January 15, 1942
ANALYTICAL EDITION
because t h e end points are usually indicated b y the position
of a very obvious inflection point in the volume-potential titration curve and not b y the realization of a definite, predetermined potential difference between the electrodes. Typical potential-volume titration curves resulting from t h e use of a glass reference electrode in redox and precipitation titrations are given in Figures 1 and 2. The conditions under which t h e titrations were made are given in Table I. T h e Beckman shielded glass electrode was used in all the titrations. The titration apparatus is similar t o t h a t described b y Lykken and Rolfson ( 2 ) ; a close-up of the electrode system discussed in the present paper is shown in Figure 3. The titration cell potential was measured with a continuous-reading electronic voltmeter ( 3 ) .
69
The authors recommend the use of a glass electrode as a reference electrode for all redox and precipitation titrations t h a t yield potential-volume titration curves with realizable inflection points and that can be made at 0" to 50" C. This conviction is based on two years of experience with the glass reference electrode in routine potentiometric titrations.
Literature Cited (1) Heintee, S. G., J . AQT.Sci., 24,28-41 (1934) ( 2 ) Lykken, L., and Rolfson, F. B., IND. ENG.CHEM., ANAL. ED.,13, 653 (1941). (3) Penther, C. J., Rolfson, 1 . B., and Lykken, L , Ibid., 13, 831 (1941). (4) Stewart, 0.J.,and Carruth, W, L., Ibid., 9, 681-2 (1937). (5) Tamele, X I . R., and Ryland, L. B., Ibid., 8,16-19 (1936). (6) Wynd, F. L., Ann. Missouri Botan. Gardcn, 22,861-5 (1936).
Laboratory Flowmeter with Interchangeable Precision-Bore Capillaries '
FRANK C. CROXTON
Battelle Memorial Institute, Columbus, Ohio
T
HE development of the laboratory gas flowmeter, par-
ticularly toward ruggedness and ease of calibration, has not been rapid. A number of devices for altering the useful range have been described but only one appears to be adaptable t o reproducibility of calibration in mass production. The capillary tube-manometer combination for measuring gas flow rates, described by Riesenfeld ( 6 ) , had no provision for use in widely different ranges. Later, however, the same investigator (7) described a flowmeter in which the capillaries were attached by means of rubber tubing and thus were interchangeable. Yoe (9) cemented capillaries in disks which were clamped between flanges when in use. Sharp (8)used a slotted stopcock as the constriction in a flowmeter, obtaining in that way a ~ i d range e of adjustments. Instead of a capillary tube or an orifice, Hofsass ( 4 ) used a rod inserted into a glass tube. The longer the annular space between rod and tube the greater the pressure drop a t a given rate of gas flow. Yuster I 10) selected one from a group of capillaries, sealed in place, by means of one or more three-way stopcocks. He also described a variation employing a large, tapered joint. As an adjustable orifice in noncorrosive applications, Marks ( 5 ) adapted the iris diaphragm to a flosmeter. Bradford (a) measured flow rates at constant pressure drop across a long-taper needle valve with micrometer adjustment. Recentlgthe Ace Glass Company has offered f l o meters ~ in which the capillaries are mounted on interchangeable ground joints and the Corning Glass Company lists one having a stopcock with four orifices of different sizes. For the fundamentals of laboratory flowmeters reference should be made to the excellent discussion by Benton ( I ) . Although the interchangeable capillaries or orifices now available extend the range of a single instrument, the conventional U-tube manometer still presents a breakage hazard and is rather difficult to lead.
In order to eliminate these disadvantages a flowmeter was designed having the following merits which, it is believed have not been described before in this connection: (1) The manometer is completely enclosed in a single glass tube 2 em. in diameter which provides strength and ease of mounting. ( 2 ) The tube is graduated so that liquid levels may be read quickly and easily. (3) The interchangeable capillaries are of precision bore and length and thus a series covering a wide range of gas flows is available. The construction of the flowmeter is shown in Figure 1.
The body, A , is graduated in millimeters over a length of 15 cm. Inside, the manometer tube, B, with a bore of 2 mm., joins the lower chamber with the upper one, C. These chambers are also connected by the inner half of a interchangeable ground joint, D, on which the capillary tubes, E, fit. The cap, F , fits the main portion of the flowmeter by means of a 19/10 i n t e r c h a n ge a b l e ground joint. Gas inlet to the lower chamber is at G . The gas outlet from the upper chamber is at H . The pressure drop across capillary E is read on manometer B. Since, as a H D result of capillarity, the height of the liquid in B above the main body of G liquid will be greater than that corresponding to the true pressure drop, it is desirable to use tube J , open at both ends and of the same bore as B , for reference. In event of surges during the operation of the flowmeter, part of the manometer liquid may be forced into the upper chamber. Tube R is sealed on at the lowest point, honever, and drainage backinto the reservoir is practically complete, Furthermore, the gas outlet tube is sealed on at such a height that chances of entraining liquid in the gas stream are slight. With sacrifice of some ease of manipulation, the outlet I I , connection may be placed b 8 cc-. a t the top of the cap, coaxial with the graduated tube. A FIGURE 1. Gas FLOWMETER third 2-mm. tube may be 4
6
INDUSTRIAL AND ENGINEERING CHEMISTRY
70
(Triplicate
TABLEI. FLOWMETER CHARACTERISTICS oapillaries. Bore = 0.676 mm. Length = 50.00 * 0.03 mm.
