The results in Figure imply that the basis for the FeCl3 Lewis acid additive increases the Brønsted acidity of HCl despite the lack of a discrete structure for "HFeCl4". This is reminiscent of the HF/BF3 pair that is sometimes referred to HBF4, for which there is no expected discrete structure. We next sought to quantify the extent of increased Brønsted acidity imparted by the inclusion of the Lewis acid additive for several Brønsted acid/Lewis acid (HA/L) pairs. We used the reaction shown in equation 5, where HA is the Brønsted acid, L represents the Lewis acid additive, and HA-L represents a complex formed between them: