Precipitation dimethylglyoxime chelates

We start with butane-2,3-dione dioxime, more commonly known as dimethylglyoxime (dmg). It is a classic reagent for the analysis of NP, the green aqueous solution of metal ions transforming into a vibrantly red precipitate of Ni(dmg)2 complex it is one of the stars of the show in Ponikvar and Liebman s analytical chemistry chapter in the current volume. Here the stereochemistry is well-established and well-known—both OH groups are found on the same side as their adjacent CH3 group on the butanedione backbone. There have been several measurements of the enthalpy of formation of this species for which we take the one associated with this inorganic analytical chemistry application, i.e. with diverse metal complexes and chelates . 

The reaction of aqneons green Np, or its bine ammonia complex, with colorless dimethylglyoxime (DMG) to form a vibrantly red precipitate of a 1 2 metaLDMG complex demonstrates an example of precise stereochemistry and oxime deprotonation in what is perhaps the archetypal analytical metal dioxime reaction (equation 1). This transformation certainly intrigued both authors early in their education. It is interesting to note that DMG is an excellent example of highly specific reagent because under the same reaction conditions only yellow palladium chelate is also precipitated. 

Dimethylglyoxime has long been used as a precipitant for nickel and palladium. Nickel is usually precipitated from an ammoniacal tartrate buffer of pH about 8. Under these conditions iron and many other metals, even in large amounts, do not interfere. Palladium is precipitated from hydrochloric or sulfuric acid solution. Nioxime ° (1,2-cyclohexanedionedioxime) has the advantage of being more soluble in water than dimethylglyoxime and therefore less subject to coprecipitation with the metal chelate. 

Nickel forms a red chelate with dimethylglyoxime (DMG), which is quite suitable for gravimetric analysis. Precipitation of the chelate is complete in an acetic acid-acetate buffer or in an ammoniacal solution. Acetate buffer is generally used when Zn, Fe, or Mn is present in the alloy. The sample given to you is a nichrome alloy that has Ni (approximately 60%), Cr, and Fe as the major constituents. Interference from Cr and Fe is removed by complexation with tartrate or citrate ions. Precipitation is then carried out in an ammoniacal solution. The Ni content is calculated from the weight of the precipitate (see Table 10.2 for the formula). 

The Tchugaev reaction [Eq. (35)] of 2,3-butanedionedioxime (dimethylglyoxime, abbreviated here as H2DMG) with nickel ions, where the chelated product is a bright red precipitate. 

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