Dropwise Testing

[TexasTwister]

Hi everyone,

On some tests, like calcium hardness and TA, you add solutions dropwise until the water in the test chamber turns from one color to another, then multiply by 10 to get your ppm.

My question is, if you have a good idea of where the number will end up, do you really have to add drops one at a time and swirl, or can you add quite a few to get you close to where you think the number will be, and then slow down to dropwise?

Example: My calcium hardness is usually around 230 ppm. Using my Taylor DPD kit, I am to add R-0012 dropwise until the solution turns from red to blue. Knowing this, is there any harm is adding 18 drops, swirl, and then add one drop at a time (and swirl) from that point forward until my solution turns blue?

Thanks for your input.

 

[rimshaker]

I've asked a similar question before and was told it really doesn't matter if you literally swirled after each drop or after whatever number. As long as you have a good idea of the target value, shouldn't be a problem. The only problem would be accidentally overshooting the target value and getting an immediate color change. Then you'd have to redo the test and waste solutions.

 

[Guest]

Actually, with the calcium hardness test it is important to add the drops one at a time and swirl for 20-30 seconds between drops (or use a magnetic stirrer) or you might get an indistinct 'purple' endpoint instead of the clear change to blue. The TA test is not that critical and if you want to add several drops at the beginning it really doesn't matter.

Bottom line is this. the calcium hardness test is not a fast one if you do it properly. Using a magnetic stirrer will speed it up quite a bit and the drops can be added fairly fast.


HOWEVER, to form each drop correctly you are supposed to form each one individually and then wipe the dropper tip off with a tissue between drops because of static charges. This is to ensure uniform drop size and an accurate titration. No one ever really does and the only test this is probably REALLY needed for is a salt (chloride) titration test and possibly for the FAS-DPD chlorine titrations.

 

[TexasTwister]

Thanks for the input, folks.

Wow. A magnetic stirrer. I had to "wiki" that one! I could see where it would come in handy, especially on the calcium hardness test.

One question about the static charge on the dropper tip though. I understood that if you discharged the tip at the beginning of your testing session, you were good to go for that session?

 

[JasonLion]

There is good and then there is better. Wiping the tip after every drop gives the most precise results. Wiping the tip once per session gives results that are usually fine. Never wiping the tip is often acceptable.

A very small charge is deposited by every drop. After several drops that charge can accumulate enough to significantly affect the drop size. But even a tiny charge has some affect. Wiping the tip dissipates the charge and results in the most consistent drop size.

 

[Guest]

One of our members sells an inexpensive, battery operated magnetic stirrer that I reviewed here. It was worth every penny!
Another member got one and reviewed it here.

 

[rhythm]

WOW! I'm new here and really glad I read this thread. I've tested my pool water hardness a few times and have always seen the "purple" that you refer to, and I had no idea that I needed to swirl the mix so long between drops for the CH test! Thank you...this will help me a lot I'm sure. And now that I know this, I think I may be interested in one of those magnetic stirrers!!

Greg