Releasably bound carboxylic acids

This invention relates to a composition for and method of storing and using acetic acid or propionic acid, the composition comprising an alkali(ne earth) metal salt of acetic acid or propionic acid and an aliphatic carboxylic acid which has a pKa value lower than that of acetic acid or propionic acid respectively. When this composition is brought into contact with a solvent system capable of allowing ionisation of the salt and the aliphatic carboxylic acid, free acetic acid or propionic acid is released in situ. This method enables the free acids to be stored and released at the point of use thereby substantially reducing the risks of corrosion and unpleasant odors.

This invention relates to a method of storing and using acetic or propionic 
acid releasably bound on a support. 
It is well known that acetic acid and propionic acids are relatively strong 
organic acids which are also corrosive. Yet these acids have a 
considerable number of uses, not least in the field of agriculture, e.g., 
for the protection of crops, preparation of silage and as a salmonella 
control agent. However, the storage and transportation of acetic acid and 
propionic acids is rather difficult due to their corrosivity and their 
pungent smell. Various methods have been tried to immobilize acetic and 
propionic acids but with inadequate success either because of the strong 
bond formed between these acids and the support thereby not being readily 
releasable at the point of use or because of a relatively weak bond 
whereby the acid evaporates from the support too readily and hence results 
in loss of the acid and risks polluting the environment surrounding the 
point of storage. 
It has now been found that these problems can be mitigated by using an 
alkali(ne earth) metal propionate with another carboxylic acid of defined 
physical properties such that free acetic acid or propionic acid is 
released at the point of use. 
Accordingly the present invention is a composition comprising an acetate or 
a propionate salt of an alkali(ne earth) metal and an aliphatic carboxylic 
acid which has a lower pKa value than that of acetic acid or propionic 
acid respectively. 
A feature of the invention is that the alkali(ne earth) acetates or 
propionates when used in a medium capable of allowing dissociation of the 
salt of free carboxylic acids into anions and cations, e.g., in aqueous 
systems, also dissociates into the alkali(ne earth) metal ions and acetate 
or propionate ions and the aliphatic carboxylic acid in turn dissociates 
into the corresponding carboxylate ion and hydrogen ions. However, due to 
the relative differences in the pKa values, the aliphatic carboxylate ion 
combines with the alkali(ne earth) metal ion to form the alkali(ne earth) 
metal salt of the aliphatic carboxylic acid and releases acetic or 
propionic acid in situ. 
The process works particularly efficiently if the alkali(ne earth) metal 
salt of the aliphatic carboxylic acid so formed is readily soluble in the 
solvent system used to generate the desired acetic or propionic acid. 
However, for practical reasons it may be preferable, though not essential, 
to form a substantially insoluble alkali(ne earth) metal salt in order to 
enable easy separation of the acetic or propionic acid solution from the 
insoluble salt by decantation or filtration immediately prior to use. Such 
an insoluble salt can be formed by the appropriate selection of reagents 
and/or solvents. It should be noted, however, that in the process of 
separating the acetic or propionic acid solution from the precipitate, 
some of the yield of available acetic or propionic acid may be lost due to 
occlusion on the precipitate. 
Particularly suitable alkali(ne earth) metal salts for use in the 
compositions of the present invention are those of sodium, potassium, 
calcium and magnesium. 
Propionic acid has a pKa value (i.e. dissociation constant) of about 4.87 
in water at 25.degree. C. Thus any aliphatic carboxylic acid which has a 
lower pKa value under comparable conditions would be suitable for admixing 
with the alkaline earth metal propionate. 
Similarly, acetic acid has a pKa value (i.e. dissociation constant) of 
about 4.75 in water at 25.degree. C. Thus any aliphatic carboxylic acid 
which has a lower pKa value under comparable conditions would be suitable 
for admixing with the alkaline earth metal acetate. 
