I am reading Charles Bamforth's (UC Davis Brewing professor) book "Beer: tap into the art and science of brewing, 2nd Ed". On page 73, I noted a damning statement regarding the validity of the chloride to sulfate ratio. His comment follows.
"The importance of this chloride to sulfate ratio is one example of the plethora of dogmatic beliefs held by many brewers. In fact, there is little if any published scientific data to justify the conclusions made concerning this ratio - which is not to say that it is not important but rather that, if it were in the dock in a court of law, it wouldn't have much of a hard-and-fast defense."
There has been similar skepticism expressed by myself and AJ DeLange in the past regarding this ratio. The contention that this ratio is applicable or appropriate for brewing without caveat is misguided in my opinion. Any respected brewing text generally cautions against having the water's chloride concentration above 100 ppm when the sulfate also exceeds 100 ppm. A review of water profiles from the historic world brewing centers shows that all profiles (Burton, Dublin, Edinburgh, London, Munich, Vienna, Pilsen) have chloride concentrations under 100 ppm. Burton and Edinburgh both have sulfate concentrations above 100 ppm.
The only historic water profile that has elevated chloride (~130 ppm) in conjunction with elevated sulfate (~330 ppm) is the Dortmund profile. Interestingly, this is a profile that is known for imparting a minerally character to beer. Also notable is that the chloride concentration is not too far above 100 ppm, so that points to the suggestion that the 100 ppm limits mentioned above are somewhat soft and possibly imprecise.
So, what I'm suggesting in this post is that the chloride to sulfate ratio may not be applicable under all conditions. But it does appear that the ratio can be useful when the chloride concentration is less than about 100 ppm. I've heard of software and other brewing water recommendations that utilize the chloride to sulfate ratio to characterize the beer flavor between bitter and malty. While I agree with the general concept of the ratio, its apparent that the 100 ppm (or thereabouts) chloride caveat needs to be inserted into its use.