348 K. DAMASCHKE, F. TODT, D. BURK, 0. WARBURG VOL. 12 (1953) 



The electrodes employed in a given vessel were (a) polarizable, a platinum wire 

 of approximately 0.5 cm length and 0.03 cm diameter, and (b) unpolarizable, a cast-iron 

 electrode of high carbon and copper content, selected for stabihty, of approximately 

 2 cm length and 4 cm^ surface area, and immersed in saturated KCl solution contained 

 in a glass tube at the end of which was a semi-permeable clay diaphragm heat-sealed 

 to the glass end. At the top center of the vessel was a ground glass joint for holding 

 a stirring device consisting of a quartz tube some six inches in length containing a close 

 fitting water-sealed quartz tube, at one end of which were four paddles at right angles 

 to each other, and at the other end of which was a short piece of rubber tubing connected 

 with an A.C. synchronous motor turned at a high constant speed of 1500 r.p.m. to 

 provide maximum electrode response, which, in the absence of stirring, could be 10-50 

 times as small. The platinum micro-electrode was placed in the culture medium as close 

 to the paddles of the stirrer as feasible, in order to take maximum advantage of the 

 stirring. When in use, a vessel was completely filled with liquid medium {Chlorella 

 suspension), and contained no gas phase. 



Calibration of the platinum electrode for response to oxygen was carried out in 

 one or both of two ways. For higher oxygen pressures, the current was measured in 

 solutions saturated with air, yielding values of 50 to 100 /x Amp. for 21% of i atm. Og, 

 the actual value in any one electrode depending mainly upon the size of the electrode. 

 At lower oxygen pressures, small quantities of air-saturated suspension medium of 

 known oxygen content (6.3 ix\jva\ at 20° C) were admitted by means of the micro- 

 burette, which contained a long open capillary permitting a forced extrusion of an equal 

 volume of liquid. By this second method, it was always possible to make a calibration 

 under the same conditions, and at essentially the same time, as the experimental 

 measurements of photosynthesis or respiration (see Fig. 4 as example). With micro- 

 electrodes of platinum the two methods of calibration gave identical results, that is, 

 galvanometer readings were directly proportional, within experimental error, to oxygen 

 content up to air-saturation, in the diffusion-current-region of the current-voltage curve 

 employed, where current is virtually independent of potential. 



Current measurements below 5 ju. Amp. were usually made with a 200 mm scale 

 Bruno Lange Multiflex Galvanometer No. 3 with a sensitivity of 2- 10^^ Amp. /mm that 

 permitted distinctions of ca. 0.0003 ^1 Og/ml solution to be made, corresponding to 

 changes of ca. 0.001% of O2 saturation at i atm. Og pressure. This w^as a far greater 

 sensitivity than could be obtained with the most sensitive polarographic or other 

 physical or chemical method available. By using larger electrodes, this sensitivity could 

 be increased several fold further. At current values above 5 ju Amp., either shunted, 

 or less sensitive, galvanometers were employed, but in all cases the time response for 

 small changes in oxygen concentration was ordinarily less than one second. 



The galvanometric responses were measured not only visually (see Figs. 2, 5, ()), 

 l)ut were usually also recorded mechanically, by means of either the Photoregistrier- 

 apparat (see Fig. 3) of Bruno Lange, or, most frequently, his Linienschreiber (see Fig. 4), 

 both of which have a time response of less than one second. In the latter recording 

 instrument, the writing pen was activated over its 200 mm ride by means of a selenium 

 cell in front of the light spot of the galvanometer, and the pen wrote upon a drum 

 that revolved once per exactly 120 seconds over a distance of 27.8 cm. 



The Chlorella cultures employed in this study were cultivated in the several ways 

 described in the accompanying publication^, for one to three days, after which they 



References p. 355. 



