162 BOTANICAL GAZETTE [OCTOBER 
out phenolphthalein. In this blank titration, the addition of N/10 
sodium hydroxide solution kept pace with the quantity added in 
the true titration, and the end-point was determined by contrasting 
the colors of the titrated extracts. The extract containing phenol- 
phthalein developed a noticeably redder brown color than the other. 
In the computations allowance was made for the water present 
in the green material. The total titrable acidity is expressed as 
cubic centimeters of normal acid per kilogram of leaf material, 
termed the ‘degree of acidity.” 
SPECIFIC ACIDITY OF WATER EXTRACT 
The specific acidity was estimated on a portion of the clear ex- 
tract by a slight modification of the colorimetric method described 
by GILLESPIE (3) after BARNETT and CHAPMAN (x1), in which use is 
made of the principle introduced by CLARK and Luss (2), following 
Sat (7), of “‘superimposing the two extreme colors of an indicator 
in determining its half-transformation point.’’ Instead of using a 
system of nine pairs of tubes having drop ratios 1:9, 2:8, etc., 
MEDALIA’s (4) system of seven pairs, having P,, exponent intervals 
of o.2 between each pair for the indicators used, was adopted. For 
convenience the procedure is briefly sketched here. 
The color comparisons are made in the small ‘‘ block”’ compara- 
tor described by GILLESPre. Seven pairs of test-tubes, selected to 
fit the comparator and for their uniformity in bore, are calibrated 
for 5 cc. capacity and arranged in a double row test-tube rack. A 
total of eight drops of the suitable indicator solution is delivered 
into each pair of tubes, 1 to 7 drops in the front seven tubes and 
7 to 1 drops in the back row, care being taken to hold the delivery 
pipette in an upright position. Sufficient alkali® (dilute acid in the 
case of the indicator thymol blue, acid range) is then added to the 
tubes in the front row to produce the full alkaline color, and suffi- 
cient acid to develop the full acid color is added to those in the rear 
row. ‘The tubes are then carefully filled to the 5 cc. mark with dis- 
sero water, , previously boiled and cooled. Similar tubes are used 
for the sol tion (the water extracts of the sorrel). 
Licht dives of! the Jaticator solution are required, of course, and the 
§ Quantity of alkali or acid varies somewhat for the different indicators. 
