Preparetion of solution

[zanio]

Hi all,

How to prepare 1 ml 0,1mg/ml solution if I have 210 g substance and 2 ml ependorph tube?

Thank you!

[Peter Apps]

Hi all,

How to prepare 1 ml 0,1mg/ml solution if I have 210 g substance and 2 ml ependorph tube?

Thank you!

If that is all you have then your task is impossible - you have no liquid to dissolve your substance in !

On a more serious note; what is the real problem ? This sounds like one of those how do you measure the height of a tower if you only have a clock and a 12 -inch ruler problems that are supposed to test people's ingenuity !

I'll go first then (on the assumption that you are actually in a laboratory with some laboratory stuff in it). Using a 5-figure balance, weigh 0.1 mg of the substance into the tube, then weigh in 1 g water (or an appropriate weight of whatever solvent you are using). Cap and shake.

A less elegant, more tedious and certainly less accurate method, but one that needs only what you specify plus the solvent (assuming that the substance is in a container and not in a heap on the bench). Use the tube to add 1050000 portions of 2ml solvent to the 210 g of substance to get a 0.1 mg/ml solution, then half fill the tube with solution (1 ml).

Peter

[zanio]

Thank you Peter.
I understand that the task is not correctly. But this was the conditions.
Ok! I think that the disiere concentration may realise that:

1.We measuring 210 g of substance in balanse.
2. We dilute it in 1000ml h2o. The concentration will be 210 mg/ ml.
3. Then we dilute 1000 x . We take 10 μl of solution and add 10ml h2o. The concentration is 0.2 mg/ ml.
4. Then we take 500μl of solution and add.500μl h2o. Then the concentration will be 0.1 mg/ ml and volume 1 ml.

Thise solution we put in 2 ml ependorph tube and shake.

Is it correct?

Thank you!!!

[Peter Apps]

You can do it one step - weigh 0.1g and dissolve in 1000 ml.

It is completely crazy to use 210 g as a starting weight. Whoever thought up the question needs to think again.

Peter

[zanio]

Thank you Peter for responce!

[dblux_]

Thise solution we put in 2 ml ependorph tube and shake.
...

May you plase explain what are you going to shake it for? Seems we solve your concentration calculations homework.

[Kreall]

Wow... I wish I could have problems like this one...

[Consumer Products Guy]

You can do it one step - weigh 0.1g and dissolve in 1000 ml.

It is completely crazy to use 210 g as a starting weight. Whoever thought up the question needs to think again.

I came across silly stuff like this several times in my career, like the crazy-large sample sizes, or the opposite- crazy low sample sizes, like the procedures were written and "approved" by folks who had no background or experience.

The most recent - from an official corporate test method - specified a 22mg sample size of a not-expensive raw material, then an automatic acid-base titration using 0.1N titrant, used a very small amount of titrant, specified standardizing the titrant each time of use. And the specification range for the material was 99.5 to 100.5%.

Of course the greatest source of error was sample weighing on the analytical balance. Increasing the sample size to 220 mg would have greatly improved the procedure at essentially no cost. But corporate was never wrong, and was not really open to suggestions from real hands-on users. Sad.

[GOM]

Hi

Peter Apps wrote
On a more serious note; what is the real problem ? This sounds like one of those how do you measure the height of a tower if you only have a clock and a 12 -inch ruler problems that are supposed to test people's ingenuity !

I appreciate that this was only meant as a humorous aside to make a point but I quite like these.

My first thought was to note the time, drop the clock from the top and note the time when it it stopped after hitting the ground. Then chew on the rule whilst I worked out the height.

My next thought was to lower the clock from the top to the ground attached to a piece of string - then measure the length of the string using the rule

That thought came from a vague recollection of a reply from a student to the question "How would you use a barometer to measure the height of a building?"

Perhaps one for the water cooler?

Regards

Ralph

[James_Ball]

You can do it one step - weigh 0.1g and dissolve in 1000 ml.

It is completely crazy to use 210 g as a starting weight. Whoever thought up the question needs to think again.

I came across silly stuff like this several times in my career, like the crazy-large sample sizes, or the opposite- crazy low sample sizes, like the procedures were written and "approved" by folks who had no background or experience.

The most recent - from an official corporate test method - specified a 22mg sample size of a not-expensive raw material, then an automatic acid-base titration using 0.1N titrant, used a very small amount of titrant, specified standardizing the titrant each time of use. And the specification range for the material was 99.5 to 100.5%.

Of course the greatest source of error was sample weighing on the analytical balance. Increasing the sample size to 220 mg would have greatly improved the procedure at essentially no cost. But corporate was never wrong, and was not really open to suggestions from real hands-on users. Sad.

We had one method here in the lab that the techs were having problems with reproducible results on their calibration check standards. After looking at the procedure they were trying to make their standard by weighing 0.0003g into 10ml each run. After changing it to weigh 3g into a liter then diluting to the final working solution their results became much more accurate.

I have always done as little weighing as possible and used volumetric flasks for dilutions. I was working with someone once at another company on a project and he was always weighing his volumetric solutions. I asked him why, and he said they had several 5 place balances but almost no glassware. Never made sense since glassware is much cheaper than all those balances. We only have two 5 place balances, the rest are 3 or 4 place units, but we have tons of volumetric flasks.

[Peter Apps]

I take exactly the opposite approach to James and weigh almost everything rather than use volumes. I record actual weights and put them into a spreadsheet to calculate actual mass fractions, mass per volume, concentration or whatever. With an ordinary 4-figure analytical balance you get accuracy and precision of 0.001% on approx 10 g of water, volumetrics gives you 0.2% at best on 10 ml if you carefully fill to the mark.

Peter

[lmh]

What bothers me, then, are the specifications on the chemical bottles. Here, for example, are some bits of information from the specification sheet for glucose 6-phosphate, sodium salt (catalogue G7879) from Sigma:
Water: <= 6%
% sodium (anhydrous) 6.1-10.2%
enzymatic purity >= 98%

so, when you calculate a particular molarity, what is the effective mass of 1 mole of this particular product? Do you assume only 98% purity? Do you assume that 6% of the mass is water, or 2%? How much does it vary from one batch to another? Does the water-content change according to who last took it out of the freezer and opened the top a little early? And what's with the very wide window on the sodium content?

There's no point in worrying about 0.2% errors on glassware or weighing if you have a 6% uncertainty on the amount of water in the raw material (in my view). This is, of course, no criticism of Sigma; it is very difficult to be sure of the actual water content of a hygroscopic compound in an ordinary working lab, and even if Sigma supplied it on Monday with a perfect analysis (something they can no doubt happily provide, for a cost), by Friday it will be different.

[Peter Apps]

Hi lmh

All the uncertainties add up - in the example you give the uncertainty in the composition just adds to the uncertainty in how much of it you have, and to the uncertaiaty in the volume that you dissolve it in. So you are right; if you have 6% uncertainty in the composition then taking a lot of trouble to weigh it out really accurately and determine the volume of the solution really accurately does not markedly improve the uncertainty of the final result.

However - weighing the final solution is not only more accurate and precise than volumetrics, it is also quicker and easier, mainly because to use volumetrics you must, without fail, make the volume up carefully to the line. If you use weight you just add roughly to the line, then weigh it and correct for the actual weight.

When I was having to do a lot of quantitative work I would weigh the solid into a vial, tare, squirt in a shot of solvent from an autopipette, then re-weigh. Result, a precisely known mass fraction (that I could calculate to concentration if I wanted to) in a screw-top sealable container with minimum exposure of the solution to potential contaminants on other glassware, and no glassware to wash.

Peter