1922] WALTON—AFRICAN SORREL 163 
5 cc. volume is completed with the “unknown” solution. The 
contents of all tubes are well mixed before making the color 
comparisons. (Mixing may be accomplished by rolling the tube 
back and forth between the palms of the hands.) 
In making the color comparisons, the tubes, held vertically in 
the comparator, are arranged in two files of three tubes each, one 
file being made up of the tube containing the “unknown,” with 
indicator solution and two tubes of distilled water, and the other 
file consisting of a pair of the standard tubes and a tube containing 
the “unknown” solution, without indicator. This arrangement is 
necessary to obviate optical differences caused by thickness of liquid 
viewed on the one hand, and on the other to offset the natural color 
and any turbidity of the extract under examination. Different 
pairs of standards are tried until the color of light passing hori- 
zontally through that file of tubes matches the color from the file 
containing the tube of ‘‘unknown,”’ with indicator. 
As stated by GILtespiE, the tubes are viewed best against the 
sky. Occasionally, in the case of certain indicators, such as brom- 
phenol blue, trouble is experienced in matching the colors because 
of a dichroic effect, especially noticeable in turbid solutions. In 
such cases the tubes may be viewed by the yellow light of a carbon 
electric lamp, screened as advised by CLARK and Luss. Only two 
indicator solutions were needed in estimating the specific acidity 
of the sorrel extracts, an 0.05 per cent aqueous solution of brom- 
phenol blue,’ and an 0.02 per cent solution of thymol blue (thymol- 
sulphonphthalein) in 80 per cent alcohol. 
To develop the full acid and “alkaline” colors respectively, in 
the standard paired tubes, the following quantities of reagents were 
used for the two indicators: 
Bromphenol blue-—To produce the acid color, 0.5 cc. of N/1o 
hydrochloric acid solution; to produce the alkaline color, 1 drop of 
N/zo sodium hydroxide solution. 
Thymol blue (acid range).—To produce the full acid color, 2 cc. 
of 1.25 per cent hydrochloric acid solution; to produce the color of 
7 Tetrabromophenolsulphonphthalein. The 0.05 per cent solution was prepared 
by diluting one volume of the indicator solution, furnished in the LaMotte field set, 
to twenty volumes, with freshly boiled and cooled distilled water. 
