Dairy Chemistry And Biochemistry

252 DAIRY CHEMISTRY AND BIOCHEMISTRY

I I I I I I

1 2 3

Eauivalentsof NaOH added

Figure 5.8 Titration curve for phosphoric acid (H,PO,); + indicates pK: (1.96), pKi (6.8) and
pK: (12.3).

The exact value of the dissociation constants which should be used
depends on the total ionic concentration and consequently, the constants
used for milk are approximate. The following values are generally used:

Acid PK,' PK,Z PK,3
Citric 3.08 4.74 5.4
Phosphoric 1.96 6.83 12.32
Carbonic 6.37 10.25
In milk, the critical dissociation constants are pK5 for citric acid, pK: for
phosphoric acid and pK,' for carbonic acid. Bearing in mind the limitations
and assumptions of the above data, the following calculations can be made
for the distribution of the various ions in milk at pH 6.6.

Phosphoric acid.
1.96

For the first dissociation, H3P04+H+ + H,PO;; pKi =

[salt]

[acid]

[salt]

[acid]

PH = p~,' + log-

6.6 = 1.96 + log-

[salt]. H,PO; 43700

1.e. ___ -

[acid]' H,PO, 1 '

--

Therefore, there is essentially no H,PO, in milk.