Salkowski's test, also known simply as Salkowski test, is a

qualitative Qualitative descriptions or distinctions are based on some quality or characteristic rather than on some quantity or measured value. Qualitative may also refer to: *Qualitative property, a property that can be observed but not measured numericall ...

chemical test, that is used in chemistry and biochemistry for detecting a presence of cholesterol and other

sterols Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol of gonane. More generally, any compounds that contain the gon ...

. This biochemical method got its name after German biochemist

Ernst Leopold Salkowski Ernst Leopold Salkowski (October 11, 1844 – March 8, 1923) was a German biochemist who was a native of Königsberg. He received his education at the University of Königsberg, later working in Berlin as an assistant in the chemical labora ...

, who is known for development of multiple new chemical tests, that are used for detection of different kinds of molecules (besides cholesterol and other sterols also for creatinine, carbon monoxide, glucose and indoles). A solution that has tested positive on the Salkowski's test becomes red and gets yellow glow.

Basic information

Procedure

For Salkowski test's procedure one needs a sample, that is to be tested for sterols, as well as

chloroform Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...

and concentrated

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

that represent Salkowski's reagent. Usually the solution of chloroform and the sample is prepared first and later treated with concentrated sulfuric (VI) acid. After that the whole solution needs to be shaken well. It is important to use only dried glassware, as dehydration reaction occurs during the test's procedure. A solution that has tested positive on this qualitative chemical test exhibits two distinct layers in a test tube; the upper layer (chloroform) gets blueish red to violet colour, while the layer of sulfuric acid becomes yellow to green, with greenish glow being visible. If a sample does not contain any cholesterol or other sterols the tested solution remains unchanged and retains its original colour. Salkowski test can also be used for determining the presence of indoles (crystalline alkaloids that are a degradation products of proteins, containg tryptophan). In such cases a sample is treated with nitric acid and 2% solution of potassium nitrite, with positive reaction being shown by presence of red colour.

Chemistry of the test

Treating a solution of a sample, containing sterols, with chloroform and highly hygroscopic sulfuric acid, leads to a dehydration reaction (two water molecules are removed from two cholesterol molecules) and formation of new double bonds. During the chemical reaction two sterols bind together and bisterol (bisteroid) is formed; bi- cholestadien (double

cholestene Cholestenes are chiral molecules that are derivatives of cholestanes that have a double bond. If there are two double bonds, the molecule is known as a "cholestadiene". Examples include fusidic acid, lanosterol, and stigmasterol. Cholestene is a w ...

with two double bonds) in a case of cholesterol. Red colour of a solution is a consequence of bi-

sulfonic acid In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is kn ...

of a bi-cholestadien, which is a product of sulfuric acid sulfonating bi-cholestadien.

References

{{Reflist Biochemistry detection methods