Water loses TDS with time?

Water analysis, treatment, and mineral recipes for optimum taste and equipment health.
alfredocalza
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#1: Post by alfredocalza »

As I mentioned in another post, I am currently brewing espresso with the following recipe: 1.5g of potassium bicarbonate + 2.8g of epsom salt, dissolved in 500ml of distilled water. Then, I use 25g of that solution to dissolve it in 975ml of distilled water. Right after making the water, I get a reading of around 80-83 ppm tds. However, after a couple of days have passed, I perform the same test with the same water and get a reading of about 67 ppm tds. I have performed this test several times with different samples and it is very consistent. It doesn't seem to get any lower than 67ppm tds. Any ideas as to why this is happening? Should I use more epsom salt and potassium bicarbonate?

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homeburrero
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#2: Post by homeburrero »

If it had gained TDS with time I would suspect that it's due to the Epsom being not fully dissolved on the first reading. I'll bet this reduced TDS reading is related to a temperature difference effect. Conductivity (which these meters measure) changes with temperature. Most meters have some rough temperature correction but the only way to be consistent is to always measure your water at the same temp. The usual calibration temperature is 25 C.
Pat
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alfredocalza (original poster)
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#3: Post by alfredocalza (original poster) replying to homeburrero »

Thank you again! That would make sense, because I store my water in the fridge. I will leave it out for a while and try the test again. I think I might have a crappy meter. Are there any good water testing kits that you would recommend? I would also like to be able to measure alkalinity, hardness and ph without spending a fortune.

Pressino
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#4: Post by Pressino »

Another possibility, which I actually think is rather likely, is that some of the dissolved solutes have precipitated out from solution, thus lowering the measured TDS. This might be temperature related, but not for the reason mentioned above.

NicoNYC
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#5: Post by NicoNYC replying to Pressino »

That was my first thought as well, but it looks like both KHCO3 and MgSO4 have a solubility of around a hundred-someodd g/L at freezing. Maybe there's some weird fractionation by density over time as the water sits, but I think the salts should be staying fully dissolved.
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homeburrero
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#6: Post by homeburrero »

I agree with Nico that precipitation would not be a factor here. You do need to consider the fact that once you add the KHCO3 and the MgSO4 together, your precipitation concern would be MgCO3 and Mg(OH)2 from chemistry of the dissolved Mg++ and HCO3- ions . But at this concentration and low temperatures you still expect no precipitation.
Pat
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