inconsistencies in FAS-DPD FC readings

[RGS]

I have been using the Taylor FAS-DPD kit for my chlorine testing. I use bleach to add chlorine. I have been using the 0.75 oz bottle of reagent R-0871 that came with my Taylor test kit. I'm trying to keep my FC in the range 4 to 6, since my CYA is 50. Recently my CYA was 4.0 in the morning. I added the amount of bleach that should have raised FC to 8.0. Later in the day I ran out of R-0871 reagent, so I went to Leslies to get some. They only had the 2 oz bottles (also made by Taylor), and the guy had to search for it until he finally found a bottle behind and under the counter.
When I tested the FC later that day, using the new 2 oz bottle from Leslies, it showed FC as 10.5. I was surprised it went up so high, given the amount of bleach I had added.
Several days later, I received a new 0.75 oz bottle of Taylor R-0871 that I had ordered online. I started testing the pool water samples using both bottles, and found that the new 0.75 oz bottle produced FC readings that were consistently 1 to 2 ppm lower than what the 2 oz bottle from Leslies indicated. I trust the lower readings from the 0.75 oz bottle more than the readings from the 2 oz bottle I got from Leslies, because my initial use of the 2 oz bottle indicated a much bigger FC increase than I had expected. My question is why the two bottles produced such different readings? I visually observed that that the droplet sizes from the two bottles appear to be the same, and the liquid is clear from both bottles. The 0.75 oz bottle I just received in the mail has an expiration date of 9/2017. The 2 oz bottle from Leslies has no expiration date on the label but just says "Lot #1122A". So I'm wondering if the bottle from Leslies was so old it has gotten weak? The color is still clear. I suspect Leslies does not sell that reagent very often since it was behind the counter, and it took the salesman a while to find it.
To add to the mystery, I did an overnight free chlorine loss test. The larger bottle from Leslies showed FC as 6.5 both the night before and the following morning, and it showed CC=0. However the smaller bottle showed FC=5.5 the night before, and FC=4.5 in the morning, and also showed CC a little under 0.5 in the morning. Has anyone had similar experiences with variability in the FAS-DPD test results? I will continue to use both bottles for a while and record the differences, but I'm thinking of just getting rid of the bottle from Leslies.

 

[Isaac-1]

It is possible that the bottle is old, Taylor started printing expiration dates on their bottles in January of 2015, so any taylor bottle without an expiration date and only a lot number is older than that. (expiration date is 24 months from pack date)

 

[RGS]

Thanks. The larger bottle from Leslies continues to shows higher FC: today it showed 8.0, whereas the smaller bottle showed 6.0. I won't use the bottle from Leslies any more. As you said, it may be too old.

One other thing that puzzles me, regarding the tests I did with the small bottle:
When I did the first test with the small bottle of R-0871, it showed FC=5.0 and CC between 0.5 and 1 (needed 2 drops to make pink completely disappear in the CC test). That surprised me so I rinsed out the tube and repeated the test with the small bottle. This time it showed FC=6.0 and CC=0 (no pink at all after I added R-0003). I repeated the test a third time with the small bottle and still got FC=6 and CC=0. I'm puzzled why I got FC=5, and CC between 0.5 and 1, the first time. I used the same water sample, and I always do the CC test immediately after the FC test, without delay.

 

[Isaac-1]

You might have had some organic contamination, the FC would react with that and form CC. as to the bottle with no expiration date, you can contact Taylor with the lot number and they can tell you if it is expired.

Ike

p.s. the CC reading may also have been from tap water if you rinsed out your container or test tube, many municipal water supplies have switched from using FC to CC for sanitation because it is cheaper

 

[RGS]

Thanks.
I've noticed on several occasions that my first reading shows a certain level of FC and a small amount of CC, but when I rinse out the tube and repeat the test, it shows a slightly higher FC and no CC. For example today, I got FC=7.5 and CC=0.5 in my first test, but when I rinsed and repeated I got FC=8.0 and CC=0

So if there was some organic contamination in the tube, it was there in my initial test before I rinsed it out. I normally don't rinse the tube before the first test, but I do rinse it between tests, and after the last test, before I put it away. After the last test, I rinse the test tube and store it inside my Taylor kit, with the cover closed, inside an outdoor cabinet. Perhaps some microscopic bugs crawl in there over night. In the future I will rinse the tube before the first test and after the last test (also between tests of course), and see if I get more consistent readings.