Such aliphatic carboxylic acids which have a pKa value lower than that of 
acetic or propionic acid may be mono-, di- or poly-carboxylic acids and 
may be saturated or unsaturated. Particularly suitable for this purpose 
are the di- and polycarboxylic acids, especially the unsaturated 
carboxylic acids due to their ability to form alkaline earth metal salts 
which have very low solubility in aqueous systems, e.g. water. Specific 
examples of the preferred aliphatic carboxylic acids include transfumaric 
acid (pKa 3.03 and 4.44), furoic acid (pKa 3.17), furan carboxylic acid 
(pKa 3.15) lactic acid (pKa 3.08), maleic acid (pKa 1.83), malic acid (pKa 
3.40), oxalic acid (pKa 1.23), malonic acid (pKa 2.38), succinic acid (pKa 
4.16), suberic acid (pKa 4.52), mesaconic acid (pKa 3.09 and 4.75), methyl 
malonic acid (pKa 3.07), methyl succinic acid (pKa 4.13), gallic acid (pKa 
4.41), alpha-tartaric acid (pKa 2.98) and meso-tartaric acid (pKa 3.22). 
For instance, if fumaric acid, which has a substantially low volatility, is 
intimately mixed with a calcium acetate or calcium propionate salt and 
stored as such the problems of odour and corrosivity are immediately 
alleviated. However, when the fumaric acid admixed with calcium acetate or 
calcium propionate is brought into contact with a suitable solvent, e.g. 
water, at the point of use and at room temperature, then a rapid exchange 
of ions takes place and the respective acetic or propionic acid is 
liberated immediately into the aqueous solution in situ leaving behind a 
substantially insoluble precipitate of calcium fumarate which can be 
readily removed either by decantation or filtration. 
The filtrate containing the acetic or propionic acid in solution and some 
calcium fumarate can then be used as desired. 
The above reaction can be represented as follows: 
##STR1## 
The alkali(ne earth) metal acetate or propionate and the aliphatic 
carboxylic acid in the composition may be combined together in various 
ways. For instance, if the aliphatic carboxylic acid is a solid, this can 
be intimately mixed with the calcium propionate and form a solid mixture. 
However, if the aliphatic carboxylic is a liquid, this liquid can be used 
to impregnate the solid calcium acetate or propionate. The admixed or 
impregnated calcium acetate or propionate can then be stored and used as 
desired. 
The amount of aliphatic carboxylic acid present in the composition along 
with the respective alkali(ne earth) metal acetate or propionate salt is 
limited only by the physical ability of the two to be admixed or for the 
former to be impregnated on the latter. The acetate or propionate salt 
may, for instance, be admixed or impregnated with 1 to 90% w/w, preferably 
40-60% w/w of the aliphatic carboxylic acid based on the total weight of 
the alkali(ne earth) metal acetate or propionate. An equimolar mixture of 
the aliphatic carboxylic acid and the acetate or propionate salt is most 
preferred. 
Each of the alkali(ne earth) metal acetate or propionate and the aliphatic 
carboxylic acid admixed therewith may be, if not a liquid, in the form of 
a powder or granules or can be shaped into any other convenient shape or 
form e.g. pellets. The physical shape of the two will be determined by the 
desired speed of release of the desired acetic or propionic acid once in 
contact with the appropriate solvent system. It would be clear that for a 
slow release system, the admixture of the two will be highly compacted. 
Whichever form of the components is used in the compositions it will be 
clear that in order for the acetic acid or propionic acid to be released 
from the salt, the admixture has to be brought into contact with an 
aqueous or non-aqueous system, e.g. water, which is capable of allowing 
the salt and the carboxylic acid to dissociate in said system. Upon 
intimately mixing the alkali(ne earth) metal acetate or propionate and the 
aliphatic carboxylic acid with the solvent system, the exchange of ions 
takes place. 
Thus according to a further embodiment, the present invention is a method 
of releasing free acetic acid or propionic acid in situ in a solvent 
system capable of allowing the respective acetate or propionate salt of an 
alkali(ne earth) metal and an aliphatic carboxylic acid having a lower pKa 
value than that of acetic acid or propionic acid respectively to ionise in 
said system, said method comprising bringing into contact with the solvent 
system a composition comprising said acetate or propionate salt and said 
aliphatic carboxylic acid, thereby generating a solution comprising free 
acetic or propionic acid respectively in said solvent system. 
The following Examples will, for the sake of convenience, be directed to 
the impregnation of calcium acetate or propionate with fumaric acid but 
should in no way be construed as limiting the inventive concept disclosed 
herein.

