Ninhydrin: Uses, Grades, and How to Specify It

Ninhydrin (CAS 485-47-2) is one of the most-used detection reagents in an analytical lab, but "ninhydrin" on a purchase order hides a decision that affects your results: which grade, and why. This guide covers how the reagent works, what it is used for, and how to specify the right grade for your application. If you already know what you need and want to buy it in bulk, see our ninhydrin product page.

How the ninhydrin reaction works

Ninhydrin (2,2-dihydroxyindane-1,3-dione, molecular formula C9H6O4) reacts with primary amines to form a purple-blue compound known as Ruhemann's purple, first observed by Siegfried Ruhemann in 1910. The reaction is useful because it does two jobs at once: the colour is visible to the eye for a spot test, and it absorbs strongly at 570 nm, so its intensity can be read on a spectrophotometer for a quantitative measurement. Proline and other secondary amines react differently and give a yellow product at around 440 nm, which is worth knowing if your sample contains them.

What ninhydrin is used for

• Amino acid analysis: the standard visualisation reagent for amino acids on thin-layer chromatography, and the quantitative detection chemistry in HPLC and automated amino-acid analysers.
• Forensic latent-print development: a primary reagent for revealing fingerprints on porous surfaces such as paper, where it reacts with amino acids left in the print residue.
• Peptide and protein quality control: used to check protein hydrolysates and to monitor whether a peptide synthesis has gone to completion.
• Clinical screening: applied in screening for aminoacidopathies, where amino-acid levels are abnormal.

How to specify the right grade

This is where the buying decision actually sits. Match the grade to the job:

• Quantitative work (HPLC, automated analysers, colorimetric assays): specify 99% purity, ACS reagent grade. Because you are reading colour intensity as a number, batch-to-batch consistency matters, so insist on a Certificate of Analysis stating the batch number, assay, and appearance.
• Qualitative spot tests and TLC visualisation: indicator grade is usually sufficient, though many labs standardise on the same 99% stock to avoid holding two grades.

Whatever the grade, check the appearance on receipt: ninhydrin should be a pale yellow to yellowish-brown crystalline solid. Darkening usually signals degradation from light or moisture exposure.

Storing ninhydrin so it lasts

Ninhydrin is light and moisture sensitive. Store it in its original amber container, tightly closed, in a cool dark place, and let a cold container reach room temperature before opening to avoid condensation. Prepared ninhydrin spray or solution has a much shorter usable life than the dry solid, so make up only what you will use.

Where to source ninhydrin

For bulk or research quantities, buy from a supplier that provides a Certificate of Analysis on every batch and can confirm the grade against your application. See our ninhydrin (99%, CAS 485-47-2) page for specifications and to request a quote, or browse the full Analytical & Indicator Reagents range.