MEASURING PHOTOSYNTHETIC ACTIVITY 239 



sulfonate (95 per cent), and for a bubbling pipette: 16 grams NasSzO*, 

 13.3 grams NaOH, and 4 grams anthraquinone-(5-sulfonate ; both the solu- 

 tions represent the amounts of the materials dissolved in 100 cc. of water. 

 One of the commonest and best absorbents for oxygen is yellow phos- 

 phorus. It reacts very readily with oxygen and since the various re- 

 action products are all soluble in water, all that is necessary is to renew 

 the water surrounding the phosphorus from time to time in order to 

 keep the surface of the phosphorus fresh. Thus the phosphorus can be 

 used for a very large number of analyses, especially if it is protected 

 from the light. For this purpose there are special pipettes of brown 

 glass, or the ordinary glass pipette can be covered with paint or a light- 

 tight box. Furthermore, the reaction products of phosphorus and oxygen 

 are solids or liquids having a very low tension so that no error is caused 

 should any of these products be present in gas residue after absorption. 

 Freshly prepared sticks of phosphorus do not absorb oxygen quantita- 

 tively. It is advisable to run air through the pipette a few times before 

 making air analysis. Gas mixtures containing more than 50 per cent 

 oxygen should be diluted with an equal volume of nitrogen, as^ with 

 such high concentration of oxygen the phosphorus may melt or ignite. 

 To dilute the oxygen, air which has previously been passed over phos- 

 phorus can be used. There are a number of substances which hinder 

 the oxygen absorption, e.g. ammonia, hydrocarbons, etherial oils and 



alcohol. 



It is essential to watch the temi^erature at which analyses are being 

 made when phosphorus is used as an absorbent. At 20° the reaction 

 proceeds normally, i.e. it requires less than 10 minutes; below 14° at 

 least 15 minutes is required to absorb the oxygen; while it is not advis- 

 able to use phosphorus below 10°. Under such conditions it is neces- 

 sary to have the absorbing bulb surrounded by a bath of warm water. 



It is best to prepare the phosphorus which is to be used shortly be- 

 fore placing it into the absorption pipette. For this purpose yellow phos- 

 phorus is melted under water in a small vessel in a water bath at 50°. 

 The operation should be carried out in as weak illumination as possible. 

 The molten phosphorus is then carefully drawni into tubes of about 

 2 mm. diameter; the upper end of the tube is closed and at once placed 

 in cold water. The phosphorus undergoes a decided contraction on cool- 

 ing so that the sticks of phosphorus can be easily pushed out of the glass 

 tubes with a rod. In working with phosphorus, of course, proper precau- 

 tions must always be taken on account of its great toxicity and inflam- 

 mability. 



Potassium pyrogallate has in recent years found greatest favor as an 

 oxygen absorbent. However, there are a number of precautions which 

 are absolutely essential to get accurate results. After Liebig announced 

 his discovery that potassium pyrogallate can be used to absorb oxygen 

 quantitatively it was found that in the interaction of oxygen with an 

 alkaline solution of pyrogallol a certain amount of carbon monoxide is 



