FAS-DPD test

[Walter2050]

Good evening.
Did some searching on here before I posted but could see a definite answer
I check my chlorine levels daily. I know I dont need to but I actually enjoy it
I usually use the 25ml test but wanted to start using the 10ml test . All my regents are up to date and I have a speed stir and sample sizer. I’m getting 35 drops (7ppm) with the 25ml and 10 drops (5ppm) with the 10ml . I’m waiting about a second between each drop as well with bottle vertically. I did the test 5 times comparing the to . Same results every time . I could understand if it was 1/2ppm between the to but 2ppm is a big difference . Does anyone have an answer for this ?

 

[Bperry]

Good evening.
Did some searching on here before I posted but could see a definite answer
I check my chlorine levels daily. I know I dont need to but I actually enjoy it
I usually use the 25ml test but wanted to start using the 10ml test . All my regents are up to date and I have a speed stir and sample sizer. I’m getting 35 drops (7ppm) with the 25ml and 10 drops (5ppm) with the 10ml . I’m waiting about a second between each drop as well with bottle vertically. I did the test 5 times comparing the to . Same results every time . I could understand if it was 1/2ppm between the to but 2ppm is a big difference . Does anyone have an answer for this ?

The tolerance for the 10ml test is 1 drop so +\- 0.5ppm. That’s assuming you fill the vial to the exact correct spot, drops all same size, making sure you have enough powder (should have a few grains of powder left undissolved in the vial).

The 25ml test is supposed to be slightly more accurate, but I’ve never used it. You might try cleaning the vial really well and seeing if that helps.

 

[newdude]

I'll wager that you are better at repeating 10 perfect drops than you are repeating 35 of them. . There is always human error involved as we aren't machines and the more drops the more it may be off.

 

[Walter2050]

The tolerance for the 10ml test is 1 drop so +\- 0.5ppm. That’s assuming you fill the vial to the exact correct spot, drops all same size, making sure you have enough powder (should have a few grains of powder left undissolved in the vial).

The 25ml test is supposed to be slightly more accurate, but I’ve never used it. You might try cleaning the vial really well and seeing if that helps.

The tolerance for the 10ml test is 1 drop so +\- 0.5ppm. That’s assuming you fill the vial to the exact correct spot, drops all same size, making sure you have enough powder (should have a few grains of powder left undissolved in the vial).

The 25ml test is supposed to be slightly more accurate, but I’ve never used it. You might try cleaning the vial really well and seeing if that helps.

I cleaned the vial after every test and also use a sampler sizer so I know the water in vial is accurate

 

[JoyfulNoise]

If you want to see how good you are at adding drops, do this test -

1. Find an old titrating bottle and fill it with distilled water or clean tap water
2. Get a very accurate measuring cylinder or pediatric medicine dosing cup
3. Hold the bottle over the measuring cup and, as evenly and consistently as possible, start adding drops from the water filled titrating bottle
4. Count off the number of drops it takes to reach 1mL of water dispensed.

The number should be about 25 … if you do the test enough times you’ll see how much your variance is.

 

[duraleigh]

Are you waiting for the last drop to produce NO further color change?

 

[Walter2050]

If you want to see how good you are at adding drops, do this test -

1. Find an old titrating bottle and fill it with distilled water or clean tap water
2. Get a very accurate measuring cylinder or pediatric medicine dosing cup
3. Hold the bottle over the measuring cup and, as evenly and consistently as possible, start adding drops from the water filled titrating bottle
4. Count off the number of drops it takes to reach 1mL of water dispensed.

The number should be about 25 … if you do the test enough times you’ll see how much your variance is.

Are you waiting for the last drop to produce NO further color change?

Yes sir . I totally understand the variance difference in drops but 2pp difference is alot

 

[Walter2050]

If you want to see how good you are at adding drops, do this test -

1. Find an old titrating bottle and fill it with distilled water or clean tap water
2. Get a very accurate measuring cylinder or pediatric medicine dosing cup
3. Hold the bottle over the measuring cup and, as evenly and consistently as possible, start adding drops from the water filled titrating bottle
4. Count off the number of drops it takes to reach 1mL of water dispensed.

The number should be about 25 … if you do the test enough times you’ll see how much your variance is.

i respect your expertise and this was a answer I was looking for . So just go with the 10ml test for more of a accurate reading?

 

[JoyfulNoise]

i respect your expertise and this was a answer I was looking for . So just go with the 10ml test for more of a accurate reading?

Yeah. Human error is pretty significant in these types of droplet based titrations. Real titrations use very accurately made glassware and modern equipment uses electronic micro-liter dispensing units.

Also, for TFP recommended levels, using the 0.5ppm/drop protocol is all the accuracy you need. And it saves reagent use as well.

 

[Walter2050]

Yeah. Human error is pretty significant in these types of droplet based titrations. Real titrations use very accurately made glassware and modern equipment uses electronic micro-liter dispensing units.

Also, for TFP recommended levels, using the 0.5ppm/drop protocol is all the accuracy you need. And it saves reagent use as well.

Thank you sir

 

[generessler]

I have always wondered why the Taylor instructions specify 2 scoops for both 25ml and 10ml tests, so am wondering if the difference is accounted for by not enough powder for the 25ml test in this case.

Obviously I'm not a chemist.

 

[Walter2050]

I have always wondered why the Taylor instructions specify 2 scoops for both 25ml and 10ml tests, so am wondering if the difference is accounted for by not enough powder for the 25ml test in this case.

Obviously I'm not a chemist.

Great question. Even in my 25ml testing I add a tad more than 2 scoops

 

[JoyfulNoise]

You add the R-0870 in excess of what is needed. Two rounded scoops for 25mL is more than enough. Since the reagent is cheap, just use two scoops in 10mL too. Too little reagent and you can risk false-low test result.