Chemistry in your cupboard

Why use thioglycollic acid? (1 of 2)

Thioglycollic acid is a reagent that you have probably not heard of. Why is this used to break the S-S bonds in cystine and why is it used in such an alkaline solution?

Cosmetic chemists have naturally carried out many experiments with different reagents at various pHs to find the best conditions for breaking the S-S bonds in cystine, Figure 12.

Figure 12: Structure of cystine

The experimental set up is simple in principle. A hair fibre is wound over a hook and then clamped in place by a machine, so that it hangs vertically and the machine applies a downward force (typically a few newtons) to stretch the hair, see Figure 13. Normally hair is elastic, so when the stretching force is removed, the hair goes back to its original length (the technical term is that it ‘relaxes’).

Figure 13: The instrument used to assess depilatory products

In the test to assess a depilatory product the hair is first stretched. Then the reagent to be tested is prepared, placed in a buffer solution to control its pH and applied to the hair. The machine adjusts the force on the hair to keep the length of the hair the same while the product works on it. As the hair becomes weaker, and wants to stretch more, then the machine reduces the applied force to maintain the hair’s length. So the force applied to the hair decreases over time as the S-S bonds break. Eventually the hair itself breaks.

In effect, this procedure enables the reaction rate to be measured, because the sooner the hair breaks, the faster the reaction. The results show that the reaction appears to be first order with respect to the concentration of reagent used, and that, up to a point, it is faster at higher pH (i.e. more alkaline conditions).