# Why is a metabisulfite / starch / HCl solution unstable?

In one variation of the iodine clock demonstration, you add two colorless solutions together and apparently nothing happens. After about 30 seconds to a minute, you get an "instantaneous" change to a dark blue color.

Solution A is $\pu{0.006 M}$ $\ce{KIO3}$ and is stable over at least a year or two.

Solution B is made with starch, $\pu{0.2 g}$ of sodium metabisulfite ($\ce{Na2S2O5}$), and $\pu{1 mL}$ of $\pu{1 M}$ $\ce{HCL}$ in $\pu{200 mL}$ of water. Starch can be from "reagent grade starch", or starch from a packing peanut, or from laundry starch bought at the grocery store. The concentration of the starch ends up at about 0.05% w/v.

Solution B must be made up no more than 24 hours in advance or the reaction doesn't work: the blue color doesn't form or the timing of the reaction is way off, depending on how old the reagent is.

What happens in solution B that makes it unstable, and is there an easy way to stabilize it so that I could have the chemicals needed for the demo ready on the shelf?

The reactions that happen in the demo are: \begin{align} \ce{S2O5^2- + H2O &-> 2 HSO3-} & \tag{1}\\ \ce{IO3- + 3 HSO3- &-> 3 SO4^2- + I- + 3 H+} &\text{(slow)} \tag{2}\\ \ce{IO3- + 8 I- + 6 H+ &-> 3 I3- + 3 H2O} &\text{(slow)} \tag{3}\\ \ce{I3- + HSO3- + H2O &-> 3 I- + SO4^2- + 3 H+} &\text{(fast)} \tag{4}\\ \ce{2 I3- + starch &-> blue complex + I-} &\text{(fast)} \tag{5} \end{align}

I don't see anything in these that tell me why the metabisulfite / $\ce{HCl}$ / starch reagent is unstable.

One potential problem is the acid-catalyzed hydrolysis of the starch into glucose, which does not form the blue complex. $$\ce{nH2O + (C6H10O5)}_n \ce{->nC6H12O6}$$