we find differences of not more than I in 6000-is distributed proportionally over the length of the tube, as being probably due to cumulative error in the meniscus-correction; and so the final table is drawn
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up. This shows, for each cm. of the tube, the total volume and the mean volume between it and the point I cm. beyond it, which interval in the case of ordinarily uniform tubee is ample for purposes of calibration.
It may, finally, be mentioned that we have re-calibrated some of our tubes after they had been wed for some months in experiments a t 25' up to 130 atmospheres pressure, without finding any significant change in capacity; nor do they suffer detectable temporary dilatation during exposure to IO atm. internal pressure. The accuracy secured by the method of calibration is thus not illusory. The Sir William Ramsay Laboratories of Inorganic and Physical Chemistry, University College, London.
THE EFFECT O F THE CONCENTRATION O F COLLOIDAL CLAY UPON ITS HYDROGEN ION CONCENTRATION’ BY RICHARD BRADFIELD
I n a recent study of the effect of the soil-water ratio upon the H-ion concentration of soil mixtures, Salter and Morgan2 found with most of the soils studied a logarithmical relationship which could be made to satisfy the adsorption isotherm of Freundlich. Since some of the solid phase remained undissolved at all dilutions, they conclude that their results “discredit any theory of soil acidity which assumes that the acid reaction is due to highly insoluble acids, either organic or inorganic which must under conditions of equilibrium form a saturated solution and give an approximately constant H-ion concentration.” They believe instead that the acidity must be due to the preferential adsorption of the OH-ion by soil colloids. Recent work in this Laboratory on the nature of the acidity of the colloidal clay extracted from an acid soil indicates that this material acts in many respects as a true acid. It has long been known that soils have many properties which are peculiar to colloidal systems. Few attempts were made until recently to separate the more active colloidal fraction from the relatively inert non-colloidal material which makes up the great bulk of the soil mass. Moore, Fry and Middleton3 have found that the amount of colloidal material in soils ie much larger than had previously been supposed. Stevenson‘, Knight6and others have shown that the buffer action and titratable acidity of soils is proportional to the amount of colloidal material present. Soil acidity is considered due to the leaching of bases from the complex mineral silicates. If that is true it would be quite logical to expect t o find that the colloidal fraction of soils would be the most thoroly weathered and consequently the most strongly acid. It has been found6 that if dilute standard solutions of strong bases are titrated with acid colloidal clays by either the conductivity method or the hydrogen electrode method, that definite end points can be obtained, that equivalent quantities of bases are neutralized by equal amounts of the colloidal acid, and that the curves obtained by both methods are of the type’that are characteristic of the neutralization of a strong base by a weak acid. By means of such measurements it becomes possible to assign a definite normality to the colloidal acid. A Contribution from the Soils Laboratory, Bgricultural Experiment Station. University of Missouri. 2 J. Phys. Chem. 27, 117-40 (1923). * J. Ind. Eng. Chem. 13, 527-30 (1921). 4 Soil Science, 12, 145 (1921). J. Ind. Eng. Chem., 12,465 (1920). 6Bradfield: J. Am. Chem. SOC.45, 2669-87 (1923).
