485-47-2

Ninhydrin, also known as Ninhydrin hydrate or 485-47-2, is a white to pale yellow crystalline powder. It has a melting point of 241°C but decomposes at this temperature. When heated to 100°C or above, it changes color to red. Ninhydrin is soluble in water and ethanol and has slight solubility in ether and chloroform. When exposed to light or air, it gradually changes color and can clump when exposed to moisture. In solution with α-amino acids, or any α-amino substances can form a deep blue or even pink or red material, so it is often used as a detection of α-amino acids and peptides color developer.

Ninhydrin was discovered in 1910 by the German-English chemist Siegfried Ruhemann (1859–1943). In the same year, Ruhemann observed ninhydrin's reaction with amino acids. In 1954, Swedish investigators Oden and von Hofsten proposed that ninhydrin could be used to develop latent fingerprints.

ChEBI: Ninhydrin is a member of the class of indanones that is indane-1,3-dione bearing two additional hydroxy substituents at position 2. It has a role as a colour indicator and a human metabolite. It is a member of indanones, a beta-diketone, an aromatic ketone and a ketone hydrate.

Ninhydrin reacts with primary and secondary amines (including amino acids, proteins, and peptides) to give a dark purple product known as Ruhemann's purple (RP) . As the eccrine component of a latent mark deposit contains amino acids, this reaction can be exploited as a means of developing fingermarks on porous surfaces such as paper and cardboard. The use of ninhydrin as a fingermark detection reagent was first proposed in 1954 by Odén and von Hofsten (1954). Since then, ninhydrin has become the most popular technique for fingermark detection on porous substrates.
Eccrine glands secrete a range of different amino acids that may ultimately be present in a latent fingermark deposit (Hamilton 1965; Ramotowski 2001). Ninhydrin is a nonspecific amino acid reagent in that it reacts in the same manner with different amino acids. In this way, each amino acid present in the latent fingermark deposit will contribute to the developed fingermark image. Amino acids are stable compounds that, due to an affinity for cellulose, do not migrate to any significant extent through dry paper substrates. As a result, very old latent marks can be developed with ninhydrin (the development of 40-year-old marks has been recorded), and the revealed marks are normally of good quality. In addition, the amino acid composition of the eccrine secretion appears to remain relatively constant. Due to these qualities, the use of amino acid reagents (ninhydrin and ninhydrin analogs, including 1,8-diazafluoren-9-one [DFO]) constitutes an effective chemical technique for the development of latent fingermarks on paper surfaces.

Synthetic Communications, 24, p. 695, 1994 DOI: 10.1080/00397919408012648
Tetrahedron Letters, 9, p. 6201, 1968

White to light yellow crystals or powder. Becomes anhydrous with reddening at 257-266°F.

Slightly soluble in water.

Ninhydrin hydrate is sensitive to prolonged exposure to light. Ninhydrin hydrate yields highly fluorescent ternary compounds with aldehydes and primary amines.

Irritant.

Symptoms of exposure to this compound may include skin irritation and sensitization, and redness of the skin.  
ACUTE/CHRONIC HAZARDS: This compound causes irritation on contact. When heated to decomposition it emits acrid smoke and fumes.

Flash point data for Ninhydrin hydrate are not available. Ninhydrin hydrate is probably combustible.

The synthesis of Ninhydrin hydrate is as follows:
A sealed-pressurised reaction vessel (5mL) equipped with a magnetic stirrer was charged with indan-1-one (1equiv), selenium dioxide (3.1equiv) and dioxane/water (3mL/0.3mL). It was then irradiated in a Biotage Initiator Microwave synthesizer 2.0 440W with microwave heating to 180°C with a maximum of 400W for 5min. Then, the vessel was rapidly forced-air cooled to room temperature. The mixture was transferred into a round bottom flask, and the vessel washed with acetone. Silica was added to prepare a solid deposit. The volatile solvents were then evaporated in vacuo before purification by flash chromatography (ethyl acetate/cyclohexane) to afford the corresponding ninhydrin.

Crystallise ninhydrin from hot water (charcoal). Dry it under vacuum and store it in sealed brown containers. [Beilstein 7 IV 2786.]