A method is described for production of a high purity compound of the formula (I) in crystalline form of the modification A. In this, starting from high purity gadobutrol, the gadolinium is removed by decomplexation with oxalic acid, and then with a calcium salt the calcium complex is produced in high purity. During the crystallization, a water equivalent of 9-11 weight % is set. The crystalline form of the modification A of the compound of the formula (I) is used in the production of Gadovist.