JoyfulNoise,
Yes, I read your post and all of the others. Yes, I literally stood next to him. He spent at least 7 "pours" to get to the 10 ml line. Yes, I am testing correctly and know when the endpoint is......it's as soon as a drop holds a salmon red color.
I just did two more tests. My wife got 4,000 and I got 3,600. I watched her do the entire test and verified she was spot on the 10 ml line. This kit is not very reliable.
I've always thought about getting a speedstir......I've always wanted one just because I like gadgets.
You are saying two things however......you are saying in your first post to test with 20 ml line and your last is saying 40 ml. Why is that? Is it the more water, the more accuarate?
I looked up that speedstir. That will work with a tube tester. What would work with the K-2006 style test tube? Would like to just get one type of stirrer for all test kits.
Thanks,
Doug
The test is accurate. In fact it
the only accurate test for chloride levels because it is actually using a chemical reaction to measure the chloride and there are no chemical interferences. This test has been around for well over a century (look up Mohr's Argentometric Method) and was developed back in the days when people had to do quantitative chemistry without the aid (or crutch) or electronic devices. Every other method of salt determination - TDS meters, capillarity strips, or hydrometers - use a proxy to "measure" the salt levels. Those proxies (electrical conductivity, capillarity, specific gravity) are much, much more subjective to interpretation, environmental influence and human error.
What you are seeing in the tests variation are variations in the humans doing it. The dropper bottles are good at forming 40 micro-liter droplets but any variation in squeeze pressure and rate can cause droplet volume variations. In fact, when creating droplets, you use a constant pressure; you should not ever do squeeze, release, squeeze, release, etc., because the changes in back pressure inside the dropper bottle can vary droplet size. This is why you, your PB and your wife can all get slightly different results - you are all measuring the sample volume slightly differently (most people don't understand the concept of a water meniscus and where the proper levels is) and you are all squeezing the droplet bottle differently.
Yes, you should get a SpeedStir as it makes testing much more uniform. You should also consider getting a SampleSizer. The SampleSizer allows you to get very exact and repeatable sample volume sizes using the standard Taylor test tube. Trying to accurately measure 10mL can be difficult and the SampleSizer makes that process very repeatable.
As for sample volume and titrant precision, yes, the larger the water sample, the higher the degree of precision. It's just simply volume/concnetration ratios. If the titrant produces a 200ppm/drop precision in a 10mL sample, then a 20mL water sample will have a precision of 200 * (10/20) = 100 ppm/drop. A 25mL sample will have a 200 * (10/25) = 80ppm/drop precision and a 40mL water sample will have a 200 * (10/40) = 50ppm/drop. The titrant is a fixed concentration so it reacts with a specific number of moles of chloride ion. The larger the water sample, the more total moles of chloride present and thus the greater number of drops needed. Since the concentrations are all fixed and the volume of titrant delivered in each droplet is fixed, then the volume ratios determine the precision of the test. This is true in many of the other tests as well - if you double the water sample volume, you half the precision of the test (e.g., a 50mL water sample with the TA test will give a 5ppm/drop precision of the R-0009 titrant). There are limitations to how large or small a sample you can test, but simply doubling or halving the test water volumes is usually not that hard.
At the end of the day, it's up to you to trust your results. As TFP always says, your results, and yours alone, are the only ones that matter. You are the only person that cares about your pool water and it is your testing that is the most accurate. As an aside, I have been doing this test for years, and it has never failed to produce repeatable and accurate results. Many others on here use this test and they will all tell you the same thing.
