Feb. 10, 1942.

E, COX ETAL 2,272,617

CALCIUM ACID PYROPHOSPHATE COMPOSITION AND METHOD OF PRODUCTION
Filed March 18, 1939

60

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Patented Feb. 10, 1942 2,272.617
UNITED STATES PATENT OFFICE
2,272,617
CALCUMAC) PYROPOSPEATE COMPOS
TON AND METHOD OF PRODUCTION
Edwin Cox and Robert E. Kean, Richmond, Wa.
and Frank B. Carpenter, Jr., and Wilbur K.
Enos, Charleston, S. C., assignors to Wirginia
Carolina Chemical Corporation, Richmond, Wa,
a corporation of Virginia
Application March 18, 1939, Serial No. 262,734
2 Claims. (C. 23-08)
This invention pertains to an acid calcium salt form of rhombic crystals. The curve in Figure 1
of pyrophosphoric acid, and to its production. shows that as heating of this hydrate proceeds,
There are several series of phosphoric acid, there is a temperature range during which an
Such as Orthophosphoric acid, metaphosphoric hydrous monocalcium phosphate is obtained rap
acid, and pyrophosphoric acid. These form se idly, namely, about 135 to 140° C. In commercial
ries of salts that vary in their degree of neutrali practice dehydration occurs below this tempera
ture range, but at slower rates; dehydration oc
zation. Thus, from orthophosphoric acid, the
formula for which is H3PO4, there are primary or curs above this temperature range, but with much
mono-Orthophosphates in which one of three greater difficulty of control of the product. Or,
available hydrogen atoms has been replaced by O one may start with anhydrous MCP, either ob
a base. Monocalcium orthophosphate is referred tained by dehydration or obtained by crystalliza
to hereinafter as MCP. There are also second tion from phosphoric acid solutions. Conditions
ary Orthophosphates and tertiary orthophos of concentration and temperature for obtaining
phates in which two and three hydrogen atoms, such crystallized MCP are well known. For ex
respectively, have been replaced by a base. 5 ample, such have been described in Mellor's
In comparison, the formula for pyrophos Treatise On Inorganic Chemistry, volume 3, page
phoric acid is HaP2O7. In the calcium acid salt 866, and references cited therein; or described
of pyrophosphoric acid only part of the hydrogen by Clark in Journal of Physical Chemistry, vol
has been replaced by calcium. Calcium acid py ume 35, page 1232 (1931). Anhydrous MCP is
rophosphate is referred to hereinafter as CAPP. a crystal of triclinic form.
As Figure 1 shows, heating of MCP above
It has been stated that CAPP is not stable and
that attempts to produce CAPP have resulted in 140° C., is accompanied by temperature rise until
Some compound of metaphosphoric acid rather another change occurs. This is a transformation
than the desired CAPP. to CAPP in which hydrogen and oxygen in pro
An object of this invention is to produce CAPP 25 portions to form water are expelled from the MCP.
as a stable salt. A special purpose is to form This conversion occurs most effectively at about
CAPP in finely divided condition, to be utilized 200 to 250 C. at atmospheric pressure.
for example Where extensive Surface is desirable. This invention is not restricted to converting
Also, a purpose of this invention is to manufac MCF of chemical purity, but may utilize con
ture CAPP in simple manner, under easily con 30 mercial varieties in which dicalcium phosphate
trolled conditions, and by inexpensive methods and tricalcium phosphate are present. However,
suitable for commercial manufacture on a large these secondary and tertiary salts result in im
scale, and particularly by a dry method of pro purities in the final CAPP and it often is desir
duction. A more specific object is to produce able to minimize the dicalcium salt.
CAPP in useful and novel combinations. With 35 Calcium acid pyrophosphate as thus formed is
MCP. These objects and others will be apparent expressed by the formula CahaP:O7. It is an
in the illustrative description of this invention anhydrous salt. It is free from phosphoric acids.
accompanying the drawing and particularly This may be determined by washing the product
pointed out in the appended claims. in acetone to dissolve any free phosphoric acid
In the drawing: 40 and testing the extracts for free phosphoric acid.
Figure 1 presents a curve showing the conver Also, chemical tests show the product to be free
sions that occur on heating hydrated MCP to from Orthoa and meta-phosphates.
different temperatures up to about 280 C. Calcium acid pyrophosphate thus formed is
