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Bromothymol blue is a pH indicator used to assign a color-based titration endpoint in the inspected solubility product measurement by titration.

A pH indicator used in the inspected titrant standardization protocol. Its visible color signals a titration endpoint rather than defining the equivalence point.

A pH indicator changes color as its protonation distribution changes. In a simple weak acid/conjugate base model, its two forms have different colors and their ratio depends on pH through the acid dissociation constant. The visible transition spans a range of pH; it is not a direct signal that the sample has reached an equivalence point. An indicator is useful when its transition lies in a steep part of the titration curve, so a modest range of pH corresponds to a small added volume.^[ref]

A titration endpoint is the experimentally assigned point indicated by a specified signal: for example, a persistent color of a pH indicator, or a feature extracted from a potentiometric titration. An equivalence point is defined by stoichiometry; an endpoint estimates its location. State the signal rule and its repeatability. A fleeting local color where a drop enters an unmixed sample need not satisfy the rule for the mixed sample.

Define the endpoint offset $\Delta V=V_{\mathrm{end}}-V_{\mathrm{eq}}$. For one-to-one neutralization, using the endpoint without correction gives inferred acid inventory ${\hat{n}}_A=c_B{V}_{\mathrm{end}}$ andThus an illustrative $0.05\mathrm{mL}$ overshoot at $V_{\mathrm{eq}}=10.00\mathrm{mL}$ overestimates the inventory by $0.5\%$. Repeatedly reaching the same color can reduce scatter while retaining this systematic offset. This example stipulates an offset; it is not a measured drop size.