If it is being used for a test for sulphates then yes, it does need to be acidified. This is because both sulfites and sulfates produce a white precipitate with BaCl. But, for sulfites the ppt dissolves in acids. read more
Barium chloride solution CAN be made not acidified, but if it is NOT acidified it quickly absorbs CO2 from the atmosphere and Barium Carbonate precipitates making the solution cloudy and not suitable for the sulfate test. read more