290 TITRIMETRIC METHODS 



acid filtrate), brought to pH 6.0, to the enzyme soln., and incubate 

 overnight under tokiene. 



2. Determine the ammonia and carry out a formol titration. 

 Muhiply the ammonia found by 1.1 and subtract this value from 

 the formol titration value at the second stage. From the figure 

 obtained, subtract the sum of the formol titer of the enzyme soln. 

 and the free amino nitrogen before the enzymatic hydrolysis to 

 arrive at the peptide amino nitrogen. 



ACID, ALKALI, AMINO, AND CARBOXYL GROUPS 



Titration of acid or alkali in the conventional manner with 

 micro apparatus, using indicators or electrometric means for deter- 

 mining the end point, requires little discussion. Usually it is 

 advisable to use a color standard for matching the end point when 

 indicators are employed. The use of standard acid in capillary 

 burettes offers no difficulties ; however, standard solutions of metallic 

 hydroxides often give rise to the complications of carbonate 

 precipitation in the fine lumen, and for this reason have been 

 avoided by the Carlsberg Laboratory group. The difficulty has been 

 circumvented in some cases by the use of tetramethylammonium 

 hydroxide for the standard solution or by the addition of excess 

 alkali and back-titration with acid. With alkaline end points, pro- 

 tection from the carbon dioxide of the air must be afforded. Methods 

 for accomplishing this have been given on page 257. For electro- 

 metric titrations, various arrangements may be employed, such as 

 the metallic wire electrode of Linderstr0m-Lang, Palmer, and Hol- 

 ter (1935) (Fig. 69, page 184) or the open-cup glass electrode of 

 Sisco, Cunningham, and Kirk (1941) (Fig. 70). 



For the direct raicrotitration of amino groups, Linderstr0m-Lang 

 and Holter (1932) developed an acidimetric acetone titration 

 method, which is described in their procedure for proteolytic enzymes 

 (page 303). Linderstr0m-Lang and Duspiva (1936) employed an 

 alkalimetric alcohol titration method for the direct micro estimation 

 of carboxyl groups, and this measurement is described in their 

 protease method (page 304). Glick (1934) (page 309) determined 

 carboxyl groups by their neutralization in an excess of alkaline 

 buffer and acidimetric titration to an acid pH. An indicator formol 



