Final answer:
Ascorbic acid is used to reduce Azure A to its colourless form in the Ehrlich Reagent method for measuring stomach acidity based on urobilinogen levels in urine.
Step-by-step explanation:
The purpose of ascorbic acid in the Ehrlich Reagent method for urobilinogen in urine is to reduce Azure A, a component used in the preparation of Diagnex blue, to its colourless form. This reduction helps in the measurement of the amount of free hydrochloric acid present in the stomach. When Azure A is extracted from test urine and reduced by ascorbic acid, the resulting color intensity is compared with standards to determine stomach acidity levels. A high color intensity indicates the presence of free HCl, suggesting normal gastric function, while lower intensities suggest hypochlorhydria or achlorhydria.