A Sealed-In Micro Glass Electrode

Abstract

A sealed-in glass electrode essentially similar to that described by Michaelis 1 but of much smaller capacity can readily be adapted for pH measurements of biological fluids when only rather small volumes are available and under conditions which exclude contact of the sample with air.

The design is shown in the accompanying diagram (Fig. 1); the inner tube of Corning O15 glass may be of capillary dimensions but care must be taken to minimize the capacity of slight but unavoidable dilations at the two ends where this capillary is sealed, through heavy walled graded glass, to the outer jacket. When this unit, the capillary electrode within its jacket, has been tested and found free of electrical leaks or other defects it is attached permanently to a stopcock of the type described by Stadie, O'Brien and Laug. 2 For testing it is convenient to connect it temporarily with rubber tubing to a spare stopcock of the same type. Redescription of this stopcock is not necessary but two points should be mentioned: (1) in order that the electrode may be permanently attached, as at X in the diagram, the stopcock must be made of soft glass similar to that of the electrode jacket. (2) In order to reduce the volume of the test sample to a minimum the diagonal bore for filling must be as fine as possible and without unnecessary widening at the ends. This bore has a diameter of 1 mm. in the electrode assemblies in use by the author; the bore of the filling and connecting tube is about 0.5 mm.

The method of filling the electrode depends. of course, on the technique and requirements of sampling. In an investigation of the digestive juice and blood of tarantulae it has been convenient to withdraw the sample directly from the animal into a 1 ml. Luer syringe (the short “Insulin” type is preferable). The needle is then removed and a short piece of haemocytometer tubing (3 mni. internal, 5 nim. esternal diameter) is fitted over the nozzle.