APPLE BY-PRODUCTS AS STOCK FOODS. 13 
tion) was estimated on a portion of the clear extract 15 by a slight 
modification of the colorimetric method described by Gillespie (i#), 
taken from Barnett and Chapman (9) , in which use is made of the 
principle introduced by Clark and Lubs (11), following Salrn (14), 
of " superimposing the twA extreme colors of an indicator in deter- 
mining its half-transformation point." Instead of using a system 
of 9 pairs of tubes, having drop-ratios 1:9, 2:8. etc., Medalia's 
system (IS) of 7 pairs, having pH exponent intervals of 0.2 be- 
tween each pair for the indicators used, was adopted. Briefly, the 
procedure is as follows : 16 
Make the color comparisons in the small "block" comparator described by 
Gillespie. Calibrate 7 pairs of test tubes, selected to fit the comparator and 
for their uniformity in bore, for 5 cubic centimeter capacity, and arrange in a 
double-row test-tube rack. Into each pair of tubes deliver a total of* 8 drops 
of the suitable indicator solution, from 1 to 7 drops in each of the 7 front 
tubes, and from 7 to 1 drops in each tube in the rear row, taking the precau- 
tion to hold the delivery pipette in an upright position. Add enough alkali 17 
(dilute acid in the case of the indicator thymol blue, acid range) to the tubes 
in the front row to produce the full alkaline color and enough acid 17 to those 
in the back row to develop the acid color; then fill to the 5 cubic centimeter 
mark with distilled water, previously boiled and cooled. Similar tubes are 
used for the solutions under examination (the water extracts of the pomace). 
Eight drops of the indicator solution is, of course, required, and the 5 cubic 
centimeter volume is completed with the unknown solution. Mix well 1S the 
contents of all tubes before making the color comparisons. 
In making the color comparisons arrange the tubes, held vertically in the 
comparator, in two files of three tubes each. One file includes the tube con- 
taining the unknown, with indicator solution, and two tubes of distilled water. 
The other file consists of a pair of the standard tubes and a tube containing 
the unknown solution, without indicator. This arrangement is necessary, on 
the one hand, to obviate optical differences caused by the thickness of the 
liquid viewed, and, on the other, to offset the natural color and any turbidity of 
the extract under examination. Try different pairs of standards until the 
color of light passing horizontally through that file of tubes matches the color 
from the file containing the tube of unknown with indicator. 
As stated by Gillespie (1%), the tubes are best viewed 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 
I such cases the tubes may be viewed by the yellow light of a carbon 
electric lamp screened as advised by Clark and Lubs. 
Only two indicator solutions were needed in estimating the spe- 
cific acidity of the pomace extracts, a 0.05 per cent aqueous solution 
of bromphenol blue 19 and a 0.02 per cent solution of thymol blue 
(thymolsulphonephthalein) 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 : 
15 Not necessarily the same extract used for the total acidity determination, but one 
for which this had been determined. 
I l6 A much simpler but less accurate color comparison may be made ou an ordinary 
" porcelain spot-plate, using 5 drops of extract and 1 drop of indicator solution. The 
color is matched against standard buffer solutions of known pH, or in the absence of 
these it may be compared with the colors in Wherry's chart (ffi). 
17 The quantity of alkali or acid varies somewhat for the different indicators. 
18 Mixing may be accomplished by rolling the tube back and forth between the palms of 
the hands. 
19 Tetrabromophenolsulphonephthalein. The 0.05 per cent solution was prepared 
diluting 1 volume of the 1 indicator solution furnished in the LaMotte field set to 20 
volumes with freshly boiled and cooled distilled water. 
