An essential function of detergent buffers is the ability to maintain the wash liquor in the pH range of 9 - 10.5. In
this alkalinity range the effectiveness of detergents is optimized and hence good pH buffering makes a direct contribution
to the washing process. Borates buffer in precisely this region, and can produce a pH jump effect, which is useful in certain
detergent applications.
Alkaline buffering
Under alkaline washing conditions the surfaces of soil particles and the substrates (e.g. fabrics, tableware or hard surfaces)
to which they adhere, acquire negative electrostatic charges and repel each other, loosening the soil into the liquor and
inhibiting redeposition.
Alkalinity also promotes the saponification of natural fatty soils such as sebum, forming soap species in situ.
Borax is among the least alkaline of many detergent buffers, because in solution its stoichiometry (Na2O:B2O3=1:2) means
that it releases equimolar amounts of the weak (Lewis) acid, boric acid and its conjugate base, the tetrahydroxy borate
anion, B(OH)4-. Borax is, in essence, half-neutralized boric acid.
This is the best condition for good buffering. The pH of borax solutions is related to the strength of the parent acid
(actually equal to its pKa, 9.14 for boric acid at 40oC).
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| Fig D2: buffering of STPP, sodium carbonate and borax |
Fig D3: buffer indices for borate and carbonate solutions |
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Dilute borate solutions (<0.1 M) contain only monomeric species, but as concentration rises polymeric (tri-, tetra-, and
pentaborate) anions are formed.
Complex equilibria determine the relative concentrations of the various borate species in solution. This, coupled with the
fact that the acid strengths of the monomeric and polyborate species differ, means that the pH of a borate solution varies
with concentration and temperature. However, at the isohydric point pH is independent of borate concentration. This occurs
at pH 8.9 and a Na2O:B2O3 molar ratio of 0.41 for a sodium borate solution.
At lower molar ratios, solutions become more acidic (and at higher molar ratios, more alkaline) with increasing concentration.
Borax has a Na2O:B2O3 ratio of 1:2 and is hence a very effective buffer, as the pH of its solutions do not vary significantly
with concentration.
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| Fig D1: distribution of boron polyanion species in solution |
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