These observations and analysis indicate that nuclear singlet relaxation may be highly sensitive to chemical exchange, even in cases where the equilibrium overwhelmingly favours one of the exchanging species. In the current case, the concentration of the Sq 2-anion exceeds that of SqH -anion by a factor of more than 10 6 at pH = 10. Nevertheless, the rare and transient SqH -species has a very significant effect on 13 C 2 singlet relaxation. This effect presents both difficulties and opportunities. The sensitivity of nuclear singlet relaxation to rare protonation events is likely to make it even more difficult to utilize long-lived states for the storage and transport of hyperpolarized spin order, especially in protic solvents. It is probably no coincidence that the longest observed singlet lifetimes have been observed in aprotic solvents , including liquid carbon dioxide . It is likely that this effect has already contributed to some observations in the literature, one example being the relatively rapid decay of 13 C 2 singlet order in 18 O-labelled oxalate .