Sciencemadness Discussion Board

Assay of Oleum by Titration

Magpie - 10-7-2011 at 20:15


By Magpie
7/10/11


I. Introduction
The purpose of this procedure is to provide a simple and safe method for the assay of oleum. It is modeled after a procedure given in Cumming (ref 1).

Oleum is an extremely powerful dehydrating and oxidizing agent and the principal concern with using it is maintaining personal safety. In the liquid phase it has a significant vapor pressure and forms a dense fog of H2SO4 when its vapor contacts the moisture in the atmosphere. This fog is difficult to condense. However it does provide a valuable visual indication of its presence.

Oleum concentration is defined as the % of free SO3. Oleum has a bimodal phase diagram, ie, at room temperature (20C) there are two concentration regions in which oleum is a liquid. The first region is 0-29% oleum, the second is 60-74% oleum. See the linked graph pdf (ref 2). Outside these ranges it is a solid at room temperature. For many applications it is highly preferable to use oleum in the liquid form. As a liquid it can be pipeted into the reaction vessel.

Diluting a given amount of pure SO3 or strong oleum will allow one to makeup an oleum of a lower concentration using high concentration sulfuric acid, such as 98% H2SO4. It is preferable to use sulfuric acid of as high a concentration as possible as the resulting oleum concentration is highly sensitive to the amount of water in the sulfuric acid.

II. Safety Equipment
The following safety equipment is required:

1. A good fume hood. I consider mine just adequate at 450CFM (765 m^3/h) capacity and 1ft/s (0.30m/s) face velocity.
2. Heavy long-sleeved shirt.
3. Heavy nitrile gloves reaching to at least mid-arm.
4. Plastic apron covering the chest and reaching to the ankles.
5. Safety goggles or face shield.

III. Equipment

1. 10 mL beaker
2. 10 mL graduated pipet
3. suction bulb, rubber or plastic, expendable
4. 1-liter Erlenmeyer flask w/rubber stopper
5. 25 or 50 mL volumetric pipet
6. 250mL graduated cylinder, or 600g x 0.1g scale
7. 25mL or 50 mL buret
8. ringstand with buret holder
9. 250 mL Erlenmeyer flask
10. digital scale, 100g x 0.01g

IV. Reagents

1. distilled water
2. ~1.0N NaOH
3. methyl orange or phenolphthalein indicator

V. Procedure

a. oleum transfer & dilution
1. Place 250mL or 249.5g distilled water in the 1-liter E. flask.
2. Place the 10mL beaker on the digital scale and press “tare.”
3. Using the 10mL pipet with suction bulb transfer 1-2mL of oleum (hood fan ON!) to the 10mL beaker, noting the weight, then quickly dropping the beaker into the 1-liter flask and stoppering. Record w = net weight of the oleum added.
4. Carefully shake the liquid in the stoppered flask until all the fog has been absorbed in the distilled water.

b. titration of the diluted oleum
1. Set up the buret with about 25mL of the 1.0N NaOH.
2. Pipet 50mL of the diluted oleum into the 250mL E. flask and add 3 drops of methyl orange or phenolphthalein indicator.
3. Titrate the diluted oleum to a yellow (methyl orange) or pink (phenolphthalein) endpoint.
4. Record n = the number of mLs of 1.0N NaOH required.

c. calculation of % oleum
1. Calculate W = w/5.
2. Calculate the % oleum with the following formula from Cumming:

% oleum = (4.9n -100W)/0.225W

VI. Discussion
This procedure differs from that of Cumming only in that a 10mL beaker is used instead of an ampoule. I felt that I didn’t have the skills necessary to ampoule oleum and that trying it would be too risky. I also cut the amount of oleum used by about 50% to conserve this hard-won reagent.

I feel that the only deficiency of this procedure is the inevitable escape of fog when the oleum beaker lands in the water in the 1-liter flask. Use of an ampoule would preclude this as the ampoule is only broken, by shaking, after the 1-liter flask is stoppered. So the % oleum determined by my procedure is going to be a little low; how low I don’t know.

I have used this procedure several times now, on both low strength and high strength oleum. My low strength oleum tested 24.5%. I ran a duplicate on the high strength oleum with the results being 51.1% and 47.6%.

The only improvement I have made since those assays is to switch from a 500mL E. flask to a 1-liter E. flask. This is an effort to minimize the escapement of fog formed at the moment the oleum is dropped in the flask. A fresh batch I analysed today tested 59.7%.

VII. References
The standards organization ISO issues (and sells) a procedure for the assay of oleum. So far I have not obtained that procedure (ref 3).

1. Systematic Organic Chemistry, 4th ed (1950), by Cumming, Hopper and Wheeler,
p. 320.
2. http://www.generalchemical.com/assets/pdf/Oleum_Freezing_Poi...
3. ISO 910:1977, Sulphuric acid and oleum for industrial use -- Determination of total acidity, and calculation of free sulphur trioxide content of oleum -- Titrimetric method


[Edited on 13-7-2011 by Magpie]

Magpie - 10-7-2011 at 20:26

Comments, corrections, and questions are welcomed.

