Indicator Selection Helper

A transparent, offline tool for students, researchers, and lab technicians to select the optimal visual pH indicator for a titration based on predicted equivalence point pH and transition ranges. Supports unit-aware pH, pKa, and concentration calculations. All computations run locally in your browser; no data is sent online.

Educational only. Follow your lab SOPs and SDS. For hazardous materials, consult safety protocols before any procedure.

Calculator

Expected Equivalence Point pH

pH units

Enter numeric value. Use dot or comma for decimals.

Selection Tolerance (± pH)

How close indicator range must be to equivalence point.

Acid/Base Type

Strong Acid – Strong Base Weak Acid – Strong Base Strong Acid – Weak Base

Why Indicator Selection Matters

Choosing the wrong pH indicator can lead to incorrect endpoint determination in titrations, causing significant errors in concentration measurements. This tool helps match the indicator's transition range to the expected pH at the equivalence point, considering the acid‑base system's strength and concentration.

Step‑by‑Step Usage

Estimate the equivalence point pH – If unknown, use the calculator's acid/base type selection. For weak systems, provide pKa and concentration.
Set a tolerance – A ±1 pH unit tolerance is common; tighten for precise work.
Review the suggested indicators – The table highlights indicators whose full range lies within your target ± tolerance.
Consider color change distinctness – Choose an indicator with a clear, perceivable color shift for your solution conditions.

Realistic Examples

Example 1: Strong Acid (HCl) – Strong Base (NaOH)

Equivalence pH = 7.0. Set tolerance ±1.0. Suggested indicators: Bromothymol Blue (6.0–7.6), Phenol Red (6.8–8.4). Avoid Methyl Orange (3.1–4.4).

Example 2: Weak Acid (Acetic acid, pKa=4.76, 0.1 M) – Strong Base

Calculated equivalence pH ≈ 8.88. Tolerance ±1.0. Suggested indicators: Phenolphthalein (8.2–10.0), Thymol Blue (8.0–9.6).

Example 3: Strong Acid – Weak Base (Ammonia, pKb≈4.75, 0.05 M)

Calculated equivalence pH ≈ 5.28. Tolerance ±1.0. Suggested indicators: Methyl Red (4.4–6.2), Bromocresol Purple (5.2–6.8).

Reference Data

Constant	Value	Unit
pKw (25 °C)	14.00	–
Weak acid equivalence pH formula	pH = 7 + ½ pKa + ½ log C	(approx.)
Weak base equivalence pH formula	pH = 7 – ½ pKb – ½ log C	(approx.)

Common Mistakes Checklist

Using an indicator whose transition range does not include the equivalence point pH.
Neglecting ionic strength or temperature effects on pKa and indicator range.
Forgetting that color perception may be affected by solution color or turbidity.
Assuming all weak acid‑base systems have equivalence at pH 7.
Using too much indicator, which can act as an extra acid/base.

Frequently Asked Questions

Q: How accurate is the equivalence point pH prediction for weak systems?

A: The simplified formula (pH = 7 + ½ pKa + ½ log C) is an approximation that ignores activity coefficients and temperature. For precise work, use a pH meter or more advanced computational models.

Q: Can I trust the indicator ranges in all solvents and temperatures?

A: No. Transition ranges shift with ionic strength, solvent, and temperature. The provided ranges are for aqueous solutions at ~25 °C. Consult literature for non‑standard conditions.

Q: What if my equivalence point pH is exactly at the edge of an indicator's range?

A: Color change becomes sluggish and less distinct. Choose an indicator whose midpoint is closest to your equivalence pH for a sharp, perceptible endpoint.

Q: Are there safety concerns with certain indicators?

A: Some indicators are toxic or irritants (e.g., phenolphthalein is a suspected carcinogen). Always review SDS, wear appropriate PPE, and dispose of waste according to your institution's regulations.

Formulas & Assumptions

Strong‑strong titration: Equivalence pH = 7.0 (assuming 25 °C, Kw=1e‑14).

Weak acid‑strong base: Equivalence pH ≈ 7 + ½ pKa + ½ log₁₀(C). C is the initial weak acid concentration.

Strong acid‑weak base: Equivalence pH ≈ 7 – ½ pKb – ½ log₁₀(C). pKb = 14 – pKa (for conjugate acid).

pH ↔ [H⁺]: pH = –log₁₀([H⁺]); [H⁺] = 10^–pH.

Rounding: pH values rounded to 2 decimal places; concentrations to 2 significant figures.

Indicator suitability: An indicator is suitable if the entire transition range lies within equivalence pH ± tolerance.

Indicator Selection Helper

Calculator

Results

Quality Notes

Why Indicator Selection Matters

Step‑by‑Step Usage

Realistic Examples

Reference Data

Common Mistakes Checklist

Quick pH / Concentration Converter

Frequently Asked Questions

Q: How accurate is the equivalence point pH prediction for weak systems?

Q: Can I trust the indicator ranges in all solvents and temperatures?

Q: What if my equivalence point pH is exactly at the edge of an indicator's range?

Q: Are there safety concerns with certain indicators?

Formulas & Assumptions

Calculator

Results 💡

Quality Notes

Why Indicator Selection Matters

Step‑by‑Step Usage

Realistic Examples

Reference Data

Common Mistakes Checklist

Quick pH / Concentration Converter

Frequently Asked Questions

Q: How accurate is the equivalence point pH prediction for weak systems?

Q: Can I trust the indicator ranges in all solvents and temperatures?

Q: What if my equivalence point pH is exactly at the edge of an indicator's range?

Q: Are there safety concerns with certain indicators?

Formulas & Assumptions

Results

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Indicator Selection Helper

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