Higher temperatures lower the pH levels of orange juice, while lower temperatures have the opposite effect, elevating the pH levels. In simpler terms, pH is a standard scale used to measure the acidity in an aqueous solution; values lower than 7 are regarded as acidic, while values higher than 7 are considered to have high alkalinity. For base comparisons, pure water has a pH level of 7.
The meaning behind the letters "p" and "H" is disputed. The Carlsberg Foundation says that pH stands for "power of hydrogen." The concept of pH was first introduced in 1909 by Danish chemist Soren Peder Lauritz Sorenzen at the Carlsberg laboratory.
The acidity in orange juice comes from citric acid, which is its major component. Orange juice has a pH level of around 3.5. Citric acid is a weak acid, which means that it does not completely dissociate in water. However, if the orange juice solution is warmer, citric acid will dissociate slightly better, which in turn slightly lowers the pH and makes the orange juice slightly more acidic. The opposite effect on the pH level occurs if the orange juice solution is colder.
It must be noted that temperature does not have this same effect on all acids. Temperature can only have the same effect on acids that have a positive H value. The exact opposite behavior will occur if the acid has a negative H value.