I am planning to make solutions containing different ratios of sulfuric acid and phosphoric acid. The purpose is to etch sapphire wafers. The etching is to be carried out at temperatures of about 180° to 200°C. What are some precautions that I need to exercise ?Like:
- Which acid should I add to which to make the solutions ? Say, I am making sulfuric acid to phosphoric acid of 3:1. Does the procedure change if the ratio is reversed ?
- Is it safe to carry out the etching in regular pyrex glassware ?
- Is if safe to use a hot plate to heat the acid solution ?
- Is there a low maximum heating rate of the solution under which I need operate under ?
Perhaps of some interest to your query ― About heating (unmixed) conc. phosphoric acid, you may check: https://www.researchgate.net/post/Is_reflux_of_O-phosphoric_acid_85_at_180C_for_8_days_in_a_glass_flask_safe
Make sure you have stirring and temperature monitoring set up in the mixing vessel.
Add the heavier acid to the lighter one (sulfuric to phosphoric), slowly with stirring. Mixing will be exothermic but not as bad as sulfuric + water, and boiling is very unlikely. However, since both components are somewhat viscous, and depending on the volumes you are trying to mix, it may take a while to get a homogeneous mixture.
Pyrex is OK for rough work, but it is slightly soluble in hot phosphoric acid (160-180C). So if you don't want contamination of the solution by Si, B, possibly Al, Na, Ca, a quartz glass vessel is preferable.
One other thing to add to my previous answer: I would not recommend heating a liquid to 180C in a Pyrex vessel on a hot plate, unless you can monitor the surface temperature and the liquid temperature at the same time and make sure the difference is never much more than 100C. Quartz glass is safer in this regard. You can also use a heating mantle (usually with a round-bottom flask).
While considering the titration of a mixed phosphoric acid / sulfuric acid aq. solution with sodium hydroxide aq. solution:
I. The acid dissociation constants for phosphoric acid correspond to pKa's of 2.15, 7.20, and 12.15. Possibly, the first equivalent point of phosphoric acid can be detected by about pH = (2.15 + 7.20)/2 = 4.68, depending on acids concentration. That can be taken as corresponding to the titration of the first proton of phosphoric acid and to the titration of both protons of H2SO4 ― 1 mole of H2SO4 is neutralized with 2 mol of NaOH, while 1 mol of H3PO4 is neutralized with 1 mole of NaOH.
II. Titration with NaOH to about the second equivalence point of phosphoric acid, by pH = (7.20 + 12.15)/2 = 9.68, allows to determine the total acid content (H2SO4 + H3PO4), as it would correspond to the titration of two protons from either acid ― 1 mol of each acid is neutralized with 2 mol of NaOH. From the first equivalent point of phosphoric acid to the second, just he second proton of H3PO4 is titrated, so that 1 mol of NaOH spent between both equivalent points corresponds to 1 mol H3PO4. So, the H3PO4 content can in principle be determined from the titrant volume spent between both the first and second equivalent points of H3PO4, while the titrant volume spent up to the second equivalent point gives the total content for both H3PO4 and H2SO4.
III. Interference from too much concentrate H2SO4 may possibly render the detection of the first and second equivalence points of H3PO4 difficult, in some cases. Pre-dilution of the solution to be titrated and/or addition of a known amount of H3PO4 may possibly help to discriminate both equivalent points.
IV. By the first equivalent point of phosphoric acid (pH ≈ 4.68), it can be accepted that sulfuric acid, which is diprotic, is already fully neutralized.
To add to Carlos Araújo Queiroz 's point #3:
You absolutely have to dilute this mixture of concentrated acids before titration. I would suggest diluting by at least a factor of 20, even 50. Otherwise the neutralization reaction is much too exothermic. ( How exothermic? Droplets of angry acid will jump at you as you add base.) Recall that reacting 1 N strong acid with 1 N strong base raises the temperature by about 13C. Reacting anything above 5 N brings the solution uncomfortably close to boiling.
For this particular type of mixture, which is viscous and clings to glass, I would strongly suggest to do the dilution by weight rather than by volume - e.g., weigh 98.00 g deionized water into your titration vessel, then on the balance add dropwise an accurately weighed approximately 2 g of the acid mixture. Then you can titrate comfortably with 1-2N NaOH.
As a general rule it's not desirable to titrate solutions more concentrated than about 1M. Exothermicity is not the only problem. Your medium will also be quite different from the medium for which acidity and complexation constants etc. are calculated. Unexpected precipitations may also occur. You will also waste a lot of expensive high-accuracy titrating reagent (very much more expensive than deionized water!), and may suffer the indignity of having to replace the titration vessel mid-titration with a larger one (I've been there).