The value of the buffer index (ß the reciprocal of the derivative of the curve of solution pH vs. added base) is at a
maximum under these conditions. Unusually for a buffer, the pH varies very little with borax solution concentration.
Should a higher pH be required, sodium metaborate may be used, but does not buffer at its natural pH of 11.06 (1% solution
at 20oC) as effectively as borax.
The mild alkalinity of borax stems from its semi-neutralized/semi salt-like state. In contrast, sodium carbonate and sodium
tripolyphosphate are more alkaline because they are fully neutralized salts.
The buffer index for sodium carbonate in solution is lower than that of borax (at equivalent molar concentrations; values
compared at their natural pH) and so, on this basis it is a less effective buffer.*
Incorporating H2O2 into liquid detergents is difficult due to its instability in alkaline solutions and its tendency to
react with other ingredients (which results in a lower pH). Borate buffering mitigates this effect well.
| *Fig D3 Fig D3 compares borax (delivering 4 moles of borate) to 1 mole of carbonate. Even when compared on the basis of equal ion molarities, borax is still a more effective buffer than carbonate when the two are compared at their natural pH. molarities, borax is still a more effective buffer than carbonate when the two are compared at their natural pH. |
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| Fig D4: effect of pH and temperature on the buffer indices of persalts simulated by use of hyrogen peroxide in borate and
carbonate buffer |
Fig D5: wash liquor pH of persalts simulated by the use of hydrogen peroxide in borate and carbonate buffer; effect of
time and temperature |
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When compared on an equal available oxygen basis, perborate solutions are better able to maintain pH in the range 9 - 10
than those of percarbonate, an observation substantiated by buffer index (ß) calculations. The large drop in ß for solutions of percarbonate ongoing from 40oC to 20oC (the perborate solution pH profiles are virtually identical at these temperatures) is probably the cause of its relative inability to maintain pH>9 at 20oC.
The pH jump effect
For reasons of stability in storage, it can be desirable for liquid formulations containing one or more enzymes to be pH
neutral or at least less alkaline than in the wash. Higher pH destabilizes lipase enzymes in the presence of protease enzymes,
and also peroxygen bleaches unless other steps are taken to prevent this (see Stabilization section).
pH neutral peroxide bleach-based liquid adjuncts are becoming popular. Formulating the products at near-neutral pH helps
reduce the corrosive nature of the formulations, enhances storage stability and enables their application directly to stains
on colored articles (i.e. for use as pre-spotters).
As this low pH is unsatisfactory for overall detergent performance, a mechanism for promoting an increase in solution pH
when such products are diluted is of great interest. Several have been described, among them borate-polyol systems.
An example is the tetrahydroxy borate anion (from borax or other borates) complexed with certain cis-1,2-diols such as
sorbitol. Upon dilution, the complex dissociates, liberating free tetrahydroxy borate anions which increase the solution
pH. Dissociation of polyborate ions also tends to promote this effect. The ratio of borate to polyol is important and in
practice compositions containing approximately 5% borax and 20% sorbitol have been found to be effective.
Borate-polyol complexes are also important in enzyme stabilization
[see Stabilization section].
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