In order to remove ketchup stains, rinse with cold water and soak the stain with white vinegar diluted to 50%.
Leave for a while.
Soak it again if necessary.
If the ketchup stain persists, dab the stain with a cloth soaked in petrol or diluted methylated or surgical spirit.
You can also use warmed glycerine to penetrate the stain.
Remember to rinse the treated area with warm water.
Ammonia diluted with a little water can also be effective.
Rinse once the stain has disappeared.