Dilution series methodology help please

Jan 2020
3
0
Uzbekistan
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.
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Last edited:
Apr 2015
63
34
The problem is that I'm very bad at all this preparations and dilution series.
No offence but you are not very good at composing a method statement - I am having trouble understanding exactly what you are doing.

Is the 20mL of dye your absorbance standard or does it serve some other purpose?

Did you not add any dye to the plain water?

In each of the 1x ; 2x etc dilutions you have a full stop after mg/L.
Is this a full stop or a decimal point ie are you adding 0.2 mL of F or 2 mL etc ?

If you are drawing 2, 4, 6, 8 and 10 mL aliquots of F then your best method is to draw of these amount with an accurate measuring burette, not a pipette.

Finally you should be reporting/dissolving 221 mg of flouride in water and making up to 1000mL.
 
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Jan 2020
3
0
Uzbekistan
No offence but you are not very good at composing a method statement - I am having trouble understanding exactly what you are doing.

Is the 20mL of dye your absorbance standard or does it serve some other purpose?

Did you not add any dye to the plain water?

In each of the 1x ; 2x etc dilutions you have a full stop after mg/L.
Is this a full stop or a decimal point ie are you adding 0.2 mL of F or 2 mL etc ?

If you are drawing 2, 4, 6, 8 and 10 mL aliquots of F then your best method is to draw of these amount with an accurate measuring burette, not a pipette.

Finally you should be reporting/dissolving 221 mg of flouride in water and making up to 1000mL.
Sorry for the bad description!
I should add dye as a absorbance standard.
I add to the 0x solution 20 mL of dye and fill with water until 100 mL.
The dot means the full stop.
The problem is that I don't know whether I'm allowed to use accurate burettes, since we only used standard pipettes (not even micropipettes). Is there any other way how I can do it? Maybe I should change the dilution factors?
Thanks for the answer!
 
Last edited:
Apr 2015
63
34
Your only other way is by weight.

This is, of course, the most accurate.

If you are presenting your results as an absorbance v concentration plot ( or log plot) then you can plot the actual concentrations calculated from the weights.
 
Jan 2020
3
0
Uzbekistan
Your only other way is by weight.

This is, of course, the most accurate.

If you are presenting your results as an absorbance v concentration plot ( or log plot) then you can plot the actual concentrations calculated from the weights.
Thank you very much.