EXAMPLE 1 
1,5 g of 63:67 (w:w) mixture of calcium propionate and fumaric acid were 
mixed, The mixture was suspended in 100 ml demineralised water in a 
beaker. After 1 or 2 hours of stirring the sample was transferred to a 250 
ml volumetric flask and water was added to 250 ml. After homogenisation, 
the solution was filtered over a 0.2 microns filter. The propionic acid 
content in the filtrate was determined by gas chromatography, in 
five-fold. Quantitative measurements were carried out by external 
standardisation, i.e. by comparison of the peak area of the sample with 
that of a propionic acid standard solution of 4 mg/ml. The quantity of 
propionic acid formed was calculated as follows: 
##EQU1## 
where C.sub.st =concentration of the standard (4 mg/ml) 
V.sub.m =volume of the flask (250 ml) 
The experiment was carried out on 3 different samples of calcium 
propionate, A, B and C, each in two-fold. 
The maximal concentration of propionic acid that can be obtained is 
calculated as follows: 
mg propionic acid max=intake (mg).times.P.sub.cp+ .times.C.sub.v 
.times.P.sub.prop .times.C.sub.c in which intake=mg sample 
P.sub.cp =% calcium propionate in the sample/100 (=0.63) 
C.sub.v =correction factor for water content of the sample (=0.95) 
P.sub.prop =% propionate in calcium propionate/100 (=0.784) 
C.sub.c =correction factor for the conversion of propionate to propionic 
acid (=1.013) 
##EQU2## 
The results are given in Table 1. 
TABLE I 
______________________________________ 
peak 
area propionic propionic 
yield 
Sample Intake (10.sup.3) 
acid found 
acid max. 
(%) 
______________________________________ 
standard 546.5 
4 mg/ml 
A 1 1561.3 402.5 
736 742 99.2 
1hr stirring 
2 1711.1 437.5 
801 813 98.5 
standard 561.5 
4 mg/ml 
A 1 1565.2 403.3 
718 744 96.5 
2hr stirring 
2 1680.1 449.2 
800 799 100.1 
standard 535.7 
4 mg/ml 
B 1 1519.3 313.6 
585 722 81.1 
2 1772.2 344.7 
644 842 76.4 
standard 535.7 
4 mg/ml 
C 1 1444.2 368.9 
689 687 100.3 
2 1458.7 383.9 
717 693 103.4 
______________________________________ 
EXAMPLE 2 
1.5g of 58:42 (w:w) mixture of calcium acetate and fumaric acid were mixed. 
The mixture was suspended in 100 ml demineralised water in a beaker. After 
1 hour of stirring the sample was transferred to a 250 ml volumetric flask 
and water was added to 250 ml. After homogenisation, the solution was 
filtered over a 0.2 microns filter. The acetic acid content in the 
filtrate was determined by gas chromatography, in five-fold. Quantitative 
measurements were carried out by external standardisation, i.e. by 
comparison of the peak area of the sample with that of an acetic acid 
standard solution of 4 mg/ml. The quantity of acetic acid formed was 
calculated as follows: 
##EQU3## 
where C.sub.st =concentration of the standard (4 mg/ml) 
V.sub.m =volume of the flask (250 ml) 
The experiment was carried out on 3 different samples of calcium 
propionate, A, B and C, each in two fold. 
The maximal concentration of acetic acid that can be obtained is calculated 
as follows: 
mg acetic acid max=intake (mg).times.P.sub.ca .times.C.sub.v 
.times.P.sub.acet .times.C.sub.c in which intake=mg sample 
P.sub.ca =% calcium acetate in the sample/100 (=0.573) 
C.sub.v =correction factor for water content of the sample (=0.945) 
P.sub.acet =% acetate in calcium acetate/100 (=0.746) 
C.sub.c =correction factor for the conversion of acetate to acetic acid 
(=1.010) 
##EQU4## 
The results are given in Table 2 below. 
TABLE 2 
______________________________________ 
peak area 
acetic acetic yield 
Sample Intake (10.sup.3) 
acid found 
acid max. 
(%) 
______________________________________ 
standard 321.4 
4 mg/ml 
A 1 1640.3 229.5 671 714 94.0 
2 1507.6 198.9 618 619 99.8 
______________________________________