1, thymolphthalein's Ka value is 10.0, that is to say, theoretically, the pH value of the titrated mixed acid solution must be greater than 10 before thymolphthalein can change color; Next question, the Ka value of methyl orange is 3.4. Theoretically, as long as the pH value of the titrated mixed acid solution is greater than 3.4, methyl orange can turn yellow.

2. The first hydrogen ion of sulfuric acid is completely ionized, the pKa2 of the second hydrogen ion is 1.99, the pKa 1 of phosphoric acid is 2. 12, pKa2 is 7.2, and pKa3 is12.36;

Therefore:

If methyl orange is used as indicator, when methyl orange changes color, as long as the pH of mixed acid solution is greater than 3.4, the solution will turn yellow, but it is still acidic at this time. That is to say, sodium hydroxide needs to react all the two hydrogen ions of sulfuric acid, and then the first hydrogen ion of phosphoric acid. Add a few drops of sodium hydroxide, the pH value will be close to 4, and the methyl orange in the titration solution will turn yellow. If the concentration and volume of sodium hydroxide, sulfuric acid and phosphoric acid are equal, the consumption of sodium hydroxide is about three times that of mixed acid solution.

At this time, the pH value of the solution is still far from thymolphthalein's theoretical color change point 10. The second hydrogen ion of phosphoric acid needs to be reacted, followed by the third hydrogen ion of a small part of phosphoric acid. According to the calculation formula of buffer solution, 0.44% of the third hydrogen ion needs to be reacted. At this time, when the pH value is greater than 10, methyl orange turns yellow and thymolphthalein turns blue.

Of course, if methyl orange and thymolphthalein exist in mixed acid solution at the same time, the mixed color of yellow and blue should be displayed at this time, and if the ratio of the two indicators is appropriate, it should be green.

In addition, the concentrations of sulfuric acid and phosphoric acid are too high, so it is best to dilute them before titration.

I hope I can help you.