Water Testing - Precision vs Saving Reagents

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smackdab

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Mar 17, 2015
155
Georgia
I'm trying to see the relationship between changing sample sizes for water testing.

Red is the instructed sample size.

FAS-DPD Chlorine Test
Sample Size... 1 Drop = ? ppm ..... Other Reagent Drops Amount ...................................... Precision
------------------------------------------------------------------------------------------------------------
2.5 mL ............... 2 .......... 1/4 heaping scoop R-0870 - 2 drops R-0003 for CC's ......... +/- one drop (2 ppm) up to 10 drops or 10% after 10 drops
5 mL .................. 1 .......... 1/2 heaping scoop R-0870 - 3 drops R-0003 for CC's ......... +/- one drop (1 ppm) up to 10 drops or 10% after 10 drops
10 mL ............... 0.5 ......... 1 heaping scoop R-0870 - 5 drops R-0003 for CC's ............ +/- one drop (0.5 ppm) up to 10 drops or 10% after 10 drops
15 mL ............... 0.33 ........ 1.5 heaping scoops R-0870 - 8 drops R-0003 for CC's ....... +/- one drop (0.3 ppm) up to 10 drops or 10% after 10 drops
20 mL ............... 0.25 ........ 2.0 heaping scoops R-0870 - 10 drops R-0003 for CC's ...... +/- one drop (0.25 ppm) up to 10 drops or 10% after 10 drops
25 mL ............... 0.2 .......... 2.5 heaping scoops R-0870 - 13 drops R-0003 for CC's ...... +/- one drop (0.2) up to 10 drops or 10% after 10 drops


TA Test
Sample Size... 1 Drop = ? ppm ..... Other Reagent Drops Amount ............ Precision
-----------------------------------------------------------------------------
10 mL .................. 25 ............... 1 drop R-0007, 2 drops R-0008 ........ +/- one drop (25 ppm) up to 10 drops or 10% after 10 drops
15 mL .................. 16.66 ........... 1 drop R-0007, 3 drops R-0008 ........ +/- one drop (16.6 ppm) up to 10 drops or 10% after 10 drops
20 mL .................. 12.5 ............ 2 drops R-0007, 4 drops R-0008 ....... +/- one drop (12.5 ppm) up to 10 drops or 10% after 10 drops
25 mL .................. 10 ............... 2 drops R-0007, 5 drops R-0008........ +/- one drop (10 ppm) up to 10 drops or 10% after 10 drops


CH Test
Sample Size... 1 Drop = ? ppm ..... Other Reagent Drops Amount ........... Precision
-----------------------------------------------------------------------------
5 mL ................. 50 ................ 5 drops R-0010, 2 drops R-0011L ...... +/- one drop (50 ppm) up to 10 drops or 10% after 10 drops
10 mL ............... 25 ................ 10 drops R-0010, 3 drops R-0011L ..... +/- one drop (25 ppm) up to 10 drops or 10% after 10 drops
15 mL ............... 16.66 ........... 15 drops R-0010, 5 drops R-0011L ...... +/- one drop (16.6 ppm) up to 10 drops or 10% after 10 drops
20 mL ............... 12.5 ............. 20 drops R-0010, 6 drops R-0011L ...... +/- one drop (12.5 ppm) up to 10 drops or 10% after 10 drops
25 mL ............... 10 ................ 25 drops R-0010, 8 drops R-0011L ..... +/- one drop (10 ppm) up to 10 drops or 10% after 10 drops


My math for the second column seems to be correct but the other reagent amounts don't seem to be right because the CH extended test kit directions says for a 25 mL size to use 20 drops R-0010 and 5 drops R-0011L.

If you double the sample size wouldn't you have to double the amount of other reagents? 25mL is 2.5 times 10 mL so multiply 10 mLs drops by 2.5 and round up as needed?

Also I assume the precision would still be "+/- one drop up to 10 drops or 10% after" when changing sample sizes or would it?
 
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I had asked a similar question about 3 weeks ago and never got a reply. My question was / is if I'm at an elevated FC level, and only using 5cc for testing, how far off will my accuracy of my test be? I'm talking FC in the 8-20 range.

In other words for the course of a SLAM if I needed to maintain FC of 16 and I was only using 5cc for the test I would use half as much reagent, as long as I was hitting 18-19FC. Was this accurate enough?
 
Your error roughly doubles with half the sample size unless you are very careful in measuring the water volume. Clearly the resolution (FC per drop) has double the width, but your percentage error also increases due to limits measuring the sample size accurately. So in your case for FC a 5 ml water sample would have +/- 1 drop so 1 ppm FC or at least +/- 10% of the FC error so at the 20 ppm range that would be 2 ppm. Your error could be closer to 20% if you are not careful -- if your 5 ml water sample were 4 ml or 6 ml instead of 5, for example. That same 1 ml error with a 10 ml water sample would only be 10%. Is this good enough for a SLAM? Probably, if you were to target a little higher than the SLAM level to be safe. You gave a range of 8-20 so at 8 ppm FC I think a 10 ml sample size would be better, but that doesn't sound like a SLAM anyway.

