|Molar mass||261.92 g/mol (anhydrous)
297.95 g/mol (dihydrate)
|Solubility in water||good|
|Solubility in ethanol||soluble|
|octahedral at Fe|
|2 minutes (metabolites: several days)|
|EU Index||Not listed|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Sodium nitroprusside is the inorganic compound with the formula Na2[Fe(CN)5NO]·2H2O. This red-coloured salt, which is often abbreviated SNP, is a potent vasodilator. The sodium salt dissolves in water and to a lesser extent in ethanol to give solutions containing the dianion [Fe(CN)5NO]2−.
- K4[Fe(CN)6] + 6 HNO3 → H2[Fe(CN)5(NO)] + CO2 + NH4NO3 + 4 KNO3
- H2[Fe(CN)5NO] + Na2CO3 → Na2[Fe(CN)5(NO)] + CO2 + H2O
- [Fe(CN)6]4- + H2O + NO2- → [Fe(CN)5(NO)]2- + CN- + 2 OH-
Structure and properties 
Nitroprusside is a complex anion that features an octahedral ferrous centre surrounded by five tightly bound cyanide ligands and one linear nitric oxide ligand. The molecular symmetry is C4v. Linear nitrosyl ligands are assigned a single positive charge, thus the iron is assigned an oxidation state of 2+. As such it has a low-spin d6 electron configuration and is diamagnetic; together with the relatively short N-O distance of 113pm and the relatively high stretching wavenumber of 1947 cm−1, the complex is formulated as containing an [NO]+ ligand.
The chemical reactions of sodium nitroprusside are mainly associated with the NO ligand. For example, addition of S2- ion to [Fe(CN)5(NO)]2- produces the red [Fe(CN)5(NOS)]4- ion, which is the basis for a sensitive test for S2- ions. An analogous reaction also exists with OH- ions, giving [Fe(CN)5(NO2)]4-.
Ball-and-stick model of part of the crystal structure of sodium nitroprusside dihydrate
Medical pharmacology 
Sodium nitroprusside (Abbreviated SNP, brand name: Nitropress) has potent vasodilating effects in arterioles and venules (venules more than arterioles, but this selectivity is much less marked than that of nitroglycerin). It is administered intravenously in cases of acute hypertensive emergency. SNP breaks down in circulation to release nitric oxide (NO). NO activates guanylate cyclase in vascular smooth muscle and increases intracellular production of cGMP. cGMP activates Protein Kinase G which activates phosphatases which inactivate Myosin light chains. Myosin light chains are involved in muscle contraction. The end result is vascular smooth muscle relaxation, which allow vessels to dilate.
In the human heart, nitric oxide reduces both total peripheral resistance as well as venous return, thus decreasing both preload and afterload. For this reason, it can be used in severe cardiogenic heart failure where this combination of effects can act to increase cardiac output. In situations where cardiac output is normal, the effect is to reduce blood pressure.
Metabolism and toxicity 
Sodium nitroprusside slowly breaks down to release 5 cyanide ions, especially upon exposure to UV light. Despite the toxic potential of cyanide, nitroprusside remains an effective drug in certain clinical circumstances such as malignant hypertension or for rapid control of blood pressure during vascular surgery and neurosurgery. The cyanide can be detoxified by reaction with a sulfur-donor such as thiosulfate, catalysed by the enzyme rhodanese. In the absence of sufficient thiosulfate, cyanide ions can quickly reach toxic levels. The half-life of nitroprusside is 1–2 minutes, but the metabolite thiocyanate has an excretion half-life of several days.
Use in research 
In physiology research, sodium nitroprusside is frequently used to test endothelium-independent vasodilation. Iontophoresis, for example, allows local administration of the drug, preventing the systemic effects listed above but still inducing local microvascular vasodilation.
Sodium nitroprusside is also used in the presence of buffers as a reagent for ketone strips, which test the ketone levels in the urine of a diabetic. Colour change on the strip indicates the relative concentration of ketones.
Sodium nitroprusside is often used as a reference compound for the calibration of Mössbauer spectrometer. It is also used as an analytical reagent in qualitative organic analysis.
In combination with acetaldehyde it is also used as a colour reagent in the development of thin layer chromatography plates, particularly for N-heterocyclic compounds.
Sodium nitroprusside is also used in a urinalysis test, called the cyanide nitroprusside test, also known as Brand's test. In this test sodium cyanide is added first to urine and let stand for approximately 10 minutes. In this time disulfide bonds will be broken by the released cyanide. The destruction of disulfide bonds liberates cysteine from cystine and homocysteine from homocystine. Next sodium nitroprusside is added to the solution and it reacts with the newly freed sulfhydryl groups. The test will turn a red/purple colour if the test is positive indicating that there was significant amounts of amino acid in the urine (aminoaciduria). Cysteine, cystine, homocysteine and homocystine all react when present in the urine when this test is performed. This test can indicate inborn errors of amino acid transporters such as cystinuria, which results from pathology in the transport of dibasic amino acids.
Use in forensic science 
Sodium nitroprusside is used by forensic chemists in a preliminary test in the identification of illicit substances. The test, called Simon's test, is performed by adding 1 volume of a solution of SNP and acetaldehyde in deionized water to a suspected drug, followed by the addition of 2 volumes of an aqueous sodium carbonate solution. The test turns blue for some secondary amines. The most common secondary amines encountered in forensic chemistry include 3,4-methylenedioxymethamphetamine (MDMA, the main component in Ecstasy) and phenethylamines such as methamphetamine.
- A. R. Butler, I. L. Megson (2002). "Non-Heme Iron Nitrosyls in Biology". Chemical Reviews 102 (4): 1155–1165. doi:10.1021/cr000076d.
- "Sodium Nitrosyl Cyanoferrate" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1768.
- Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 22: d-Block metal chemistry: the first row metals". Inorganic Chemistry, 3rd Edition. Pearson. p. 721. ISBN 978-0-13-175553-6.
- Coppens, P.; Novozhilova, I.; Kovalevsky, A. (2002). "Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes". Chem. Rev. 102 (4): 861–883. doi:10.1021/cr000031c.
- A. Navaza, G. Chevrier, P. M. Alzari, P. J. Aymonino (1989). "Single-crystal neutron diffraction structure of sodium pentacyanonitrosylferrate(2−) (sodium nitroprusside) dihydrate". Acta Cryst. 45 (6): 839–841. doi:10.1107/S0108270188013691.
- Nitropress (Nitroprusside Sodium) Drug Information: User Reviews, Side Effects, Drug Interactions and Dosage at RxList. Rxlist.com. Retrieved on 2010-12-11.
- Barraud, N; Hassett, DJ; Hwang, SH; Rice, SA; Kjelleberg, S; Webb, JS (2006). "Involvement of nitric oxide in biofilm dispersal of Pseudomonas aeruginosa". Journal of bacteriology 188 (21): 7344–53. doi:10.1128/JB.00779-06. PMC 1636254. PMID 17050922.
- Carol L. O’Neala, Dennis J. Croucha, Alim A. Fatahb (2000). "Validation of twelve chemical spot tests for the detection of drugs of abuse". Forensic Science International 109: 189–201. doi:10.1016/S0379-0738(99)00235-2. PMID 10725655.