Graduated cylinder incorrect, or am I doing it wrong?

soundguy

LifeTime Supporter
Jun 22, 2009
62
0
Gulf Coast, AL
#1
I just pruchased some 1mL plastic pipettes and some glass graduated beakers for testing chlorine concentrating using 10,000:1 dilution.

Just for kicks, I took a 1 mL pipette and filled up the graduated containers that come with the tfp test kit. I filled the pipette up to the 1 mL mark and squirted it into the chlorine only cylinder 5 times. Once finished the cylinder was filled to the 7 mL mark.

That's off by 40%.

The pipettes are graduated in 0.25 mL increments.

To verify the accuracy of the pipettes, I measured the weight of 1 mL tap water with a beam scale. The weight is 17.2 grains or ~1.115 grams at approx room temp. Distilled water should weigh approx .9966 at a close temperature which is only 11% difference.

I do not have distilled water to test.

I am performing these experiments as I type...

This is very interesting:
The cylinder with part # 9198 seems to be very accurate.
I filled this one 5 times with 1 mL tap water and it came to just above the 5 mL mark.
Then I removed 11%, or approx .6 grams (10 grains) and the water is touching the 5 mL line. Since there is no meniscus in the cylinder, it is hard to determine where the mark really is, but it is extremely close.

So, I guess I answered my own question using the cylinder part # 9189.

The chlorine cylinder and the other one is off by as much as 29% or around 1.45 mL.

I guess that is not too bad considering how expensive (or lack thereof) they were.
 

jjslinger

LifeTime Supporter
Sep 22, 2011
97
0
64
Santa Rosa, CA
#2
I have a small electronic scale, which I used to check this out. I can place the empty cylinder on the scale and calibrate it, then add the water. I used tap water.

For the "chlorine only" cylinder, filled to the 10 mL mark (bottom of meniscus), I got 11.5 grams, so 15% high. I then filled it to 25 mL and got 24.9 grams.

For the other cylinder (the one for TA etc.), I got 10.4 grams at the 10 mL mark, and 25.1 grams at the 25 mL mark.

For the cylinder that came with the speed stir, I got 9.5 grams at the 10 mL mark, and 23.8 grams at the 25 mL mark.

I think that is sufficient accuracy for testing our home pools. If I chlorinate my pool to 5.75 ppm instead of 5 ppm, it doesn't really matter since I didn't know my CYA that accurately anyway. If you want more accuracy in the chlorine tests, it seems a bit better to measure at 25 mL, but that wastes reagent, and I don't think the added accuracy is really needed for this sort of thing.
 

wetchem

Well-known member
#3
Be Careful with the plastic pipette markings.

In this case, as jjslinger said, is more than likely close enough for what we're after at home.

If you have ones that look like these (both of these are example only): http://www.coleparmer.com/Product/C...Wrapped_Paper_Plastic_Banana_Peel/EW-13001-20 Then you can more than likely trust the markings "to deliver" the correct amount.

If you're looking at the "eye dropper" style such as these: http://www.coleparmer.com/Product/Transfer_Pipette_1_5ml_Graduated_500_pk/EW-06226-37 they can be really very inaccurate... +/-15% or more depending on how much you want to pay :)

Then there are arguments in the labs about "to deliver" versus "to contain." One would think they'd be the same; however, there can be error associated with using the "wrong" one - depending on who you talk to... :chuckle:

In Analytical Chemistry we were always told... ask your client: What is it you seek, Do you need to just know it's there (cut off level too) or do you need an amount too, and how much error can you tolerate.

If anyone is really interested, I'll take the ones from my kit into the labs and run them against the calibration solutions. I have saline solutions of a known weight/volume that I check certain glassware with for very trace work (the results are down in the part per billion). It will be a day or two before I could do this as the end-of-month crush has started.

-WC
 

soundguy

LifeTime Supporter
Jun 22, 2009
62
0
Gulf Coast, AL
#4
Thanks for checking your cylinders jj.

Wetchem, I wish could spend $335 on a 2 mL pipette.
I would have all kinds of cool chem lab stuff if I could.

In my case, I need to both deliver and contain. First deliver 1 mL from the pipette into 100 mL distilled water.

My purpose for testing is not chlorine ppm in my pool. I'm testing the strength of the chlorine from the pool store.

