Process for the preparation of a sterile injectable physiologically acceptable solution of an X-ray contrast agent and solutions of the X-ray contrast agent and a buffer

A solution having a physiologically acceptable pH comprising a non-ionic m-carboxamido-o-iodo-N-(.beta.-hydroxyalkyl)-aniline X-ray contrast agent in a concentration of from 100 to 450 mg I/ml of the solution and a physiologically acceptable buffer system containing an amine buffer of formula II

• NR.sup.1 R.sup.2 R.sup.3 (II)
• wherein R.sup.1, R.sup.2 and R.sup.3, which may be the same or different, each represents a hydrogen atom, a sugar residue, an alkyl group with 1 to 6 carbon atoms which may carry one or more hydroxy, mercapto, carboxyl, carboxamido, imidazolyl, indolyl or hydroxy substituted phenyl groups, alkylthio groups with 1 to 6 carbon atoms and/or groups of the formula:
• --NR.sup.4 R.sup.5 in which R.sup.4 and R.sup.5, which may be the same or different, each represents a hydrogen atom, a carboxamido or a ##STR6## group or an alkyl group with 1 to 6 carbon atoms;
• or any two of R.sup.1, R.sup.2 and R.sup.3 may, together with the intervening nitrogen atom, represent a pyrrolidine or piperidine ring which may carry hydroxy, carboxyl or carboxamido groups;
• the pH of which decreases with increasing temperature, the pKa of said buffer system at 15.degree. C. being less than or equal to 9.5 and the concentration of said buffer in the solution being no greater than 50 m.molar.