Selection of Capillaries with Predetermined Characteristics for the Dropping Mercury Electrode N o m o g r a m a n d Directions
State 1 ciitersity of Yew
OTTO H. M~ULLER Y o r k .Medical Center at Cs\.rwii.sc, Syracuse, Y. I-.
PROBLEM facing everyone in polarography is the choice of
From this may be seen t h a t pressures greater than 60 cm. of mercury are needed to produce a n y flow a t all through capillaries with p values of less than 10 microns (or a diameter of < 20 microns). Such capillaries are therefore not feasible for practical polarography. On the other hand, capillaries with p values greater than 40 microns would continue flowing in potassium chloride solution or distilled water until the mercury pressure is diminished below 14 cm. This may be considered the upper limit in the size of useful capillaries for polarography, since capillaries of larger bore will use too much mercury in their operation, and require means for stopping the flow of mercury other than simple lowering of the reservoir. Hence, one can conclude t h a t suitable capillaries for polarographic analyses must have internal diameters ranging from 20 t o 80 microns. The most useful capillaries for all-around work will probably have diameters varying between 30 and 60 microns
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proper glass capillary for the dropping mercury electrode. It is known t h a t t o be suitable for analytical work, the latter should have a drop time between 2 and 6 seconds in 0.1 iV potassium chloride a t pressures between 30 and i o em. of mercury, Imt the select,ion of such a capillary has been a hit-or-miss proposit ion in which experience was the best guide. When i t comes to laboratory-drawn capillaries, the selection will probably always wmain t h a t way unless the experimenter is sufficiently expert in their manufacture. However, in the case of the more widely used capillaries of uniform bore it is possible t o calculate the exact cliniensions of a capillary which will have a desired drop time at a desired pressure of mercury, so that experience is no longer necessary for it8 selection. This paper calls attention t o this fact and gives thc equations needed for the calculations as well : glam for simplifying their application. .Icapillary of uniform bore can be defined by its length and its internal diameter or radius. If a capillary is Iaborutory-drawn, this is not possible; either the bore is not uniform or the length of ariy existing uniform bore is not measurable. Severtheless, such c:cpillaries can still be characterized and compared by means of the radius of their orifice, p, and their capillary constant,, K , which represents t h a t pressure of mercury necessary t o cause the flow of 1 ing. of mercury per second through the capillary. Simple metho(1s for the determination of these constants have been describrd ( 1 ) . The following equation defines K in terms of the length, I , of' :I capillary of uniform bore and of the radius of it,
