Detection of marked mineral oils and novel azo dyes

A method of marking a mineral oil, comprising adding to said mineral oil an effective amount therefor consisting of an azo dye of the formula Ia ##STR16## where m is from 1 to 4,

• X.sup.1 is hydrogen, cyano or nitro,
• X.sup.2 is hydrogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy or C.sub.1 -C.sub.13 -alkoxycarbonyl,
• X.sup.3 is hydrogen or C.sub.1 -C.sub.4 -alkyl, and
• X.sup.4 is hydrogen or C.sub.1 -C.sub.4 -alkyl, which may be phenyl-substituted.

A method of marking a mineral oil, comprising adding to said mineral oil an effective amount therefor of an azo dye of the formula Ib ##STR18## where X.sup.1 is hydrogen,

• X.sup.2 is hydrogen,
• X.sup.3 is hydrogen or C.sub.1 -C.sub.4 -alkyl,
• X.sup.4 is hydrogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -dialkylamino or C.sub.1 -C.sub.16 -monoalkylamino,
• X.sup.5 is hydrogen or C.sub.1 -C.sub.4 -alkyl, and
• X.sup.6 is hydrogen, C.sub.1 -C.sub.4 -alkyl or C.sub.1 -C.sub.4 -alkoxy,
• with the proviso that a) at least one of the three radicals X.sup.1, X.sup.2 and X.sup.3 is different from hydrogen, b) X.sup.2 may also be cyano when X.sup.4 is C.sub.1 -C.sub.4 -dialkylamino, c) X.sup.2 may also be C.sub.1 -C.sub.4 -alkoxycarbonyl when all the radicals X.sup.4, X.sup.5 and X.sup.6 are different from hydrogen, and d) one of the radicals X.sup.1, X.sup.2 or X.sup.3 may also be nitro when X.sup.4 is C.sub.1 -C.sub.16 -monoalkylamino and X.sup.6 is C.sub.1 -C.sub.4 -alkyl.

Azo dyes of the formula II ##STR19## where Y.sup.1 is hydrogen, or C.sub.1 -C.sub.4 -alkyl-substituted phenylazo,

• Y.sup.2 is hydrogen,
• Y.sup.3 is hydrogen or C.sub.1 -C.sub.4 -alkyl,
• X.sup.4 is hydrogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -dialkylamino or C.sub.11 -C.sub.16 -monoalkylamino,
• X.sup.5 is hydrogen or C.sub.1 -C.sub.4 -alkyl, and
• X.sup.6 is hydrogen, C.sup.1 -C.sub.4 -alkyl or C.sub.1 -C.sub.4 -alkoxy,
• with the proviso that a) at least one of the three radicals Y.sup.1, Y.sup.2 and Y.sup.3 is different from hydrogen, b) Y.sup.2 may also be cyano when X.sup.4 is C.sub.1 -C.sub.4 -dialkylamino, c) Y.sup.2 may also be C.sub.1 -C.sub.4 -alkoxycarbonyl when all the radicals X.sup.4, X.sup.5 and X.sup.6 are different from hydrogen, and d) one of the radicals Y.sup.1, Y.sup.2 or Y.sup.3 may also be nitro when X.sup.4 is C.sub.1 -C.sub.16 -monoalkylamino and X.sup.6 is C.sub.1 -C.sub.4 -alkyl.