As for the CH test, the titrant drops scale exactly but the amount of the other drops is not super critical so they were just doubled for the larger sample size rather than multiplying by 2.5. That is not a big deal since having a little less or a little more indicator dye doesn't matter and the same is true for the calcium buffer since it is added in excess to precipitate any magnesium in the water so that you measure calcium hardness instead of total hardness. The error is +/- 1 drop or 10% whichever is greater, so what you wrote is correct since 1 drop out of 10 is that 10% transition point.
 
Great. Thanks Chem Geek. I was hoping you would drop in and give us an answer that I could understand.

So, if I understand correctly, the most important part is to make dead sure the test volume is accurate. Then if I'm testing above 15 or more the 20% margin is not enough to really worry too much about as long as I'm shooting for a few ppm over my slam target anyway.

But for everyday maintenance, I should use 10ml to get back to the 10% error margin.

Does this sound right or am I still not really understanding it all.
 
Yeah, that's basically it. There's normally really no need to skimp on reagents, especially for the regular FC testing where the 10 ml sample size should work fine with it's 0.5 ppm FC resolution and 1 drop or 10% error, whichever is greater. The only time to consider skimping is for a SLAM at higher FC levels where the 5 ml sample is OK with the drop resolution of 1 ppm, though for an OCLT you'll need to use the 10 ml sample again since you need greater accuracy and resolution. For the CH test, if one knows the CH is going to be rather high, then using a smaller sample size is OK since the CSI where the CH is used is a logarithmic scale so a 50 ppm CH resolution is OK when the CH is rather high (say, 500 ppm or more).

Basically, the +/- 1 drop error comes from the fact that you don't measure at any finer resolution than that. So that error is obviously larger with smaller sample sizes since what 1 drop represents is higher. When measuring levels that use more than 10 drops, the 10% error comes into play and that's the sum of many different sources of error that are each less than 10% (most are 5% or less errors). Some of the errors are bias errors meaning they are fixed for the specific reagents and dropper tips that you are using.

The concentration of the reagents may have some error and the quality of the dropper tip may have some error producing too small or too large a drop (static electricity can make drops squirt out, but that's something you easily fix by wiping the dropper tip with a damp cloth). The Taylor standard is 24 +/- 1 drops per milliliter so there should be 120 drops in 5 ml and this error is in the range of +/- 1/24 = +/- 4.2%. You do have some control over your part of this error by not squeezing too fast and hard and let the drop fully form and hang before it drops. There may be a bias error in the markings for measuring the sample size.

Measuring the sample size is mostly a variable error that you can only improve to a limited degree unless you use different more precise calibrated glassware (like lab equipment). It's basically how careful you are at having the bottom of the meniscus at the sample size line. Your error is going to be an absolute one such as +/- 0.5 ml if you are careful or +/- 1 ml if you are sloppy. A smaller sample size has such error be a higher percentage than when using a larger sample size. A +/- 0.5 ml error with a 5 ml sample size is +/- 10% while with a 10 ml sample size it's +/- 5% and with a 25 ml sample size it's +/- 2%.
 
Thanks for replying. I updated the graph above and added the FC and TA test. Maybe that will help answer dieseltims question.

From what I understand from your reply is what I updated the graphs with. Are the graphs correct?

The fractional heaping scoops are just there for mathmatical purposes and also I see no need to do a CC test with a sample smaller than 10 mL since the presision is too far out but I put the info for the sake of mathmatics.
 
Yes, that looks OK. In practice, the heaping scoops need enough for a decent color transition at the end of the test. There's some debate of how much is needed. I often test using a 25 ml water sample and find it doesn't really take 2.5 heaping spoons. 1 heaping spoon is on the edge and probably 1.5 works reasonably (probably similar to the 2 dippers Taylor recommends) but this also depends on whether the powder is still fine and more fully dissolves. But mathematically you've got it right.

The same could be said for CC's where technically what you wrote is correct but normally CC's are so low that 5 drops is sufficient even for a 25 ml water sample. You just need to add excess potassium iodide more than the amount of CC's one has. This is probably why Taylor (see their instructions) just says 5 drops independent of 10 vs. 25 ml sample size. In fact, one could add R-0003 drops until one no longer sees a color change which would most often result in only two drops being used most of the time when the CC is low.

For the CH test, Taylor says 20 and 5 for the 25 ml sample size since they are just making it simple doubling the 10 even though it technically should be multiplied by 2.5 as you show. Again, this is because the reagents really just need to be in excess for the calcium buffer and the indicator dye just needs to be intense enough to visibly see a clear transition. So again, while technically correct, there's leeway in the actual instructions.

For my own testing, I'm usually using 25 ml sample sizes for everything so measuring FC with 0.2 ppm resolution and TA and CH to 10 ppm. My FC isn't that high and my daily chlorine losses are low (around 1 ppm) so I like to see more resolution to tell if I've got a problem. For CH, mine isn't at 500 ppm or more so I get some more accuracy though it's probably the least important to be that accurate. For me, it's a way of checking on water dilution or plaster dissolving and a secondary reference compared to salt and CYA levels and I only do these TA, CH, CYA tests infrequently. But then, I'm a chem geek.
 
Oh ok I got ya.

If one expects a big number such as 30 ppm FC, 800 ppm CH, 300 ppm TA then one could use a smaller sample size to save reagent at the expense of being less accurate.

If one expects small numbers such as 2 ppm FC, 100 ppm CH, 60 ppm TA then one could use a larger sample size to be more accurate at the expense of using more reagent.
 
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