pH: pH pH is a unit of measure which describes the degree of acidity or alkalinity (basic) of a solution. It is measured on a scale of 0 to 14. The formal definition of pH is the negative logarithm of the hydrogen ion activity. pH = -log[H+] pH value: pH value The pH value of a substance is directly related to the ratio of the hydrogen ion and hydroxyl ion concentrations. If the H+ concentration is higher than OH- the material is acidic. If the OH- concentration is higher than H+ the material is basic. 7 is neutral, < is acidic, >7 is basic The pH scale: The pH scale The pH scale corresponds to the concentration of hydrogen ions. If you take the exponent of the H3O+ concentrations and remove the negative sign you have the pH of the solution. For example pure water H+ ion concentration is 1 x 10^-7 M, therefore the pH would then be 7. pH: pH The addition of acid to water increases the concentration of hydrogen ions and reduces the concentration of hydroxyl ions The addition of a base would increase the concentration of hydroxyl ions and decrease the concentration of hydrogen ions Acids and Bases: Acids and Bases An acid can be defined as a proton donor, a chemical that increases the concentration of hydrogen ions in solution. A base can be defined as a proton acceptor, a chemical that reduces the concentration of hydrogen ions in solution. pH Measurement: pH Measurement A pH measurement system consists of three parts: a pH measuring electrode, a reference electrode, and a high input meter. The pH measuring electrode is a hydrogen ion sensitive glass bulb. The reference electrode output does not vary with the activity of the hydrogen ion. pH Meter: pH Meter A sample is placed in a cup and the glass probe at the end of the retractable arm is placed in it. The probe is connected to the main box. There are two electrodes inside the probe that measure voltage. One is contained in liquid with fixed pH. The other measures the acidity of the sample through the amount of H+ ions. pH Meter: pH Meter A voltmeter in the probe measures the difference between the voltages of the two electrodes. The meter then translates the voltage difference into pH and displays it on the screen. Before taking a pH measurement the meter must be calibrated using a solution of known pH. Temperature and Buffers: Temperature and Buffers Temperature compensation is contained within the instrument because pH electrodes are temperature sensitive. Temperature compensation only corrects for the change in the output of the electrode, not for the change in the actual solution. Buffers are solutions that have constant pH values and the ability to resist changes in pH. They are used to calibrate the pH meter.