Dissociation Constant of an Indicator by UV-Visible Spectroscopy

1. Aim

To complete the Year 3 syllabus practical dissociation constant of an indicator by uv-visible spectroscopy using an instructor-approved method or dataset. Actual analyte, measurements, and result must be entered in the student record.

2. Requirements

Instructor-approved sample and reference standard, validated method, instrument and calibration records, blank and controls, observation sheet, calculator or graphing aid, protective equipment, and chemical waste route. Confirm sample identity from its label and use the college method sheet for reagent quantities, concentrations, wavelengths, pH, and temperature. Acceptance criteria are to be recorded from the assigned current monograph or approved college method in the student record.

3. Theory

An acid-base indicator may have different spectra in its protonated and deprotonated forms. At a selected wavelength, a validated calibration or absorbance-ratio method can estimate their relative amounts. Henderson-Hasselbalch then relates pH, form ratio, and pKa for the relevant equilibrium. The exact wavelength and method must be confirmed.

4. Procedure

Confirm sample identity, method version, hazards, instrument calibration, and required controls.
Record raw standards, blanks, sample readings, units, and any preparation or dilution factors.
Follow the verified laboratory method under instructor supervision, or analyze its supplied dataset.
Check acid-form and base-form reference spectra, pH calibration, and sample absorbance. Use the approved spectral model to estimate the form ratio and calculate pKa only from valid readings.
Check the fit, control recovery, and measurement range before applying a formula.
Report the calculation with units, method limitations, and unresolved details recorded in the student record.

5. Observation and calculation

Student record field	Entry
Case or product	Record here
Method	Record here
Observation	Record here
Inference	Record here
Supervisor check	Record here
Outcome	Record here

Assumed values for calculation practice only: Assumed pH is 7.0 and assumed base-to-acid ratio is 1.0. Assumed pKa = 7.0 − log10(1.0) = 7.0. These invented inputs are not an indicator value. Each invented input and output is assumed, not a reagent instruction or actual laboratory result.

6. Output or final result

Submit a labelled indicator spectra and pKa, calculations where justified, and a conditional conclusion. Record the supervised outcome here.

Teaching-lab safety: Use instructor-supervised chemical handling and the approved disposal method. No assumed value here authorizes a preparation or analytical release decision.

This page is a supervised-activity guide; it does not report a completed case.

7. Sources

← Comparison of UV Spectrum of a Compound with Its DerivativesConductometric Titration of a Mixture of Acids with a Strong Base →

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