"pH" stands for "power of Hydrogen" (it has to do with the concentration of hydrogen ions in a sample- since these concentrations are usually very small they are measured in "powers" i.e. 0.001 mol/L of hydrogen would have a power of 3 and 0.0000001 mol/L of hydrogen would have a power of 7). The H is capitalized because it is the atomic symbol for the element hydrogen and the first letter of atomic symbols are always capitalized.

pH is an old abbreviation for a french description of the acidity of water. The French term is "puissance d'hydrogen", which means "power or strength of Hydrogen". The p is small because it refers to a word. The H must be capitalized because the first letter of abbreviations for chemical elements (in this case Hydrogen) are always capitalized. Examples would be Ni for nickel, O for Oxygen, etc.

In the scientific world, p has come to refer to the inverse logarithm of the concentration of an element, so people now refer to pOH as -log[OH] , when in reality this means "puissance d'hydroxide" (the brackets mean concentration, or amount per volume, OH is hydroxide, the ion which combines with H to form water, HOH or H₂O).

The 'H' in pH refers to the concentration of hydrogen ions in a solution. The chemical symbol for hydrogen is H, and it's always capitalized.

The 'p' is just a mathematical symbol that means'negative logarithm.' Logarithm is usually abbreviated 'log'; logs are an efficient way of dealing with exponents. If y = 10^x, then log y = x. So if the concentration of H = 10^-6 M, then log H = -6. If you multiply this by -1, you get the negative logarithm, so pH = 6.

Using negative logs is just a convenient way of dealing with the large range of concentrations that chemists have to deal with. Unfortunately I'm not sure why 'p' is the abbreviation for negative logarithm.

"pH" stands for "power of hydrogen", and the atomic symbol for hydrogen is H. Because the first (or, in this case, the only) letter of an elemental symbol in chemistry is always capitalized, the H is capitalized. The p is not capitalized because it is not an element, and capitalizing it would indicate that we were talking about phosphorous, for which P is the elemental symbol.