Micro pH meters are available for samples as small as 0.1 drop. The smaller they get, the more they cost ($1132, Ref 1, Ref 2).

Ref 1. https://www.thomassci.com/Instruments/Electrodes-pH/_/MICRO-COMBINATION-pH-ELECTRODES?q=Micro%20Ph%20Meter

Ref 2. https://www.horiba.com/int/water-quality/support/electrochemistry/measurement-of-ph/measurement-method-for-single-micro-liter-sample-using-compact-ph-meter/

Micro pH meters are available for samples as small as 0.1 drop. The smaller they get, the more they cost ($1132, Ref 1, Ref 2). The OP mentions a fairly low concentration of acid: "We know the acid solution at the input, and for safety and ease of testing at this point using a lower concentration, 0.12N solution."

Ref 1. https://www.thomassci.com/Instruments/Electrodes-pH/_/MICRO-COMBINATION-pH-ELECTRODES?q=Micro%20Ph%20Meter

Ref 2. https://www.horiba.com/int/water-quality/support/electrochemistry/measurement-of-ph/measurement-method-for-single-micro-liter-sample-using-compact-ph-meter/