1. Stable isotonic formulation of a solution having a pH of 4.5 - 7.5 consisting of - xylometazoline hydrochloride and/or oxymetazoline hydrochloride as active substance, - a solvent which is pharmacologically acceptable for nasal administration, - an adjuvant selected from among sorbitol and/or glycerol, - a buffer selected from among the inorganic pH buffers or the organic buffer trometamol and - optionally hydrochloric acid and/or sodium hydroxide solution, - optionally substances needed for adjusting the isotonicity, - optionally an oligodynamically active metal or oligodynamically active metal ions in a pharmaceutically acceptable amount, the formulation containing no other organic additives. 2. Formulation according to claim 1, characterised in that the buffer is an inorganic buffer. 3. Formulation according to claim 2, characterised in that the solution contains a sodium and/or potassium phosphate buffer or a sodium and/or potassium borate buffer. 4. Formulation according to claim 3, characterised in that the solution contains a monosodium dihydrogendisodium monohydrogen phosphate buffer and/or monopotassium dihydrogen-dipotassium monohydrogen phosphate buffer. 5. Formulation according to claim 1, characterised in that the buffer is trometamol. 6. Formulation according to one of claims 1 to 5, characterised in that the solution is adjusted to a pH of 5.0 - 7.2. 7. Formulation according to one of claims 1 to 4, characterised in that the solution is adjusted to a pH of 5.5 - 6.8, preferably 5.8 - 6.0. 8. Formulation according to claim 5, characterised in that the solution is adjusted to a pH of 6.1 - 6.3. 9. Formulation according to one of claims 1 to 8, characterised in that the active substance is present in a concentration of between 0.01 and 1.0% by weight, preferably between 0.01 and 0.5% by weight and most preferably between 0.05 and 0.1% by weight. 10. Formulation according to one of claims 1 to 9, characterised in that the solvent is water. 11. Formulation according to one of claims 1 to 10, characterised in that the solvent is a mixture of ethanol and water. 12. Formulation according to one of claims 1 to 11, characterised in that the proportion of adjuvant in the solution is 1 to 10% by weight, preferably 2 to 6% by weight. 13. Formulation according to claim 12, characterised in that the adjuvant is 3.5 to 4.5% by weight, preferably 4.0% by weight, of sorbitol. 14. Formulation according to claim 12, characterised in that the adjuvant is 2.0 to 2.8% by weight, preferably 2.4% by weight, of glycerol. 15. Formulation according to one of claims 1 to 14, characterised in that the formulation does not contain any oligodynamically active metal or oligodynamically active metal ions. 16. Formulation according to one of claims 1 to 14, characterised in that the formulation contains an oligodynamically active metal or oligodynamically active metal ions. 17. Formulation according to claim 16, characterised in that the oligodynamically active metal or oligodynamically active metal ions is silver or silver ions. 18. Formulation according to one of claims 1 to 17, characterised in that the formulation contains only xylometazoline hydrochloride as active substance. 19. Formulation according to one of claims 1 to 17, characterised in that the formulation contains only oxymetazoline hydrochloride as active substance. 20. Use of a formulation according to claims 1 to 19 as a rhinologica agent together with an inhaler having silver-containing elements in the region between the active substance reservoir and the sprayhead. 21. Use of the formulation according to one of claims 1 to 19 for the preparation of a rhinological agent. 22. Pharmaceutical composition in the form of a formulation of a solution according to one of claims 1 to 19.