Marshall’s acid is:
(A) ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
(B) ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{7}}}$
(C) ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{3}}}$
(D) ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_8}$

Hint: Marshall’s acid is also known as peroxydisulfuric acid. Peroxy indicates that the ${\text{O}} - {\text{O}}$ linkage is present.

Step by step answer: ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ is known as sulphuric acid. Sulphuric acid is also known as hydrogen sulphate or oil of vitriol.
Thus, ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ is not Marshall’s acid.
Thus, option (A) is not correct.
${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{7}}}$ is known as disulphuric acid. Disulphuric acid is also known as pyrosulphuric acid.
Thus, ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{7}}}$ is not Marshall’s acid.
Thus, option (B) is not correct.
${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{3}}}$ is known as thiosulphuric acid. Thiosulphuric acid is also known as hydroxidioxidosulfido sulphate.
Thus, ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{3}}}$ is not Marshall’s acid.
Thus, option (C) is not correct.
${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_8}$ is known as peroxydisulfuric acid. Peroxydisulfuric acid is also known as Marshall’s acid.
Thus, ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_8}$ is Marshall’s acid.
Thus, the correct option is (D) ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_8}$.

Additional Information:
-Marshall’s acid is an inorganic compound. It is solid and colourless in appearance. It is a powerful oxidising agent. The salts of Marshall’s acid are industrially important.
-Marshall’s acid is named after Professor Hugh Marshall who invented it.
-Ways of production of Marshall’s acid are as follows:
-From potassium sulphate or ammonium sulphate in acidic solution.
-Oxidation of oleum with hydrogen peroxide or ozone.
-Electrolysing aqueous solution of sulphate ions.
-Reaction of chlorosulfonic acid with hydrogen peroxide.
-Electrolysis of sulphuric acid with platinum electrodes at high voltage.



Note: Marshall’s acid in structural terms is written as ${\text{H}}{{\text{O}}_{\text{3}}}{\text{SOOS}}{{\text{O}}_{\text{3}}}{\text{H}}$. The oxidation state of sulphur is ${\text{ + 6}}$.
The structure of Marshall’s acid is as follows:
Peroxy indicates that the ${\text{O}} - {\text{O}}$ linkage is present.