Can carbonate reducing solution lead to off flavours?

CRS is a 50:50 blend of sulphuric/sulfuric and chloric acids. In reducing the carbonate alkalinity, it therefore produces sulphates/sulfates and chlorides. Being key desirable salts, this will not produce any off-flavours. As it is a 50:50 mix, CRS produces roughly equal amounts of these salts and therefore is unlikely to upset your sulphate:chloride ratio unduly and therefore should not significantly affect flavour.

The trick is to reduce your alkalinity without exceeding the desired sulphate or chloride limit. I use CRS regularly, living in a hard water area, but occasionally have to limit its use in a particular brew and achieve the alkalinity reduction via acidulated malt, lactic acid or, more likely, phosphoric acid.

To reduce alkalinity from 320 (6.4 meq/L) to 40 (0.8 mEq/L) you will need 6.4 - 0.8 = 5.6 mEq/L protons. I assume your finding that 1.5 mL of CRS will do this for you comes from the Brupaks table. Thus tells us that the normality of CRS is 5.6/1.4 = 4 N (a nice round number). Now we know that CRS is equimolar HCl and H2SO4 so that 2/3 of the protons (8/3 mEq/L) come from sulfuric and 1/3 (4/3 mEq/L) from HCl. The equivalent weight of sulfate is 48 so the sulfate addition is 8*48/3 = 128 mg/L sulfate and the equivalent weight of chloride is 35 so the chloride addition is 4*35/3 = 47 mg/L. Those anion additions are not enough to be tasted directly but the sulfate will make the hops bitterness appreciably more assertive and the chloride is enough to improve the body of the beer and round its flavors out.

Also, 5.2 is worthless.

I've been looking at using CRS for a partial mash effort, but before I potentially trash all that hard work I decided to try it with an IPA kit brew.

Foolishly, I didn't taste test the water after treating it. I would say that is very sound advice.

Botttled last night and the inevitable quick taster seemed to harbour a very subtle salty/tart flavour IMO. It is very subtle thought and to be honest, without having tasted the treated water first, it's difficult to gauge if I'm imagining it.

Conclusion: As is so often the case, if it tastes good it's probably fine: Taste your treated water (as I didn't) and if it's salty/sour/tart try a lower dosage.

You know what, as it will only take 5 minutes, I'm going to treat some water this evening with same dosage as for the kit and see what its like... to be continued.

I would avoid pH 5.2 with high carbonate levels since the quantities needed leave a salty taste.

Although the Brupaks page doesn't list the ingredients, it mentions that the CRS is an acid blend. This is a much better start than pH 5.2, which adds lots of sodium to your beer, which in excess gives a harsh saltiness.

Managing the mash pH can be a pain, so a simple solution like CRS that works will make your brewday considerably simpler. I would consider getting a bottle, and adding to some water the amount of CRS to reduce the carbonate to typical levels for a light Pils - this style has little acidity in the mash so you need more compensation for the high carbonates. Do a blind 3 way comparison with treated and untreated water - if the treated water still tastes good, then there's a good chance the beer will be fine.

Other alternatives are to use lactic or phosphoric acid to reduce the pH, or cut with water with low carbonates, such as distilled/RO water.

There is no way to answer this question without knowing what's in the CRS and I don't see any listings for that. I would not use it without knowing what's in it.

I'll take a stab at this since there are no answers yet. I also live in a high alkaline water area (although probably not as high as yours) and I regularly use 5.2 stabilizer to get my mash ph in the right area. It is simple to use (1 Tbl / 5 gal batch) and uses various buffers to bring the water ph either down or up to around 5.2, which is the general ph you want for most beer styles. I have used lactic acid in the past to bring the ph down, and have gotten very mixed results since it is very hard to gauge how much I need to add and how much I am adding (unlike you I do not know exactly how much carbonate I have in my water, but I am in a very limestone-ridden area so I know it is up there).

You could also use acidulated malt to bring your water ph down or simply brew a beer with a lot of heavily roasted malt in it since that should also contribute acids to your mash ph. There is a reason beer styles evolved in the regions they did and a lot of that is due to water ph. Stouts in Ireland (heavily roasted malt), Pilseners in the Pilsener Czech region, etc...

Oh yeah to answer your question, no you should not get any off flavors from the CRS. An easy test to check is brew the same batch of beer with and without CRS added (or split your mash and do 1/2 with CRS and 1/2 without). I have actually brewed with pond water before and there weren't any very detectable off-flavors in that beer. Beer styles also hide off-flavors differently as well with darker beers hiding them better than pale beers. (The pond beer was an Irish Red Ale)

Crap I forgot to add that if you are that concerned with your water, then just use bottled or otherwise known water source.