pH Titration of Phosphoric Acid

Introduction
Knowing pH value of different systems can give us a lot of information about the state of the systems and processes going inside it. The purpose of the exercise is to familiarize us with the theory behind pH value and one of the ways of measuring it, where pH value is defined as negative logarithm of hydrogen activity. Determining the pH is achieved by measuring the potential of the solution with unknown [H+] and comparing it to some reference potential (for which [H+] is known and/or constant). Additionally, the behavior of various acids can be eluded simply by adding a base through pH measurement which is exemplified here for the case of H3PO4.

Instrumentation
Measurement of the pH is accomplished by the use of pH meter which determines the potential difference between the reference half-cell and the sensing half-cell. Both potentials are determined by the Nernst equation (Eq. 1) which is at the heart of all electrochemical systems.

E=E^0+RT/nF ln ([Ox])/([Red]) (1)

Reference half-cell is made of Ag wire covered by AgCl immersed in a solution of known [H+] and potassium chloride solution of fixed concentration. Writing the Nernst potential for this electrode (Eq. 2), in the case the activity of Cl– ions is kept constant, gives us confirmation that the potential will be constant which is desirable.
E_(Ag/AgCl)=E_(Ag/AgCl)^0+RT/nF lna_(〖Cl〗^- ) (2)

The end of the measuring half-cell is made of a conductive glass membrane in the shape of a bulb to increase contact surface area when measuring the unknown solution insuring more precise measurement. The sensing half-cell also contains and Ag/AgCl electrode immersed in a buffered electrolyte solution. When the glass membrane contacts the solution of unknown [H+] Li+ are exchanged with the H+ ions from the solution and a concentration/potential gradient is formed through the glass which in turn changes the potential of the sensing electrode also given by Nernst formula (Eq. 3).

Lab report guide: pH Titration of Phosphoric Acid The full lab rep...