Figure 2 is a chart showing curves for rates of White or Colorless. It is insoluble in acetone. .
evolution of Carbon dioxide gas from sodium bi 45 In water it is soluble less than 4 parts in 100 at
carbonate solution by various acid phosphates. 20 C., somewhat less than either hydrated or
The processing procedure under this invention anhydrous MCP. Calcium acid pyrophosphate is
rests on our observation that CAPP may be ob Odorless. It is of acid taste and exhibits a pH
tained by controlled heating of MCP to tempera in 0.5% aqueous solution of 3.18 to 3.20, as com
tures within the range of 200 to 250' C., or even 50 pared with a pH of 3.9 to 4.3 for 0.5% solutions
about 265 C. to 300 C. of RCP.
Transformations that occur on heating MCP The neutralizing value of CAPP for sodium bi
are shown in Figure 1. Warious forms of MCP carbonate to an endpoint of pH 8.2 is about 60
may be treated, for example, the Crystaline hy to 64 as compared with about 88.8 for MCP.
drate, of formula Ca(H2PO4)2.H2O, having the 66 Neutralizing value is a weight of sodium bicar
 2 2,272,617
bonate expressed as percentage of the Weight minutes, while anhydrous crystal MCP required
of acid phosphate to neutralize the bicarbonate. heating from periods of 75 to 120 minutes. The
Neutralizing value is determined by adding to physical character of the MCP influences also
acid phosphate under test an excess of standard the amount of CAPP required to adequately cover
NaOH. The solution is boiled for one minute; the surfaces. This is illustrated in the following
then excess NaOH is determined by titrating the representative examples of partial conversion of
solution hot to a phenolphthalein endpoint with MCP particles to CAPP.
standard acid.
Rate of release of gas from sodium bicarbonate Conversion to CAPP
by CAPP is considerably slower than by either 10
hydrated or anhydrous MCP. At room tempera Temp. Time CAPR
tures, CAPP released from sodium bicarbonate
the following percentages of the total available C.O. Min. Perce
carbon dioxide gas: Commercial MCP-...---------------- { 2: 6
Per cent 5 .Commercial
M. MCP free from di- and 225-233 5 23
2 minutes-------------------------------- 2 tri-Orthophosphate------------------ 58; 8
5 minutes-------------------------------- 9 Crystal MCP, hydrated, pure-------. 2: 18
30 minutes------------------------------- 50 230-234 10 6
Completion at temperature of boiling water. Anhydrous crystal MCP------------- 221-23
- 231-238
5
O
The reaction goes to rapid completion at 100° 20 Anhydrous agglomerate.------------- 230-235 O
C. Comparative rates for MCP are relatively
rapid though varying slightly with the physical While conversion within the temperature range.
character of the material. They attain: of about 230 to 235 C. is described in this table,
Per cent 25 slower conversion can be obtained at tempera
tures as low as 200. However, above 250 other
1 minute.---------- -- - - - - - - - - - - - - - - - - - - - - -
2 minutes-------------------------------- 50
48 phosphates than acid pyrophosphates are ob
30 minutes------------------------------- 60 tained; that is, calcium metaphosphates and
polyphosphates begin to appear. Hence, from
Figure 2 also compares CAPP with sodium acid 250 to 300° C. one may obtain coatings of CAPP
pyrophosphate, 3) admixed with other phosphates. These coated
Anhydrous calcium acid pyrophosphate takes products are characterized by lower neutralizing
up moisture from a very humid atmosphere, but value and a lower pH than for CAPP. For ex
loses moisture when exposed to a relatively dry ample, at 265 C., a product is formed having a
atmosphere. This moisture is apparently, largely pH of 3i and a neutralizing value of 51.
superficial and in large part governed by the In these various new products, it has bee
physical character of the CAPP, found that although total acidity as evidenced
When CAPP is prepared from MCP the re by neutralizing value declines, so also does pH,
sulting macroscopic appearance is characteristic though free acids are absent. This increased hy
of the material heated and consequently is . . drogen ion concentration is a definite and un
rounded nodular from MCP of that character, t usual characteristic of the products of this inven
rhombic in gross appearance from monohydrate tion.
MCP crystals, or triclinic from anhydrous MCP It has been stated that CAPP is less soluble
crystals. The rate of conversion to CAPP varies than MCP. Consequently, the control made pos
somewhat with the physical character of the sible by the present invention makes feasible
