Students may already have been introduced to DMG chemical, and have some idea about its structure and properties. It is a white crystalline powdered solid which is used in analytical chemistry as a reagent for nickel. It was one of the initial organic reagents that had found application in analytical chemistry.
We will go into an elaborate discussion on dimethylglyoxime and also focus on its important attributes. By the end of this discussion, you will also have an idea about its structure, properties along with uses.
Let us start!
Dimethylglyoxime or DMG is a white-coloured powder like chemical which is substantially less soluble in water but soluble in sodium hydroxide solution or methanol. The formula of dimethylglyoxime is C4H8N2O2, and it is also known as 2,3-Butanedione dioxide.
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DMG is used as a reagent in analytical chemistry, and its multiple derivatives have been synthesised. It helps in the detection of various other metal ions apart from nickel. The anionic form is termed as DMGH, and the neutral form is DMGH2 (here H indicates hydrogen). DMGH2 is used for the identification of nickel and palladium. It is also a bidentate ligand, that is, two donor arms.
Note: Ligands are neutral ions attached directly to the central metal ion through coordinate bonds.
Do You Know?
Nickel ions chelation reaction with that of organic bidentate ligand dimethylglyoxime produces a raspberry or cherry red colour precipitation. This reaction finds widespread use for the purpose of detecting nickel metal ions on account of the formation of its striking colour.
It is a sensitive reaction which is used as a test for confirming any presence of nickel cations, even if it is in particularly low concentration. This procedure is readily applied for any metal object that may come in contact with our skin such as a wristwatch, jewellery, coin, spectacle frames etc. The process is highly useful in the identification of a particular kind of dermatitis or skin sensitivity known as Nickel itch.
Dimethylglyoxime and nickel cation reacts to form red precipitation of nickel dimethylglyoxime which is insoluble in nature.
Ni2+ + 2C4H8N2O2 → Ni(C4H7N2O2)2 [red precipitate] + 2H+
It is a common reagent in the gravimetric estimation of nickel. The broad principles in the gravimetric estimation are – (1) precipitation of nickel from solution by addition of dimethylglyoxime, (2) filtering out the precipitate, and (3) calculation of the mass of nickel from the precipitate mass. In this instance, the organic functional groups that precipitate with the metal ion is facilitated by chelating agents. Here, dimethyglyoxime is the chelating agent.
Note: Chelating agents are such organic compounds that form coordinate covalent bonds with metal ion through cationic side chains.
The precipitation of nickel dimethylglyoxime is obtained with the addition of alcoholic solution of dimethylglyoxime. Aqueous ammonia solution is subsequently added.
NiSO4 + 2C4H8O2N2 → Ni(C4H7O2N2)2 + H2SO4
Red chelate can be noticed in the solution when the pH is in the range of 5-9. Here, the electron pairs of four nitrogen atoms are donated, as opposed to the electrons of the oxygen atoms. The ammonia solution helps in preventing the pH from falling below 5. In the case of lower pH, the formation of nickel (II) ion takes place, causing the dissolution of Ni(DMG)2.
While a little excess of the reagent will not have any substantial impact, a significant excess can cause the reagent to precipitate. It may further crystallise out with the chelate. The amount of reagent to be added is directly proportional to the quantity of ions present.
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Ferrous sulphate and ammonium hydroxide react in the presence of dimethylglyoxime to form a compound of iron and ammonium sulphate, and at the same time releasing water.
FeSO4 + 2NH4OH + 2C4H8N2O2 → Fe(C4H7N2O2)2 + (NH4)2SO4 + 2H2O
Dimethylglyoxime helps in the precipitation of palladium which is utilised to prepare a standard palladium solution out of palladium chloride. Palladium chloride is purified with the removal of any trace of platinum in the form of ammonium chloroplatinate. Subsequently, palladium is precipitated with dimethylglyoxime.
The complex is treated with aqua regia, and the nitrate ions are eliminated from the resulting solution by concentrated hydrochloric acid. Complete precipitation takes place after the solution stands overnight.
Different Applications of Dimethylglyoxime
It may be utilised as a precipitant for palladium and nickel. In case of palladium, the precipitation takes the form of a yellow compound after diluting it with hydrochloric acid. On the other hand, precipitation from ammoniacal nickel solution takes the form of a bright red voluminous compound.
DMG helps in the detection of nickel to dermatitis or similar skin ailment. It identifies the nickel released from a watch, jewellery etc. that come directly in contact with our skin. Various countries have such regulations where diemthylglyoxime test can be purchased over the counter in pharmacies.
Dimethylglyoxime is used extensively in analytical chemistry in the form of detecting reagent, precipitating reagent, and photometric reagent for different metal ions such as platinum, palladium and nickel among others.
It plays a role in sustainable recycling of dumped lithium-ion batteries which is considered as an environmentally hazardous substance. The hydrometallurgical process that is employed for leaching lithium, cobalt, nickel from waste cathode materials of these batteries requires precipitation by dimethylglyoxime.
DMG also has application in the investigation of iron (III) reduction for determining dissolved iron in seawater. It involves kinetic experiments for examining the efficiency of the reduction of inorganic iron with sulphite under various conditions. Transition metals such as cobalt are present in seawater which required DMG to be added to luminol solution.
Question: Red precipitation takes place when ethanol solution of dimethylglyoxime is combined with ammoniacal nickel (II). Which of the following statement is incorrect?
(a) There is tetrahedral geometry in the red complex
(b) There is symmetrical hydrogen bonding in the complex
(c) There is square planar geometry in the red complex
(d) Dimethylglyoxime acts as a bidentate ligand
(a) There is tetrahedral geometry in the red complex
Hazards of Dimethylglyoxime
DMG can cause severe health effects in case of unregulated exposure. Few potential health hazards are –
It may cause eye and skin irritation
Ingestion can be particularly harmful causing irritation in digestive tract
Inhalation can cause irritation in respiratory tract as well as mucous membranes
Moreover, owing to the flammable nature, dimethylglyoxime is a direct fire hazard. Such a threat is aggravated in its finely divided state as it may be ignited by sparks alone. In the instance of a fire, highly toxic gases may be emitted with combustion or thermal decomposition. Fine dust with reduced particle size can cause a dust explosion.
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1. What is the type of DMG ligand?
Ans. Dimethylglyoxime exhibits bidentate ligand, that is, having two donor arms (N) by which it coordinates in order to form stable metal chelates.
2. What is the formula of Dimethylglyoxime?
Ans. The dimethylglyoxime formula is C4H8N2O2. The expanded formula is CH3C(NOH)C(NOH)CH3. Another name of dimethylglyoxime is 2,3-Butanedione dioxide.
3. What is the role of Dimethylglyoxime in analytical chemistry?
Ans. Dimethylglyoxime is a popular chelating reagent utilised for the gravimetric determination of nickel. It helps in the precipitation of Nickel for its aqueous solution. Even though it has been found to be remarkably effective in terms of completeness and selectivity, but it is not industrially used on a wider scale. It is due to the fact that there is a significant loss of reagent in case of calcination.
4. What are the properties of DMG?
Ans. DMG is transparent crystalline reagent which is soluble in alcohol, acetone, ether but insoluble in water. Its molecular weight is 116.12 and the melting point is 238-240°C. It does not have any odour. Its reaction with metal ions produces coloured precipitates such as bright red precipitate with nickel and blue precipitate with copper among others.