Theoretical expressions for the macroscopic polarizability and quadrupolarizability of a quadrupolar mixture are derived. The theory is demonstrated on the example of a liquid mixture of methane and nitrogen (non-quadrupolar plus quadrupolar component). It turns out that the dielectric permittivity (the "dipole strength" of the liquid) of this mixture changes little with the composition, while the quadrupolar length ("quadrupolar strength") almost triples as the fraction of nitrogen approaches one. A set of such mixtures can be used as standard quadrupolar solvents to study systematically phenomena such as quadrupolar solvatochromism, the effect of the solvent on the rate of a reaction etc.