480 
Journal of Agricultural Research 
Vol. XXVIII, No. 5 
At first it was thought that a standard glucose solution in saturated picric 
acid might be a convenient source of color, since a standard tube could be pre¬ 
pared each time a series of analyses is run. It was found, however, that such a 
solution gradually increases in color-producing value, being about 115 per cent 
of its original value at the end of a year. The change is rather slow, however, 
and the solution may be used for about a week without any appreciable change 
being noticed. Solutions of pure picramic acid, or of sodium picramate, match 
the sugar colors exactly, but in the writers’ experience they fade too rapidly to 
be satisfactory. It was thought that the change might be hastened to comple¬ 
tion by treating with heat or with intense light or with both. But instead of 
fading, the color became more intense during the treatment, and then faded 
again, so this method proved useless. The usual method is to make a standard 
glucose solution each time a set of analyses is to be run. This is time-consuming 
but accurate, and is possibly the best procedure available at present, especially 
where analyses are not run daily. 
Another procedure that was tried was to make the color in bulk from sugar, 
and then store in amber bottles. Without giving the data in detail, it is suf¬ 
ficient to say that the colors from both sucrose and glucose, made in quantities 
of 500 cc., and stored in both clear and in amber bottles ; faded rapidly. After 
30 days the fading was much slower, but it was still too marked even after 100 
days to warrant using the material as a standard. 
From the above experiences, the writers recommend as a standard an 0.08 per 
cent glucose solution in saturated picric acid, making a new solution each week. 
A reduction in sugar tubes is made with each set of analyses run, using 1 cc. of 
sugar, 1 cc. of water, 1 cc. of saturated picric acid, and 1 cc. of 20 per cent 
sodium carbonate solution. This is the equivalent of using 1 cc. of unknown 
sugar solution and 2 cc. of picric acid. This is the most convenient standard 
where daily analyses are run. Where the analyses are intermittent, it is simpler 
to make an 0.08 per cent glucose solution in water, and make reductions of it in 
the usual way. 
EFFECT OF PICRIC ACID ON GLUCOSE AND FRUCTOSE 
It was mentioned above that glucose, on standing in picric acid at room tem¬ 
perature, gradually increases its chromogenic value. It was also stated that 
when sucrose is to be determined by the present method the solution of sugar 
is first heated for 10 minutes with the picric acid alone, for inversion, and then 
heated again with sodium carbonate, for reduction. (Rose (16) found 10 min¬ 
utes was sufficient for inversion, and this has been corroborated by the writers.) 
It was noticed that sucrose always gave more than its theoretical color value. 
These facts suggested that either the heating in the presence of picric acid has 
some effect on the glucose and fructose, or the fructose constituent has a greater 
chromogenic value than the glucose. The latter alternative was proved not to 
be the case, as will be mentioned below. 
It was then decided to investigate the heat effect. Okey (18) found that both 
glucose and fructose have enhanced color values when heated with picric acicf^ 
and she abandoned the attempt to use picric acid as a hydrolyst for inulin. A 
standard glucose solution was treated both ways: that is, the color developed 
both with and without the 10-minute heating with picric acid. These two treat¬ 
ments duplicate the procedure for total sugars and for reducing sugars, respec¬ 
tively. The results are shown in Table I. It is apparent that glucose gives a more 
intense color when subjected to a preliminary heating with picric acid. Heating 
for 10 minutes causes a maximum development of color. In further trials this 
color was found to be constant regardless of the ratio of reducing sugars to sucrose 
