V. ESTIMATION 245 



relatively small weight of material required and the wide variation in con- 

 centration of ascorbic acid within one plant organ, or between different 

 individual fruits and vegetables.*- s- S". sb (ggg ^Iso p. 261.) Hand grinding 

 with acid and sand is not only laborious and requires several extractions, 

 but, since only a limited amount of material can be extracted by this pro- 

 cedure, it needs care to deal adequately with this difficulty of represen- 

 tative sampling. 



The use of homogenizing or emulsifying apparatus, such as the Waring 

 Blendor, has been valuable in helping to overcome this sampling difficulty 

 as well as ensuring more complete extraction. ^-^^ Emphasis has, however, 

 been laid on the importance, during homogenizing, of avoiding the oxida- 

 tion of ascorbic acid, which may be accelerated by the whipping of air into 

 the acid-sample slurry (blending in an atmosphere of an inert gas has been 

 suggested to avoid this'^'^) or by the dissolution of copper from worn metal 

 parts of the apparatus.'* In addition to these precautions for avoiding 

 oxidation of ascorbic acid by atmospheric oxygen during the extraction 

 stage, the concentration of acid must be kept sufficiently high to inactivate 

 oxidative enzymes set free during the process'^- '^'^ (usually 5 to 6 % meta- 

 phosphoric acid is preferred); similarly the tissue must not be minced, 

 ground, or shredded except under the appropriate acid,^- "• ^^ and it should 

 be completely immersed in the extractant."- '^ Distilled water and other 

 reagents should be free from copper.^^- 20. 21 To stabilize the vitamin, 

 crystalline sodium chloride has also been suggested in place of sand when 

 plant tissue is extracted by hand grinding.^i^ The ratio of extractant to 



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