July 08, 2020, 09:37:52 AM
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Topic: Very bad at analytical chemistry, have an final lab tomorrow, help please :(  (Read 276 times)

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Offline Alberto12

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Good day for everyone, bad day for me, I'm doing chemical analysis course and have a lab tomorrow. The problem is that I'm very bad at all this preparations and dilution series. I did some experiments, but most of them were below average, due to the lack of experience and knowledge. Can someone help me please?
I was told to make fluoride solutions with concentrations between 0 and 1.6 mg/L.
Stock solution is 221 mg of fluoride dissolved in 1000 ml of water. So concentration is 221 mg/L.
In order to get a suitable concentration, I dilute it 165.75 mg/L and then to 16.6 mg/L.
Then there are some problems. I have calculated the concentration and how many mL of final solution F (16.6 mg/L) I should add to obtain 5 dilutions.
0x- 0 mg/L.
1x-0.332 mg/L.2 ml of F, add water, 20 ml of dye and fill with water until 100 ml
2x- 0.664 mg/L. 4 ml of F, add water, 20 ml of dye and fill with water until 100 ml
3x-0.996 mg/L. 6 ml of F, add water, 20 ml of dye and fill with water until 100 ml
4x-1.328 mg/L. 8 ml of F, add water, 20 ml of dye and fill with water until 100 ml
5x-1.66 mg/L. 10 ml of F, add water, 20 ml of dye and fill with water until 100 ml
By the end measure absorbances.
Now I'm bothered with the question, how I will pipette such a small amount of stock solution, since I remember that Pasteur pipette+ measuring cylinder is not good idea. What should I do or how to improve the methodology?
Thanks in advance, I know that this is easy for cool guys like you and I'm sure that you can give some tips.
« Last Edit: January 30, 2020, 09:14:25 AM by Alberto12 »

Offline Corribus

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You would usually use a calibrate micropipette appropriate to the solvent. Or, if you want to do it gravimetrically, use an analytical balance and the known density of the solvent.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Alberto12

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Are there any other options, since I think it is not allowed to use the micropipettes. I'm beginner, so not familiar gravimetrical method unfortunately. Or I should change the dilution factor and use various volumes of volumetric flasks?

Offline Corribus

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First you need the density of your solvent. You can assume ideal density of 1 g/mL for pure water if you want, but this will introduce some error because your water isn't pure, and the density does change slightly depending on, e.g., temperature in your lab. Best way is to use a volumetric flask or something with a volume indicator you can trust, and just weigh, say, 100 mL of your solvent. Calculate the density. From there you can do all your dilutions by weighing out volumes, using the fact that density = mass * volume. If you have good quality volumetric glassware, you can just weigh out the substance you're diluting, and then fill to the mark with solvent.

Make sure your balance is calibrated and has sufficient precision to weigh the volumes you are trying to weigh.

Since your concentrations are pretty small compared to your stock solution, you can also dilute your stock solution 1:10.

Be aware there are lots of ways to prepare calibration standards, and every one has its own sources and patterns of error. Every dilution introduces a new source of error. Be aware of what those sources of error are. This way if your results don't make sense you can better figure out where something went wrong.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Alberto12

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Thank you very much, I will try and give a try!

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