Detection limit for Wal-Mart kit CH test??

[Henrys514]

This is a question about Calcium Hardness...Pool info: 12,000gal IG exposed-aggregate surface.

Pool store test says hardness is 185ppm. My Wal-Mart test taken on the same day says 0ppm (I think. It says to add two drops of this reagent, and if hardness is present, water will turn a violet color. There was no color change, so I assumed hardness was not present)

I tested my fill water, to make sure the reagents were working, and the result was 200ppm.

Is there a Method Detection Limit for the Wal Mart Hardness test, so my number may not be 0ppm, it may just be below the ability of the test to detect? Should I add indicator until I get a violet color, then adjust the amount of titrant added accordingly?

I need to add hardness either way, so I'm planning on adding 8lbs of Calcium Chloride (i think that's what the ingredient is) tomorrow evening. According to the bucket, that should increase my hardness by ~55ppm. 55ppm increase is great if my hardness is 185ppm. If it's really 0ppm, it's not nearly enough.

Another question...it's supposed to rain here Monday and Tuesday...a lot (darn tropical storm ) Should I wait to add the calcium chloride until Wed? The hardness has been low for a while now...we battled algae for a while, now that the water is clear we need to get everything else in line.

[waterbear]

There has been a problem with the walmart hardness test that has been documented in a prevoius post by several people including myself. I suspect that it is being caused by high chlorine levels interfering with the indicator used in this test. A work around that I have used is to keep adding drops of indicator until the water turns either pink indicating hardness or blue indicating totally soft water. I have had to add up to 15 drops. this does not seem to have much effect on the titration results.

[Henrys514]

Hmmm...what is considered "high chlorine levels" for this test? At the time, my TC was 3-5ppm. My TC is 5ppm+ now because I added bleach. Come to think of it, when I tested my make up water, the TC of that was 1.5ppm and the test worked fine. So should I trust the pool store numbers in this case? (until I get a better test kit, that is). 55ppm of Calcium really isn't that much, and I'll always have my FC/TC around 5ppm.

[waterbear]

If you are testing the TC with the OTO test in the walmart kit be aware that OTO is not the most accurate test to read. It is possible that your TC was higher. The test takes about 2 minutes to develop fully.

[Henrys514]

I usually wait 2 minutes before doing the reading. So if I keep adding indicator, if I have no hardness, the water will eventually turn blue. If I do have hardess, it will turn pink (instead of purple like the instructions say), and then I titrate from pink to blue, then multiply drops by 50 to get my reading?

[waterbear]

I usually wait 2 minutes before doing the reading. So if I keep adding indicator, if I have no hardness, the water will eventually turn blue. If I do have hardess, it will turn pink (instead of purple like the instructions say), and then I titrate from pink to blue, then multiply drops by 50 to get my reading?

Basically, yes. The color is sort of a pinkish magenta when hardness is present and blueish when there is none.

[Henrys514]

Ok, thanks. I'll try that tomorrow and see what happens. Oh yeah, and I'll work on getting a better test kit too.

[prh129]

I had a similar problem with the CH test in the WalMart (HTH) 5-way test kit. My TC was in the 3-5 ppm range at the time and the hardness was actually pretty low (10-20 ppm) but the solution turned yellowish when I added the 5 drops of the dark reagent. The CH test in Ben's kit worked without a problem.

Peter

[Henrys514]

Well, i tried the test yesterday...My TC was 5ppm+ (I did some dilution, and got about 9ppm). With 8 drops of the indicator, the water turned this funny pinky yellowish color, so I titrated as usual and got 250ppm CH. I will test again today after work and see what I get. I can hopefully add some calcium today if the rain stops.

[waterbear]

250 ppm CH is fine. Why add more?