Capillary No.
1
Air a t 20° C. and 760 mm.) Manometer Observed Air Calculated Air Reading, Mm. Hz0 Rate, Ml./Min. Ratea, hll./Min. 117.5 118.9 51.7 210.4 211.2 98.7 281.2 281.9 139.6
2
53.2 101.2 139.6
123.7 217.2 283.2
122.1 215.7 281.9
3
50.1 100.0 144.1
115.3 213.2 288.8
115.5 213.5 289.2
a Caloulated from equation of best curve through pointe. V 4.24 x IO-shE 5.44 x 10-6ha
+
=
2.505k
-
included within the envelo e with its up er end sealed through the wall at about the lever of G. Vente: to the atmosphere in this way, it will read upstream pressure above atmospheric. When water is used as the manometer liquid, the addition of a small amount of a selected wetting agent will improve drainage both during normal operation and after a surge. Foster (3) suggested methyl salicylate as a manometer liquid in order to avoid the drainage difficulties characteristic of water. It possesses certain disadvantages, however, which make it less desirable than water containing a trace of surface-active material. Agents which have been found suitable include Wetsit, Du onol W A, and Tergitol 7, but many others are probably equalyy effective. One drop of the wetting agent is sufficient since, in most cases, the total amount of manometer liquid will be approximately 3 ml. This is not enough to alter specific gravity significantly but will improve wetting properties considerably. The outer half of the 7/15 joint is sealed to the capillary in such a way as to leave the capillary end unchanged and unobstructed. In addition, considerable care is taken in the sealing process to avoid altering the bore of the capillary. To determine what degree of reproducibility could be attained in the construction of these capillaries, a set of three, 50 mm. in length, was prepared
I
/
Vol. 14, No. 1
from Pyrex precision-bore tubing of 0.676-mm. internal diameter. Lengths of slightly more than 50 mm. were ground true on both ends in a Thompson surface grinder, using a 2 per cent aqueous solution of soluble oil as coolant. No deviation greater than 0.03 mm. from 50.00 mm. was permitted in the finished capillaries. Examination revealed that the ends of the bores were completely free from appreciable chipping or fusing as a result of grinding. Table I gives the manometer readings corresponding t o several rates of flow for each. The average deviation of the measurements from the best curve through the group is but k0.53 per cent. This is satisfactorily low and indicates t h a t dimensional reproducibility is easily attainable to the desired accuracy. I n its practical aspects this means that a given capillary might be duplicated whenever necessary and the original calibration figures would apply to the new tube as Tyell as to the old one. With the question of reproducibility of capillaries clarified, a series of tubes covering reasonable gas flow rates was designed. It was assumed that the usable portion of the manometer scale would be 10 to 150 mm. and the liquid would be water. Capillary lengths were established at 50.00 * 0.03 mm. Table I1 shows the bore for each member of the series and the minimum and maximum flow rates. The calibration curves for the four capillaries are shown in Figure 2. A range of 0.002 to 4.9 liters of air per minute a t 20" C. and'760 mm. is covered and may be extended upward with either larger capillaries or manometer liquids of higher specific gravities. Further work should lead to complete calibrations a t higher pressure differences and with empirical corrections for specific gravity and viscosity of gases. There will then be available a rugged and compact flowmeter with but four interchangeable, standard capillaries which will cover the range of laboratory flow rates most used and whose calibration curves will be accurately characteristic within the limits of experimental error, thus allowing replacement or duplication without confusion.
TABLE 11. FLOWMETER CAPILLARY BORES Flowmeter
Flow Rate, Air at 20' C. and 760 Mm. Pressure Pressure difference, difference, 10 mm. HpO 150 mm. HnO Liters per minute
Capillary Bore iMm.
Acknowledgment The author wishes to thank R. Shutt for his continued interest during the course of this work. Grateful acknowledgment is made to the Battelle Memorial Institute for permission to publish this material.
Literature Cited
50
PRESSURE DIFFERENCE-mm
IO0
H&
FIGURE 2. FLOWMETER CALIBRATIONS Air at 20' C. and 760 mm.
(1) Benton, A. F., J. IND.ENG.C H E M . 11, , 623 (1919). (2) Bradford, B. W., Chemistry & Industry, 52, 363 (1933). (3) Foster, R. H. K., IND.ENG.CHEM.,18, 82 (1926). (4) Hofsass, M . , Gas-u. Wasserfach, 70, 293 (1927). (5) Marks, G . W., Rev. Sci. Instruments, 3, 764 (1932). (6) Riesenfeld, E. H., Chem.-Ztg., 42, 510 (1918). (7) Ibid., 51, 678 (1927). , 646 (1925). (8) Sharp, J., IND.ENQ.C H E M .17, (9) Yoe, J. H., J . Soc. Chem. I n d . , 44, 432T (1925). (10) Yuster, S., IND.E m . C H E M .ANAL. , ED.,4, 224 (1932).
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