Dimethyl glyoxime gives a red precipitate with Ni 2 + , which is used for its detection. To get this precipitate readily the best pH range is

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Nickel(II) forms a precipitate with the organic compound dimethylglyoxime, $C_{4} H_{6} (NOH)_{2}$ . The formation of the red chelate occurs quantitatively in a solution in which the $pH$ is buffered in the range of 5 to 9 . The chelation reaction that occurs is illustrated below.
The reaction is performed in a solution buffered by either an ammonia or citrate buffer to prevent the $pH$ of the solution from falling below 5 . If the $pH$ does become too low the equilibrium of the above reaction favors the formation of nickel(II) ion, causing the dissolution of $Ni (DMG)_{2}$ back into the mother liquor.

Chemistry - Ionic Equilibrium Question with Solution | TestHub

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