How can I find out the concentration of chlorine in an acid bath?
The addition of chlorine to the bath will produce hydrochloric acid.  If the chloride ion concentration in the bath sample could be determined, then you could back-calculate to the amount of hydrochloric acid added.  The standard method to determine the amount of chloride ion in an aqueous sample is to precipitate it by adding silver nitrate solution followed by collecting the precipitate by filtration and then drying and weighing it.  You could use that approach here because silver nitrate and silver fluoride (which would be created from silver nitrate and the hydrofluoric acid) are soluble* in water, so only the silver chloride would precipitate if silver nitrate were added to your acid bath sample.  Here's a link to an actual gravimetric procedure for determining chloride ion content.


Note: silver fluoride is soluble to the extent of 1.8 kg/L, therefor silver fluoride should not precipitate upon addition of silver nitrate to your chlorine-contaminated sample because the hydrofluoric acid content of the bath was only 4-5%.