SULFUR DIOXIDE-SELECTIVE OPTODES

Plasticized poly(vinyl chloride) (PVC) membranes based on a hydrogen ion-selective chromoionophore optically sense gaseous sulfur dioxide in the presence of humidity. If the membrane additionally contains a lipophilic alcohol or its matrix consists of hydroxylated PVC, it also responds to dry SO2. In either case, sensitivity and selectivity of the optodes can be improved by incorporating a lipophilic aldehyde into the membrane phase. The chemical equilibria involved in the different sensor types are discussed. With octadecyl 4-formylbenzoate (ETH 5444) as the lipophilic aldehyde and 9-(diethylamino)-5-[(2-octyldecyl)imino]-5H-benzo[a]phenoxazine (ETH 5350) as the basic chromoionophore, SO2 concentrations as low as 4 ppb(v) are determined. The response times (t90%) of the sensors are in the order of 0.5 min for membranes without aldehyde but 10-20 min for those with aldehyde and a secondary amine as a catalyst. Very good reproducibilities can be achieved, the relative standard deviation for repeated measurements being less-than-or-equal-to 1.2%.