Laboratory Exercise 2: Introduction to pH and Buffers Background and Introductio

Question

Laboratory Exercise 2: Introduction to pH and Buffers Background and Introduction to pH. pH electrodes and buffer solutions (pg. 1-5): Measurement of pH Although the pH of a solution may be estimated using indicator dyes, the most accurate method of determining pH is electrometrically using a pH meter. This instrument is composed of an electrode connected to a voltmeter, which records very small differences in electrical potential due to differences in hydrogen ion concentration (Figure 1). Figure 1: Ohaus starter 2100 bench pH meter and a pH electrode. This is the pH meter that will be used for making measurements in this lab The tip of the electrode is immersed in the sample and the electrical potential is sensed in volts, but is reported in pH units. While the electrical resistance of distilled water is high, as the concentration of dissolved ions (the ionic strength of the solution) increases, resistance decreases. The pH meter measures changes in electrical potential difference (AE) that occur in response to changes in resistance at constant current (). The relationship among these term is expressed in Ohm's Law: : IR At constant current, as pH decreases, the concentration of hydrogen ions in solution increases and resistance decreases. The pH meter senses this as an increase in potential difference, measured in volts. The pH meter is standardized using commercial standard solutions that were prepared from buffers with pH values that are accurate to 0.01 pH units. Typically, a three- point standardization is performed, using three buffers whose pH values cover the pH range of

Explanation / Answer

a). the goals are as follows..... 1.Understnd weak acid (base) equilibria and conjugate acid-base pairs

2.Investigate the acidity/basicity of solution containig aquaeous ions.

3.Performe calculations involving ionic equilibria

4.understand buffer solutions and how they work

b) Henderson-Hasselblach equation describes the measuring of pH as a measure of acidity in biological and chemical systems

this equation is useful in estimating the pH of buffer solution

it is widely used for calculating the isoelectric point of proteins.

c) for acid base titrations the formulae is M(acid) X V(acid) = M(base) X V(base)

where M(acid)= Molarity of acid

V(acid)= volume of acid

M(base)= molarity of base

V(base)= volume of base

Handersan- hasselbalch describes the relationship between pH and pKa

pH= pKa +log([conjugate base]/[weak acid])

pH is measure of hydrogen ions in aquaeous ssolution

pKa tells you what the pH needs to be in a order for a chemical species to donate or accept a proton

d)Digital pH meter describes the pH of the solution

stirring plate or also known as magnetic stirrer that emplyoes a rotating magnetic field to cause a stir barnimmersed in a liquid to spin very quickly

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