Bridge, Agar

Apparatus. Use the apparatus shown in Figs. 14.2(a) and (b). The generator cathode (isolated auxiliary electrode) consists of platinum foil (4 cm x 2.5 cm, bent into a half cylinder) and the generator anode (working electrode) is a rectangular platinum foil (4 cm x 2.5 cm). For potentiometric end point detection, use a platinum-foil electrode 1.25 cm x 1.25 cm (or a silver-rod electrode) in combination with an S.C.E. connected to the cell by a potassium chloride- or potassium nitrate-agar bridge. 

Prepare an approximately 0.1 M silver nitrate solution. Place 0.1169 g of dry sodium chloride in the beaker, add 100 mL of water, and stir until dissolved. Use a silver wire electrode (or a silver-plated platinum wire), and a silver-silver chloride or a saturated calomel reference electrode separated from the solution by a potassium nitrate-agar bridge (see below). Titrate the sodium chloride solution with the silver nitrate solution following the general procedure described in Experiment 1 it is important to have efficient stirring and to wait long enough after each addition of titrant for the e.m.f. to become steady. Continue the titration 5 mL beyond the end point. Determine the end point and thence the molarity of the silver nitrate solution. 

A poly (vinylchloride) membrane electrode was described for local anesthetics, based on dibenzo-24-crown-8 as the electroactive material, and di(2-ethyl)hexyl phthalate as the plasticizer [74]. It was reported that the electrode exhibited a Nemstian response to procaine, and other electrode properties were also presented. The analysis was performed at pH 6 to 6.5 vs. S.C.E., with a 0.2 M lithium acetate agar bridge. Less efficient crown ethers studied at this time were benzo-15-crown-5, dibenzo-18-crown-6, and dibenzo-30-crown-10. 

In nimen-listulaled experimental animals, it has been observed that for certain distribution ratios of chloride in ihc ruminal fluids and hlood plasma, the calculated equilibrium potential Tor chloride is relatively the same as that measured directly with KCI-agar bridges and calomel electrodes However, in many circumstances, the calculated and measured values have been found lo be significantly different, an observation that could only be accounted for by the presence of an active transport mechanism responsible for the movement of chloride out of its equilibrium distribution. 

More elaborate entries, such as "23CD5", ere occasionally necessary this one denotes the use of a three-electrode cell with an agar bridge, deaeration by both hydrogen and sulfite, and digital data-acquisition. When insufficient information was given to permit completing any part of this three-character code, a dash always appears in the appropriate position, as in the entry "0-0". This permits easy differentiation between, for example, "—0" and "0--". 

The products of electrolysis accumulate around the electrodes and change the pH of the buffer locally. To avoid ill effects on the paper strip a baffle system is used, the paper strip dipping into one chamber, the electrode into another, the two chambers being so connected by wicks of material such as glass wool (K22) or by an agar bridge (M8) that electrical current is maintained but physical flow of buffer solution is restricted. 

The reduction potential is an electrochemical concept. Consider a substance that can exist in an oxidized form X and a reduced form X . Such a pair is called a redox couple. The reduction potential of this couple can be determined by measuring the electromotive force generated by a sample half-cell connected to a standard reference half-cell (Figure 18.6). The sample half-cell consists of an electrode immersed in a solution of 1 M oxidant (X) and 1 M reductant (X ). The standard reference half-cell consists of an electrode immersed in a 1 M H+ solution that is in equilibrium with H2 gas at 1 atmosphere pressure. The electrodes are connected to a voltmeter, and an agar bridge establishes electrical continuity between the half-cells. Electrons then flow from one half-cell to the other. If the reaction proceeds in the direction... 

Figure 18.6. Measurement of Redox Potential. Apparatus for the measurement of the standard oxidation-reduction potential of a redox couple. Electrons, but not X or X% can flow through the agar bridge.

Place the agar bridges to connect the beakers to each end of the electrotaxis chamber. Two holes in the cell culture dish lid made by cutting through with a hot scalpel blade will minimize evaporation of culture medium during long experiments (Fig. 3b). 

Freshly prepared agar bridges are suggested for each experiment. A check of the electrical connections is made before proceeding to the next step (tee Note 7). 

Prepare agar bridges, complete the electric circuit, and apply electric fields to the cells as detailed in Subheading 3.2. 

Good electrical connections are essential for the electrotaxis experiments. Air bubbles are easily trapped in the electrotaxis chamber or in the agar bridge. It is necessary to check carefully the connections before switching on the power. Short circuit due to overflow of the medium outside the chamber needs to be carefully avoided. 

Figure 14. Experimental setup for measurements of in vitro streaming potentials during electrolyte flow through canine aorta or carotid artery (1) nitrogen tank (2) graduated aluminum foil-covered bottles (3) aorta or carotid artery (4) sponge soaked with electrolyte (5) insulation (6) agar bridge (7) calomel electrode (8) Keithley electrometer and (9) grounded aluminum plate. ...

Figure 19. Apparatus for determinations of sedimentation potentials of blood cells (1) glass tube (2) Tygon tube (3) agar bridge (4) KCl chamber (5) Ag-AgCl electrode (6) Keithley electrometer and (7) clamp. " ...

Calomel I 1 M NH4NO3 agar bridge reference solution (NaDecS04 + NaCl) liquid memtoane I sample solution (polypeptide + NaDecS04 + NaCl) I 1 M NH4NO3 ar bri( e I Calomel... 

An Ag/AgCl reference electrode is connected to the bath solution. The reference electrode can be put directly into the recording chamber, or connected via an agar bridge. In the last case, the use of an agar bridge allows to use chloride-free external solutions, with a significant smaller interference with the electrode potential. 

Fig. 1, Effect of reference electrode on potassium determination. Single measurements in NaCl or KCl with reference electrode (glass micropipet with 5 y m tip) against a 3 M KCl agar bridge 2 mm in diameter, upper lines, and with liquid ion-exchanger electrode against the reference electrode, lower lines. The reference electrode was filled with a solution of 3 M lithium acetate with or without addition of agar 2 g/100 ml.

Fig. 2. Effect of tip size and filling medium on reference electrode tip potentials. Single measurements in NaCl or KCl with reference electrode against a 3 M KCl agar bridge. Left panel, lithium acetate filled electrodes with 1 y m tip, broken tip (40 y m), and 5 y m tip (with and without agar). Right panels, pipets filled with KCl or NaCl with or without agar 2g/100 ml.

A saturated potassium chloride agar bridge is made by adding 3-5 g of powdered agar, in small portions at a time, so that the solution does not froth and boil over, to 100 cm of a saturated solution of potassium chloride at 100 °C on a steam-bath. The solution is kept at 100 until... 

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