204 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
Solubility of Iron Salts 
Duplicate one-liter portions of the nutrient solution were prepared from 
the special salts provided for collaborators (3). To one portion was added 
enough M/2 KOH to increase the pH value from 4.2 to 6.0. To each of 
four aliquot portions of the solution at each pH value were added the 
equivalent of ten milligrams of iron in the form of one of the following com¬ 
pounds: ferric citrate, FePC>4, Fe2(S0 4 )3, and FeS 0 4 . The solutions were 
allowed to stand for 24 hours at a temperature of about 22°C. They were 
then filtered and evaporated to a volume of about 100 cc. After oxidizing 
by boiling with the addition of HNO3, the ferric iron was precipitated from 
the hot solution with NH 4 OH. A white, flocculent precipitate was obtained 
in each case. The precipitate was filtered and washed until free from 
nitrate. 
As the amount of iron present in each case was very small, an attempt 
was made to use the colorimetric method of determining the iron as sulfo- 
cyanate (1). Probably because of interference by the salts of the nutrient 
solution, the results were unsatisfactory. 
The following procedure was therefore employed: After dissolving the 
precipitate in dilute H2S0 4 , the iron was reduced by the use of metallic 
tin and finally titrated with potassium permanganate. The results ob¬ 
tained by this method were fairly satisfactory. The agreement of dupli¬ 
cate solutions may be ascertained from the data given in table 1. 
Table i. 
Table i. Solubility of Iron in Livingston-Tottingham Solution RsCi when Amounts 
of Salt Equivalent to 10 Milligrams of Iron per Liter were Employed 
Ferric Citrate 
FeP0 4 
Fe 2 (S0 4 ) 3 
FeS0 4 
pH 
Mg. Fe 
Mg. Fe 
Mg. Fe 
Mg. Fe 
per 1. 
per 1. 
per 1. 
per 1. 
4-2. 
5-7 
2.3 ~ 
5-6 
8.6 
4-2. 
6.6 
2.3 
5-6 
9.2 
6.0 . 
5-7 
2.3 
4-7 
i-7 
6.0 . 
6-3 
i-7 
4-5 
1.4 
The outstanding feature of these data is the fact that ferric citrate 
is as soluble in the nearly neutral solution as in the more acid one. FeS 0 4 
is actually the most soluble form of iron at pH 4.2, but its solubility is de¬ 
pressed greatly when the pH value is increased to 6.0. Fe2(S0 4 )3 compares 
favorably with ferric citrate at pH 4.2, and is not greatly depressed in 
solubility at pH 6.0. FeP 0 4 is relatively insoluble at both pH values. 
Availability of Iron Salts in Solution Cultures 
A series of cultures were arranged to compare the availability of organic 
and inorganic sources of iron, namely, ferric citrate and Fe 2 (S 0 4 ) 3 ; and also 
