API GH and KH test kit results

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yertchuk
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#1: Post by yertchuk »

Reminded me of high school chemistry experiments!

I first tested water out of the tap (San Francisco peninsula):
GH, one drop and already green, so very low hardness, maybe 20 ppm?
KH, 2 drops to yellow, so maybe 30-40 ppm?
TDS test with cheap resistivity tester, 25 ppm

Then I added a small amount of potassium bicarbonate:
GH still green at 1 drop
KH, 4 drops, so maybe 60-70 ppm
TDS test, 65 ppm

The above ppm values come from the table included with the test kit instructions.

Not sure what to make of this. I would like to think that the bicarbonate mix would not give rise to scale, but I really don't understand the chemistry. Would appreciate some guidance!

pinky-and-me
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#2: Post by pinky-and-me »

Have you looked at this: https://web.archive.org/web/20080526072 ... erfaq.html maybe a bit too much but it got me started.

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homeburrero
Team HB

#3: Post by homeburrero »

That ^^^^ link to Jim Schulman's Insasnely Long WAter FAQ is an excellent reference about hardness, alkalinity, and scale.

One thing to be aware of is the fact that the API 'KH' test is a measure of alkalinity. If used per instructions on a 5 ml sample each drop corresponds to about 17.9 mg/L of alkalinity. So your 2 drops to yellow is roughly 20 - 50 mg/L alkalinity in CaCO3 equivalents. (or ppm. ppm and mg/L are the same thing in this case.) 4 drops would be 60 - 85 mg/L.

The API 'GH' test is a measure of general or total hardness, so your one drop or less would indicate a roughly less than 20 mg/L total hardness. This is the total amount of calcium and magnesium in the water, expressed in units of CaCO3 equivalence.


yertchuk wrote:I would like to think that the bicarbonate mix would not give rise to scale, but I really don't understand the chemistry. Would appreciate some guidance!
Limescale is a result of calcium cation and carbonate/bicarbonate anion forming a calcium carbonate precipitate when the water is heated. At this low hardness level your calcium is so low that your additional bicarbonate is not going to cause scale. The Insane FAQ can be used to make scale estimates for a given boiler temp, hardness, and alkalinity numbers.

Note: For very soft water like San Francisco you can get better precision out of the API test kits by using a larger water sample. Find a test tube or vial that holds 10 ml rather than 5 ml of water and do the test per instructions. Then when converting, each drop corresponds to a half degree, or 8.9 mg/L CaCO3 equivalent. An ideal alkalinity at or near 40 mg/L would correspond to 4-5 drops on a 10 ml sample.
Pat
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yertchuk
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#4: Post by yertchuk »

Thanks Pat and 'Pinky-and-me'. I had not seen the linked FAQ. There's a lot of information there to absorb.

For a while I was using straight tap water, but decided to add a small amount of bicarb to ensure sufficient alkalinity. As suggested, I'll repeat the KH test with a larger water sample to get a better idea of what, if anything, needs to be done to be sure I'm in the 40-50 ppm range.

I have checked the city water quality report, but it gives wide ranges on most of the parameters. Alkalinity, for example, is quoted as 3.5-97 ppm! Similarly, CaCO3 hardness is quoted at 8.9-77 ppm. Hence my desire to understand what actually comes out of the tap.