An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 269

measure the pH of a sample. The hydrogen ion concentration of a sample is measured
using a pH electrode in combination with a reference electrode or solution. The pH
electrode usually consists of a glass tube that contains a buffer solution of a known pH
and a reference electrode. In operation, pH measurements are made using a single-tube
measuring device, containing both a pH and reference electrode, which is immersed in
the sample to be tested. The glass wall of the electrode at the end of the tube acts as a
complex membrane and allows hydrogen ions from the sample to penetrate into the glass
electrode. Sodium ions are displaced by hydrogen ions and a potential difference
develops at the glass/water interface. The exchange rate between the sodium and
hydrogen ions on the surface of the glass of the electrode is pH-dependent. Therefore the
different potentials between the reference solution and the sample are linked to different
pH values, and it is this difference that is measured and interpreted as the pH of the
sample [4].
In conductometry, the presence of ions in solution allows a current to flow when an
alternating current voltage is applied. The conductance or resistance of the cell is the sum
of the ions across the sides of a reference cell in comparison to the sample cell.
Measurements using conductometry are mostly used in analytical methods in which a
large change in conductance of a sample cell is compared to a reference conductance.
Conductance measurements are used to measure the salt content of aqueous solutions or
extracts.
Any error in sampling that changes the pH or salt content of the sample will adversely
affect these measurements. It will also affect the proposed cleanup and remediation
procedures, and it may interfere with other analytical procedures [17].
10.8. ANALYTICAL METHODS—SOME SPECIFICS
In determining the actual methods (described above) to be used in the sample analysis the
first thing to consider is the nature of the sample; that is, is it solid or liquid and what is
the client's question? This consideration is a way to start the logic tree for the analysis,
which is a relatively simple, logical consideration of the sample and what you want to
learn from it. Historically it has been repeatedly found that a few minutes' time given to
such a basic consideration of common sense before the sample is taken (much less logged
in for analysis) can save time, effort, and frustration later.
Ideally this consideration should be second nature to the PM in discussing the problem
or analysis needed with the client. (Remember that the PM is the first and last contact
with the client and hopefully can get the complete picture.) If a client thinks his well is
contaminated with organics from the next-door neighbor or some governmental or
regulatory agency has requested monitoring of an underground storage tank (UST) the
base method to be used for analysis of the sample is determined by this request. If the
sample is being submitted for initial characterization (i.e., what it is or what is in it), basic
analytical methods for the general component determination are used. The use of GC/MS
methods in both these cases is highly recommended because of the ability of this method
to isolate and determine many different analytes with one analysis.