I'm doing an investigation to see the effect of temperature on the solubility product of strontium hydroxide.

I made a supersaturated solution left in the oven overnight at 85°C for equilibrium to be reached. Now, I want to titrate it against $\ce{HCl}$, but when I try to pipette a 25 mL sample of the (very hot) solution, it quickly crystallises in the pipette due to the large difference in solubility at 85°C compared to room temperature.

(NOTE: doing at lower temperature is not an option.... )

One idea I can think of is to keep the glassware in the oven overnight with the solution so that it's more or less at the same temperature. (Then, I'd have to handle with a cloth, of course). However, I do not have access to an oven that is big enough to fit a pipette.

Hence, the only option I have is to use a cylinder rather than a pipette, but that would increase my uncertainties by A LOT.

Can anyone think of anything else that I could do? Would really appreciate any help.

  • $\begingroup$ You can get glass measuring cylinders for that volume range (~25 mL), which should be smaller than your pipette in length. $\endgroup$ – Eashaan Godbole Mar 19 at 10:56
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    $\begingroup$ @Mohamad Anyway, if the problem is in the measurement, you can move measurement to happen after titration, if you can tolerate/eliminate uncertainties due to evaporation. Yes, you won't be able to titrate exactly 25 ml of your solution, but that's acceptable for analytical titration as long as you can deduce the value from other means. Another option is to prepare solution in 25 ml batches to begin with. $\endgroup$ – permeakra Mar 19 at 11:55
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    $\begingroup$ You are concerned about about this one uncertainty (volume error when removing 25 mL of supernatant). You should also worry about volume changes with temperature, and systematic errors of using volumetrics at temperatures they are not designed for. For these reasons, it is common to switch from concentration to molality for studies where temperature is varied by a lot. The density of water changes by about 3% from your equilibration temperature (d = 0.96859 g/cm3) to room temperature. $\endgroup$ – Karsten Theis Mar 19 at 14:28
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    $\begingroup$ Oh but the volume is technically changing because the water is expanding... ok got it. $\endgroup$ – Mohamad Mar 19 at 15:01
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    $\begingroup$ Yes, I am saying there will be some systematic errors. It would be interesting to hear how you plan to use your data (is this one of many temperatures, for example). $\endgroup$ – Karsten Theis Mar 19 at 15:11

Just use your room temperature volumetric pipette, and wash out the precipitate with water. The exact density at 85°C is going to be tricky anyway. The expansion coefficient of borosilicate you can find, and you need to measure or calculate (heat capacity of borosilicate, and your solution?!? IR thermometer?) the temperature of the solution.

You might think about using a second volumitric pipette with dest water, and wash everything into a beaker with known empty weight.

Have fun with calculating the uncertainties! ;-)

Afterthought: Fill the RT volumetric pipette from the hot solution. Wait until cooling (the filling should get lower, because water has a larger expansion coeff. than glass) and precipitation is complete. Now completely fill the pipette, the additional amount of solution will be rather small, and you don't need to wait again for quillibration.

Now weigh and titrate the solution&precipitate (after washing out with a known volume of water). You know the density and molarity of cold strontium hydroxide solution, you know the density of solid hydroxide. I think that is sufficient information to calculate the concentration at 85°C.

  • $\begingroup$ I think you misunderstood something here. The problem is not with having to wash the precipitate away.. It's that just when the solution gets into the bottom of the pipette, it crystallises clogging it up so no more can even go in. Your advice would work if I could get the solution up into the pipette in the first place... $\endgroup$ – Mohamad Mar 19 at 23:06
  • $\begingroup$ OK, that's a problem. Didn't think it was that bad. ;-) Heat the lower end of the pipette in a hot water bath? $\endgroup$ – Karl Mar 19 at 23:34

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