1070-4272/05/7810-1651 C 2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 10, 2005, pp. 1651!1655. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 10, 2005,
Original Russian Text Copyright C 2005 by Paleckiene, Sviklas, Šlinkšiene.
AND INDUSTRIAL ORGANIC CHEMISTRY
Reaction of Urea with Citric Acid
R. Paleckiene, A. Sviklas, and R. Šlinkšiene
Kaunas Technological University, Kaunas, Lithuania
Received May 12, 2005
Abstract-Reaction of urea with citric acid in aqueous solutions at molar ratios of 1 : 1, 1 : 2, and 2 : 1
and the solubility of urea citrate were studied. The data obtained can serve to optimize use of citric acid as
a physiologically active substance or nitrification inhibitor and in production of liquid fertilizers containing
Urea is a frequent component of many liquid inte-
grated fertilizers. Interaction of urea with citric acid
can affect phase equilibria in the systems constituting
liquid complex fertilizers.
Urea is highly reactive. Many urea compounds ob-
tained in reactions with mineral or organic acids are
known . Citric acid is a necessary component in
the system of biochemical reactions of the cellular
respiration. Along with other tricarboxylic acids, citric
acid is contained in small amounts in mitochondria of
all the cells. Another function of citric acid is to sup-
port the acid3base equilibrium and ionic composition
in a living body. Citric acid is a typical chelate form-
ing agent in reactions with metal ions. It can also in-
hibit nitrification  and hence is of interest as a com-
ponent of liquid complex fertilizers.
There is no reliable information on how urea reacts
with citric acid. Fragmentary tentative results have
been published previously .
In this study, we examined the reaction of urea
with citric acid in aqueous solutions and determined
the main physicochemical properties of the reaction
and citric acid C
of chemically pure grade. The compositions of the sub-
stances were determined by chemical and physico-
chemical analyses. The nitrogen content was found
in solutions and in the solid phase .
A thermal analysis of solids was performed on
a Du Pont Instruments 990 Thermal Analyzer at
a heating rate of 10 deg min
The IR spectra were recorded on a Spectrum GX
(Perkin Elmer) FT-IR spectrometer in KBr pellets.
An X-ray phase analysis was performed on a DRON-6
diffractometer with copper radiation and nickel filter
within the 0o3166.5o range of diffraction angles with
a relative error at no more than 0.5%.
Mixtures of urea (Ur) and citric acid (HCit), taken
in molar ratios of 1 : 1, 2 : 1, and 1 : 2, were dissolved
in water taken in amounts corresponding to the weight
ratio of the sum of the reactants to water of 1 : 0.25,
1 : 0.5, 1 : 0.75, and 1 : 1. The solutions were pre-
pared at room temperature and, in a number of cases,
were heated, if required, for complete dissolution of
the initial substances. The results are listed in Table 1.
As can be seen, a precipitate is formed at a 1 : 1
molar ratio; its chemical composition corresponds to
urea citrate with a nitrogen content of 11.1 wt %. Prac-
tically no nitrogen remains in solution. At Ur : HCit =
2 : 1, a substance with ~11 wt % nitrogen content
precipitates, but nitrogen remains in solution in
Table 1. Chemical composition of aqueous solutions
containing urea and citric acid and of precipitate
Ur/HCit ³ Weight ratio ³ Nitrogen content, wt %
³ ÃÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄmolar ³ of Ur + HCit ³
ratio ³ to water ³ precipitate ³ solution
1:1 ³ 1 : 0.50 ³ 11.0 ³ 3
³ 1 : 0.75 ³ 10.9 ³ 0.1
³ 1 : 1.00 ³ 11.1 ³ 3
2:1 ³ 1 : 0.50 ³ 10.9 ³ 12.1
³ 1 : 0.75 ³ 11.0 ³ 10.1
³ 1 : 1.00 ³ 3
No precipitate formed.