The Denigés' reagent was developed in 1898 by G. Denigés is a reagent

Reagent

A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

 used for qualitative analysis.

Uses

Denigés' reagent is used to detect isolefin or tertiary alcohols which can be easily dehydrated

Dehydration reaction

In chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions...

 to form isoolefin in the presence of acid. Treatment of solutions containing either isolefin or tertiary alcohols with this reagent will result in the formation of a solid yellow or red precipitate.

Synthesis

Despite the different stoichiometry in these mixtures which varies the concentration of the reagent, they all follow the same idea of adding HgO to distilled water and concentrated sulfuric acid

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

. The Denigés' reagent is ultimately mercury(II) sulfate

Mercury(II) sulfate

Mercury sulfate, commonly called mercuric sulfate is the chemical compound Hg S O4. It is an odorless solid that forms white granules or crystalline powder...

 in an aqueous solution.

• 5 grams of mercury(II) oxide
  Mercury(II) oxide
  Mercury oxide, also called mercuric oxide or simply mercury oxide, has a formula of HgO. It has a red or orange color. Mercury oxide is a solid at room temperature and pressure...
  
   (HgO) is dissolved in 40 mL of distilled water. The mixture is slowly stirred, while 20 mL of concentrated sulfuric acid is added. After adding an additional 40 mL of distilled water, the solution is stirred until the HgO is completely dissolved.

• The Denigés' reagent can also be prepared by dissolving 5 grams of HgO in 20 mL of concentrated sulfuric acid and 100 mL of distilled water.

• The Denigés' reagent can be modified by using nitric acid
  Nitric acid
  Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
  
   in place of sulfuric acid
  Sulfuric acid
  Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
  
  

Health issues

Inhalation of the Denigés' reagent can result in acute poisoning: causing tightness in the chest, difficulties breathing, coughing and pain. Exposure of mercury sulfate to the eyes can cause ulceration of conjunctiva and cornea. If mercury sulfate is exposed to the skin it may cause sensitization dermatitis. Lastly, ingestion of mercury sulfate will cause necrosis, pain, vomiting, and severe purging. Ingestion can result in death within a few hours due to peripheral vascular collapse.