bbartlog - 11-7-2011 at 08:22

Any estimate of the precision of the procedure? Just looking at the volume of oleum used (1-2ml) and the use of a scale with +/-0.01g precision it looks like you can't do better than about +/- 0.5%, even if the rest of your execution is flawless. Obviously this is part of the tradeoff between using up valuable oleum and getting a precise figure. Actually on further consideration it looks like the use of 1-2ml of oleum (only one fifth of which is actually titrated) is an attempt to increase the precision, that is, if you want more precision you can just weigh a somewhat larger quantity and adjust the procedure. Or alternatively you could just weigh out 0.50g drop by drop and get a really rough reading...
Also, the dropping of ampoules or beakers into a larger flask seems weird to me (what if the beaker breaks?) and in any case my smallest beaker (50ml) would not fit through the neck of my largest flask. So I wonder: if you have a scale that goes to 300g (+/-0.01g), couldn't you pipette the oleum directly into water contained in a smaller flask (say 100g of water in a 250ml flask, which together should weigh no more than 250g).

unionised - 11-7-2011 at 11:58

" couldn't you pipette the oleum directly into water "
Sooner you than me.

Magpie - 11-7-2011 at 12:44

Quote: Originally posted by bbartlog  
Any estimate of the precision of the procedure? Just looking at the volume of oleum used (1-2ml) and the use of a scale with +/-0.01g precision it looks like you can't do better than about +/- 0.5%, even if the rest of your execution is flawless. Obviously this is part of the tradeoff between using up valuable oleum and getting a precise figure. Actually on further consideration it looks like the use of 1-2ml of oleum (only one fifth of which is actually titrated) is an attempt to increase the precision, that is, if you want more precision you can just weigh a somewhat larger quantity and adjust the procedure. Or alternatively you could just weigh out 0.50g drop by drop and get a really rough reading...


I would be happy with +/-0.5% accuracy. I have no way to know how accurate my results are. The only measure I have is that of repeatability as shown by the duplicate analysis reported above. I plan to do more of that in the future the next time I need an analysis. Oleum is something I don't mess around with casualy as it takes 5hours to make and is risky to handle and store.

Weighing out "drop-by-drop" would be counterproductive IMO. When handling oleum you want to minimize the surface area and time of exposure to the atmosphere as it is evaporating rapidly all the while forming a dense fog. That is why I tried to emphasize in the procedure that the weighing/transfer be conducted as one swift step.

Quote: Originally posted by bbartlog  

Also, the dropping of ampoules or beakers into a larger flask seems weird to me (what if the beaker breaks?)

It seemed weird to me also when I first read it in Cumming. But I have not been able to think of a better way. The beaker landing is cushioned by the 250mL of water in the flask and is a reasonably soft landing. I've had no breakage, so far.
Quote: Originally posted by bbartlog  

... couldn't you pipette the oleum directly into water contained in a smaller flask (say 100g of water in a 250ml flask, which together should weigh no more than 250g).


I tried that and show this on YouTube, ie, http://www.youtube.com/watch?v=XJDohfwbffU
This didn't work out very well due to the copious fog escape, and possibly due to not knowing the volume of transfer accurately. Oleum, although having a high density of ~1.9, has a higher viscosity than water, ie, it's somewhat "oily."

I have also tried just evaporating the SO3 off a weighed amount of oelum in the 10mL beaker by setting it on a hotplate. When evaporation of the SO3 is complete the beaker is reweighed. Thereby the weight of the remaining pure H2SO4 is determined. For some reason this did not work out well either.

chornedsnorkack - 12-12-2014 at 05:21

Quote: Originally posted by Magpie  
Comments, corrections, and questions are welcomed.


The graph ends at 64 %. How is the freezing point of oleums at higher concentrations?

Magpie - 12-12-2014 at 10:56

Quote: Originally posted by chornedsnorkack  


The graph ends at 64 %. How is the freezing point of oleums at higher concentrations?


I don't have that data; sorry. If you find it please post it in this thread.

subsecret - 13-12-2014 at 08:26

Perhaps I'm missing something basic here, but why did you use such an exact amount of water to dilute?

Magpie - 13-12-2014 at 11:00

Quote: Originally posted by Awesomeness  
Perhaps I'm missing something basic here, but why did you use such an exact amount of water to dilute?


Thanks for your question. I have gone through the calculations to derive the formula and I do see a slight error. Instead of specifying the addition of 250 mL of distilled water, it should be:

Add 245 mLs of distilled water to the flask. Then, after the oleum is added bring the volume to 250 mLs.

Let me know if this is not clear and I will provide a complete derivation.

subsecret - 13-12-2014 at 13:37

Ah! Thank you, I understand now. I missed that you had added only 50 mL of the solution to the Erlenmeyer flask, and was wondering why you chose to dilute the solution with such an exact amount of water. Thanks for the explanation.

chornedsnorkack - 13-12-2014 at 13:46

1978´s Kirk-Othmer encyclopedy complains about lack of references for strong oleum crystallization - but does present data from 1950 Russian book and discussion from an 1930 English book.

shadow - 13-8-2016 at 17:35

I've read about using a dely tube or a snake tube to lift the oleum out of your vessel prior to weighing for the assay of oleum.
The tube keeps the oleum from the atmosphere.
The tube and oleum are weighed then the exact amount of liquid is passed into the water where it is titrated.
Here is a link to the procedure:

https://www.cornerstonechemco.com/ckfinder/userfiles/files/s...

They also show specific gravities of H2SO4 and oleum, so I thought the strength could easily be measured with a hydrometer, but somewhere I read no go over 92% sulphuric acid. Does anyone have any information on the accuracy of this method?
Thanks,
shad