The last time I purchased chlorine from the pool store, what was supposed to be close to 12% was actually closer to 6%
 

wetchem

Well-known member
#6
soundguy said:
I would have all kinds of cool chem lab stuff if I could.
In my case, I need to both deliver and contain. First deliver 1 mL from the pipette into 100 mL distilled water.
Isn't that the truth! AHhhh the toys .... kind-of-what keeps me coming back to the lab. :mrgreen:

Anyway, contact your local community college or the university Chemistry/Biology Department, tell them what you're doing and ask them if there is anyway you could get one or two of the "disposable" serological pipettes. Some will ask for a small fee. Hmm... some Pharmacies might have them too if they do inhouse compounding... doesn't hurt to ask and then there are the industrial labs and the hospitals... good luck there.

-wc
 

jjslinger

LifeTime Supporter
Sep 22, 2011
97
0
64
Santa Rosa, CA
#7
soundguy said:
Thanks for checking your cylinders jj.

Wetchem, I wish could spend $335 on a 2 mL pipette.
I would have all kinds of cool chem lab stuff if I could.

In my case, I need to both deliver and contain. First deliver 1 mL from the pipette into 100 mL distilled water.

My purpose for testing is not chlorine ppm in my pool. I'm testing the strength of the chlorine from the pool store.

The last time I purchased chlorine from the pool store, what was supposed to be close to 12% was actually closer to 6%
In this other thread
testing-bleach-strength-t48083.html
JasonLion mentions that the Taylor K-1579 kit is designed to directly measure bleach. I'd probably check into that before spending big bucks on fancy pipettes and whatnot.

Don
 

soundguy

LifeTime Supporter
Jun 22, 2009
62
0
Gulf Coast, AL
#8
jjslinger said:
testing-bleach-strength-t48083.html
JasonLion mentions that the Taylor K-1579 kit is designed to directly measure bleach. I'd probably check into that before spending big bucks on fancy pipettes and whatnot.

Don

Too late. :p

Well, I didn't spend big bucks though.
I already purchased a set of 5 beakers and 100 1 mL pipettes which was way cheaper than the taylor test kit.
My purpose is to get a rough idea of the strength of chlorine and to have fun with some chemistry.
Besides, I do not use liquid chlorine regularly as I have an swg.
 

chem geek

LifeTime Supporter
TFP Expert
Mar 28, 2007
12,082
2
San Rafael, CA USA
#9
soundguy said:
The chlorine cylinder and the other one is off by as much as 29% or around 1.45 mL.

I guess that is not too bad considering how expensive (or lack thereof) they were.
They shouldn't be that far off. 10% maybe; 15% possibly; but not much more than that. Errors come from multiple sources already and though they are not additive, they can have biases that occasionally fall in the same direction. Taylor makes their dropper tips to have 24 drops/ml +/- 1 drop (so that's +/- 4%). They have the concentrations of their titrating reagents be well within 10% accuracy (usually within 5%). The measuring cylinders similarly should be on the order of 10% accuracy or so.

If one had a bunch of these errors be that extreme and line up (30%, 10%, 10%), then we're looking at 50% or more errors and that is obviously completely unacceptable and means the difference between maintaining a proper chlorine level vs. getting algae in pools with sufficient algae nutrients. It would also mean having a saturation index that could be off by -0.6 if both TA and CH were each off by a 50% (i.e. half) too low.

If you believe your volume tests are accurate and demonstrating a nearly 30% error, let Taylor or Dave know (depending on which test kits we're talking about).
 

CaOCl2

LifeTime Supporter
May 23, 2007
326
0
Montreal Canada
#10
and to have fun with some chemistry.
Well the bleach test kit is loads of fun, you can use a pipettor to draw the 0.5 mL of bleach sample precisely, there's 2 endpoints to the test (watch that you don't go past the first one). You add potassium iodide to the diluted sample then drop the pH then titrate to a yellow endpoint with thiosulfate then add starch indicator and continue titrating to the clear endpoint. So lots of reagents, two endpoints, precision and cleanliness are required, lots of fun. Keep the labware clean so you don't contaminate. Test your bleach twice and if you get the same number you're doing something right. So yeah lots of fun with that kit.