Answer:
Dr. Toru Ozeki , who is studying pH measurement, kindly answered the question above.
 
 
The relationship between the concentration of sulfuric acid and pH
(A) would be correct. And the first part of (b) would also be correct. In other words, I can say, “pH decreases as the concentration increases. However, the rate of pH decrease becomes smaller as the concentration increases. Thus, pH approaches constant value, but it never increases again.”
 
Since sulfuric acid is a dibasic acid, which has two hydrogen atoms, the relationship between the concentration of sulfuric acid and pH is:
 

 
In these cases, all sulfate ion in solution is SO₄^2- 
 
By the way, the pKa of the following reaction
 
HSO₄^– ↔ H⁺ + SO₄^2–   (1)
 
is 1.9.  So [HSO₄^–] = [SO₄^2–] at pH 2 (pH ≈ 1.9). This means that half of the sulfuric acid releases two hydrogen ions, and the other half of the sulfuric acid releases only one hydrogen ion at pH 2.
 
Since 90 % of sulfuric acid works as a monobasic acid at pH 1, only the following reaction
 
H₂SO₄ ↔ H⁺ + HSO₄^–   (2)
 
occurs, and the reaction (1) is negligible. Thus, the relationship between the concentration of sulfuric acid and pH is:
 

  
At pH 1, almost all of the sulfuric acid becomes HSO₄^–.
 
Although I don’t know the pKa of reaction (2) because there is no reference, sulfuric acid becomes oxidative at more than 2 – 3 M. So it is thought that the reaction (2) would become
  
H₂SO₄  ← H⁺ + HSO₄^–    (2)
  
at more than this concentration.
  
In the case of nitric acid, the reaction also becomes
  
HNO₃ ← H⁺ + NO₃^– 
  
at more than 3 M. At such a high concentration, sulfuric acid and nitric acid are not only acid but also oxidative. This implies that both the H₂SO₄ molecule and the HNO₃ molecule would be formed at this concentration.
  
pH of concentrated sulfuric acid
However, pH of concentrated sulfuric acid hardly has any meaning, even if it can be measured. 
 
The definition of pH is: 
 
pH = - log aH⁺ (aH⁺ = f [H⁺])
 
Here, aH⁺ is activity of hydrogen ion concentration, and f is activity coefficient.
  
f is nearly 1 in diluted water solution. However, the value of f increases as sulfuric acid concentration becomes higher. Also, its property as an oxidizing agent becomes more dominant than as an acid. Therefore, there would be no practical meaning in measuring the pH of highly concentrated sulfuric acid.
 
 
Acknowledgement
Dr Toru Ozeki
He is studying fundamental pH measurement and its application. 
  
  

*This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.