Refractive Index to Concentration Tool

Convert refractive index measurements to solution concentration (mass%, molarity, etc.) for aqueous and common solvent systems. Built for students, researchers, and lab technicians. Works 100% offline — no data leaves your device.

Educational only. Follow your lab SOPs and SDS.

Calculator

Refractive Index (n_D)

Enter measured refractive index at 20°C (Na D-line). Accepts dots/commas.

Solute

Output Concentration Units

Solvent Refractive Index (n₀)

Pure solvent reference. Water at 20°C ≈ 1.3330.

Results

Concentration

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Intermediate Values

Δn (n - n₀): —
dn/dc used: — mL·g^-1
Linear model: —

Using Refractive Index for Concentration

Refractive index (RI) is a rapid, non‑destructive method to determine solute concentration in transparent solutions. It relies on the linear relationship Δn = (dn/dc)·c for dilute solutions (typically <10 %). This tool uses this principle with built‑in dn/dc values for common systems.

Step‑by‑Step Usage

Measure refractive index of your solution at 20°C (use temperature correction if needed).
Select the solute from the dropdown (or enter custom dn/dc).
Verify the solvent RI (default is water at 20°C).
Choose your desired concentration units.
Click Calculate Concentration.

Realistic Examples

Solute	Measured n_D	Solvent n₀	dn/dc (mL·g^-1)	Result (g/100 mL)
Sucrose	1.3478	1.3330	0.00145	10.2
NaCl	1.3410	1.3330	0.00175	4.57
Ethanol	1.3390	1.3330	0.00140	4.29

Common Mistakes Checklist

Not temperature‑correcting RI (≈ ‑0.0001/°C for aqueous).
Using linear model beyond its range (high concentrations).
Ignoring solvent impurity or evaporation.
Mismatching dn/dc value with actual solute/buffer.
Forgetting to blank with pure solvent.

Quick Concentration Converter

From Value

From Unit

To Unit

Result:

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Frequently Asked Questions

What are the accuracy limits?

Linear model works best for c < 10 %. Accuracy depends on dn/dc precision (± 0.0001 mL·g^-1 gives ~ ± 0.5 % relative error). Temperature must be controlled within ± 0.5°C.

When does the linear approximation fail?

At high concentrations (>20 %), intermolecular interactions cause non‑linearity. Also with mixed solvents, pH‑sensitive solutes, or when the solute affects solvent structure (e.g., salts).

How do I measure dn/dc for a custom solute?

Prepare a series of known concentrations, measure RI for each, plot Δn vs. c. The slope is dn/dc. Use at least 5 points across your expected range.

What safety precautions are needed?

Always wear safety glasses when handling solutions. Clean the refractometer prism gently after each use. Avoid corrosive or viscous samples unless the instrument is rated for them.

Formulas & Assumptions

Linear relationship: Δn = n − n₀ = (dn/dc) · c (c in g/mL)
Conversion to other units: c (g/100 mL) = 100 · Δn / (dn/dc); Molarity = (c in g/L) / Molar mass
Built‑in dn/dc values: Sucrose: 0.00145, NaCl: 0.00175, Ethanol: 0.00140, Glycerol: 0.00138 (mL·g^-1)
Assumptions: Dilute solution, no significant solute‑solute interactions, constant temperature (20°C), solvent RI known.
Rounding: Results rounded to 4 significant figures. Intermediate Δn to 6 decimal places.

For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. This tool does not replace professional judgment, lab SOPs, or safety procedures. Always follow SDS guidance and institutional policies.

Sources: Common chemistry constants: R, N_A, F; Standard unit conversions and lab measurement best practices; General analytical chemistry and uncertainty concepts; Laboratory safety basics and SDS principles.

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Refractive Index to Concentration Tool

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