OXYGEN RESPIRATION 



31 



To every liter is added i gram of barium chloride.^ The large 

 vessel of baryta water is closed with a stopper fitted with a 

 soda lime tube, so that no carbon dioxide from the air can enter 

 the vessel. Through another hole in the stopper the vessel 

 is provided with an accurate burette. After any possible 

 precipitate has settled the baryta water always remains clear. 

 The titration of the baryta water is best done with hydrochloric 

 acid for the standardization of which pure cryst. oxalic acid is 

 used to advantage." 



With the tubes filled with baryta water and fastened to the 

 frame, one tube is connected with the plant container and 



Fig. 5. — PettenkofYer tubes. 



the experiment starts as follows. One starts the air pump, 

 completely opens the pinchcock at the rear end of the tube 

 which is connected with the plant container, and then gradually 

 unscrews the pinchcock at the fore end. With the aid of this 

 pinchcock the stream of gas is admitted at the desired rate. 

 The gas bubbles must rise in the Pettenkoffer tube at a uniform 

 rate and must not fuse but pass singly through the whole tube. 

 After the lapse of a certain time, that tube is cut out and the 

 plant container at once joined to another fresh Pettenkoffer 

 tube. In cutting out the tube, first the rear and then the fore 

 pinchcock is closed and the rubber tubing communicating with 

 the plant container is removed from the entry tube. In this 



1 Clausen, H. Landwirtschaftl. Jahrb. 19 : 898. 1890. 



« As an alternative to the use of baryta water, Nishi (Jour. Biochem. Tokyo 

 4:473-480. 1925) describes the use of N/50 XaOH which is titrated in the 

 presence of the NaHCOg. Since thymolphthalein becomes blue at the pH of a 

 dilute solution of NaHCO^ its use permits the titration of the NaOH to completion 

 without a bicarbonate error. — Ed. 



