Test results discrepancy

rainman168

Well-known member
Apr 2, 2007
46
#1
Hi, I tested my water using the FAS-DPD test kit and my free chlorine reading was 4.2. I used the 25 ml sample which needed 21 drops of R-0871 before it turned colorless. I took the water into Leslie's to confirm my test results. Leslie's informed me that I had 1.5 for free chlorine. I believe they use the DPD method.

I don't know whether or not I messed up. Does the amount of R-0870 impact the amount of R-0871 needed to make the water colorless by that much? I would be interested to hear other people's experience / thoughts. Thanks.
 

TarheelPool

LifeTime Supporter
Mar 29, 2007
86
Burlington, NC
#2
Someone else with more knowledge will weigh in I'm sure but I don't think the amount of r-0870 will impact the test as long as you put in the general amount specified by the kit (2 scoops using the Taylor K2006). If you are able to repeat your results with your kit, I would trust your own numbers.

Kevin
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#3
Chlorine can lower in concentration during the trip to the pool store. This is especially true if the sample is not capped or is in sunlight or if the sample jar or container contains any contamination (something that the chlorine would react with). Or course, they could just be measuring wrong at the pool store.
 
G
#4
Also, since the endpoint of the FAS-DPD test is much easier to read than comparing colors as used in the DPD test it is more accurate. However, the test has to be done correctly. The dropper bottle needs to be held upside down and not tilted to produce uniform drops. Also, wiping the dropper tip with a damp tissue between drops will eliminate static that can affect drop size. Let the drop from slowly and don't squeeze it out. If this is not done then it is possible to produce smaller drops and get a higher reading.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#5
Taylor has an excellent Chemistry Topics section here and if you click on the topic "Two Ways To Improve Drop-Test Accuracy", you will see the points waterbear is making. This page of helpful tips answers a lot of questions about test kits. Hopefully it's consistent with waterbear's experience :)
 
G
#6
chem geek said:
Taylor has an excellent Chemistry Topics section here and if you click on the topic "Two Ways To Improve Drop-Test Accuracy", you will see the points waterbear is making. This page of helpful tips answers a lot of questions about test kits. Hopefully it's consistent with waterbear's experience :)
Yep, pretty much! :-D