MCP utilized as source material. Somewhat 45 substantially any rate of reaction of calcium
longer heating or heating nearer the higher tem phosphate with carbonate intermediate the rate
perature of the described ranges is necessary for hydrated MCP and that for CAPP. With
with crystal materials than materials of more
open structure. MCP forming the interior of the partially con
verted particles described, CAPP acts initially as
This invention admits of controlling the con a sheath to prevent contact of liquid. This may
version of MCP to CAPP to obtain composite be termed 'retarded MCP'.
particles in which the central portion is of anhy Although it has been indicated that this con
drous RACP, while the surface contains a greater version by heating MCP occurs under atmos
percentage of calcium and phosphorus than the f
pheric conditions, it is contemplated that dimin
anhydrous MICP. Partial conversion to CAPP s ished pressure or vacuum is of assistance.
on the surfaces can be controlled by the amount A preferred application of this invention is pro
of heat applied, by the time of heating, and by duction of CAPP by heating open structure or
the physical character of the MCP heated. porous MCP at 230-240° C. Important utility
Utilization of hydrated phosphate as source ma results from partial conversion of MCP into
terial permits introducing considerable range in 60 CAPP at the surfaces of the particles being heat
porosity or potentially reactive surface into the ed, when anhydrous crystalline MCP is heated
final product, since prior to conversion to CAPP for a limited time, say 15 minutes or less, at ten
heating of crystalline hydrated MCP or com perature of 230-240° C. The total composition
mercial MCP evolves moisture to. leave the mass
in porous condition.
may be about 75-90% anhydrous MCP and about
65 25-10%. CAPP. Also, it is useful in many ap
Anhydrous MCP in crystalline form may con plications of this invention to heat, open struc
sist of dense, non-porous, diserete crystals. ture MCP to a temperature above 250, say 265 C.,
Utilization of such anhydrous MCP crystals as to form a mixture of CAPP and metaphosphate
source material in the process of this invention or polyphosphates. These operations can be con
results in final particles having a dense, non 70 trolled by concurrent determination of pH value,
porous crystal core coated with calcium acid of loss of weight, of loss in neutralizing value,
pyrophosphate. or of rate of reaction with sodium bicarbonate
In transforming MCP to CAPP, agglomerated solution. Reaction is stopped by cooling below
MCP of relatively large Surface area was con the selected temperature. -
verted to CAPP in a period of the order of 5 to 15 s These new products are of various utility.
 2,272,617 3
They may be used in leavening compounds with substantially pure anhydrolls monocalcium phos
materials adapted to evolve carbon dioxide, such phate individually coated with calcium acid py
as sodium bicarbonate or substantially any other rophosphate, the coating being free from phos
carbonate. They may be used as buffer salts in phoric acid and free from ortho- and meta
the control of bacterial Organisms known as 5 phosphates, and from alkali metals, the coating
"rope' organisms in bread making; in buffering consisting of calcium acid pyrophosphate consti
the alkalinity of dentifrices; in clarification of tuting about 10% to 25% of the entire particle.
Sugar and various other organic solutions. Also, 2. A process comprising heating anhydrous
they supply pyrophosphate ion. Further, they monocalcium phosphate crystals free-from excess
afford a desirable source of calcium and phos 0. calcium and from free acid and from alkali metal
phate and are particularly well adapted for the and from moisture for about 5 to 15 minutes at
practice of phosphating flour, and such cereals. about 230 C. to produce particles of monocal
In accordance with the patent statutes, we cium phosphate individually covered with Sub
have disclosed the purposes of this invention and stantially pure calcium acid pyrophosphate to
have illustrated the preferred practices thereof, 5 the extent of about 10% to 25% of the entire
but it will now be apparent to those skilled in particle.
the art that specific variations may occur within EDWIN COX.
the scope of the appended claims. ROBERTH, KEAN.
What we claim is: FRANK.B. CARPENTER, JR.
WBUR. K. ENOS.
1. As an article of manufacture, particles of 20

.. s.