[Julle]

I'am making some tests to crosslink starch using sodium trimetaphosphate in alkaline aqueous solution. The alkaline agent is NaOH and pH is 10-10.5. In some tests I use CaCl2 as an additive to improve crosslinking. Improved crosslinking means that the starch is more resistant to gelatinization (lower viscosity during heating and less degradation).
Trends in my tests shows that Ca is significantly improving crosslinking compared to the same conditions only with Na ions (from my base). Thats very nice, but I also see pH decrease very rapidly during reaction when Ca present and not when only Na is present.
My best explanation is that Ca will catalyse the hydrolysis of the remaining phosphate dimer to H2PO4- and Na is not!
But is it really so?
Does anyone have an idea?

[Borek]

What is solubility of calcium phosphate?

[Julle]

As I can see calcium metaphosphate Ca(PO₃)₂ is insoluable.
So this could indicate that Ca²⁺ is removing the metaphosphate dimer and thus increase reaction rate for the esterification of phosphate and starch.
But why the decrease in pH?

[Borek]

Balance reaction equation:

P₃O₉^3- + Ca²⁺ + H₂O -> Ca₃(PO₄)₂ + H⁺

[Julle]

OK, I see!

2P₃O₉^-3 + 9Ca²⁺ + 6H₂O -> 3Ca₃(PO₄)₂ + 12H⁺

But according to this reaction I waste my reagent on precipitation and not for crosslinking. Fact is, I still see a better degree of crosslinking with the use of Ca than Na ??

[Borek]

It may be all in equilibrium (while calcium phosphate is weakly soluble it is still soluble, there are other precipitates like CaHPO₄, plus some calcium/phosphate complexes) and kinetics (complexation, hydrolysis and precipitation). Thus you may have some some low but constant concentration of calcium for some time - long enough to catalyze crosslinking. But that's just guess.

You may try to check how fast pH changes, as that will give some insight into preicpitation kinetics.

[Julle]

I'll try to monitor pH decrease in connection with different TMP levels.

Thanks a lot, Borek