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Dear stack exchange,

For my next lab session we have been tasked to come up with two different methods for determining the concentration of an unknown sample of phosphoric acid.

Design at least two different experimental methods to determine the concentration of the phosphoric acid solution using the materials and equipment provided. (NB: two different volumetric titrations are not two different methods.)

For reference, the "materials and equipment provided" are (along with the standard burettes, pipettes, etc.):

A sample of the phosphoric acid solution
A selection of pH indicators:
- Bromocresol green (range 3.8-5.4)
- Phenolphthalein (range 8.0-10.0)
- Thymolphthalein (range 9.3-10.5)
A standardised NaOH (approximately 0.1 M) solution
A pH meter

One obvious method is an acid-base titration, which is fine, but I have no idea what to do as a second method since we cannot do two different volumetric titrations - perhaps there is some finnicky definition of "volumetric" which means there is another type of titration we can do? It seems as though it has to be something along those lines based on the provided equipment.

Many thanks!

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Specific gravity would be a useful technique; perhaps not as accurate as titration. Weigh 50 mL of the unknown and check vs tables in the CRC Handbook.

Do a reaction: add weighed amounts of NaHCO3 (slowly, carefully) until the foaming stops. I think this will just neutralize 2 of the 3 acidic protons on H3PO4. Knowing how much NaHCO3 reacts, you can determine how much H3PO4 there was.

Evaporation to "dryness": in a porcelain dish, weigh a decent amount of the unknown and heat at about 110C for an hour or so. The liquid at the end will be highly concentrated, but perhaps not 100% H3PO4. Check with a known concentration of H3PO4.

If you can get the apparatus, you could check conductivity. You could even do it yourself (roughly) with a multimeter and stainless steel wire (of course, platinum wire is preferable).

You could try to determine viscosity; check vs known standards. Very rough for analysis.

You could offer the lab director $10 for the correct answer.

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  • $\begingroup$ I think the $10 might be the way forward ;) thank you very much for the extent of the reply! Should be able to find the equipment for at least one of those :D $\endgroup$ – Sensei_Stig Jan 30 '18 at 19:28
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You can add a single dose of NaOH to a sample of the acid and measure the pH with the pH meter. You can easily calculate the charge on a mole of phosphate at a given pH and as the moles of NaOH you added is equal to the moles of phosphate multiplied by the charge change per mole you can get the number of moles of phosphate.

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  • $\begingroup$ Ooh thank you! Hadn't even thought of that, would be an interesting write-up $\endgroup$ – Sensei_Stig Jan 30 '18 at 19:29

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