# 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.

I can partially answer the question. I know what is happening to solution B. I don't know of a more stable version.

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

For the timing to be off, the metabisulfite must also be decomposing, either by reducing glucose or by reacting with dissolved oxygen. I'm not sure what the specific process is, nor do I know how to stop it.

• Will starch hydrolyze that easily? The HCl concentration is only 0.0005 M. Thanks for the answer, this gives me something to explore. – Janice DelMar Jun 3 '12 at 19:03
• Ah, I only the saw the 1 M. I missed that it was 1 mL of 1 M HCl diluted to 200 mL. Then I expect that the main problem is the metabisulfite decomposing. – Ben Norris Jun 4 '12 at 20:00