3) Processing the Sample
Under certain circumstances, no sample processing is required prior to the measurement step. For
example, once a water sample is withdrawn from a stream, a lake, or an ocean, its pH can be
measured directly.
However, under most circumstances, we must process the sample in any of a variety of different
ways. The first step is often the preparation of a laboratory sample.
Preparing Laboratory Samples
A solid laboratory sample is ground to decrease particle size, mixed to ensure homogeneity, and
stored for various lengths of time before analysis begins.
Absorption or desorption of water may occur during each step, depending on the humidity of the
environment. Because any loss or gain of water changes the chemical composition of solids, it is
a good idea to dry samples just before starting an analysis. Alternatively, the moisture content of
the sample can be determined at the time of the analysis in a separate analytical procedure.
Liquid samples present a slightly different but related set of problems during the preparation step.
If such samples are allowed to stand in open containers, the solvent may evaporate and change the
concentration of the analyte.
If the analyte is a gas dissolved in a liquid, the sample container must be kept inside a second
sealed container, perhaps during the entire analytical procedure, to prevent contamination by
atmospheric gases.
Extraordinary measures, including sample manipulation and measurement in an inert
atmosphere, may be required to preserve the integrity of the sample.
Defining Replicate Samples
Most chemical analyses are performed on replicate samples whose masses or volumes have been
determined by careful measurements with an analytical balance or with a precise volumetric
device. Replication improves the quality of the results and provides a measure of reliability.
Quantitative measurements on replicates are usually averaged and various statistical tests are
performed on the results to establish reliability.
Preparing Solutions: Physical and Chemical Changes
Most analyses are performed on solutions of the sample made with a suitable solvent. Ideally, the
solvent should dissolve the entire sample, including the analyte rapidly and completely. The
conditions of dissolution should be sufficiently mild so that loss of the analyte cannot occur.
Unfortunately, many materials that must be analyzed are insoluble in common solvents.
Examples include silicate minerals, high-molecular-weight polymers and specimens of animal
tissue. Under this circumstance, conversion of the analyte in such materials into a soluble form is
often the most difficult and time-consuming task in the analytical process.
The sample may require heating with aqueous solutions of strong acids, strong bases, oxidizing
agents and reducing agents, or some combination of such reagents. It may be necessary to ignite
the sample in air or oxygen or to perform a high-temperature fusion of the sample in the presence
of various fluxes.
1
, Once the analyte is made soluble, we then ask whether the sample has a property that is
proportional to analyte concentration and that we can measure. If it does not, other chemical steps
may be necessary to convert the analyte to a form suitable for the measurement step. For
example, in the determination of manganese in steel, manganese must be oxidized to MnO4
before the absorbance of the colored solution is measured.
At this point in the analysis, it may be possible to proceed directly to the measurement step, but
more often than not, we must eliminate interferences in the sample before making
measurements.
4) Eliminating Interferences
Once we have the sample in solution and have converted the analyte to an appropriate form for
measurement, the next step is to eliminate substances from the sample that may interfere with
measurement.
Few chemical or physical properties of importance in chemical analysis are unique to a single
chemical species. Instead, the reactions used and the properties measured are characteristic of a
group of elements of compounds.
Species other than the analyte that affect the final measurement are called interferences, or
interferents. A scheme must be devised to isolate the analytes from interferences before the final
measurement is made.
No hard and fast rules can be given for eliminating interferences; indeed, resolution of this
problem can be the most demanding aspect of an analysis.
NOTE: SEPARATION TECHNIQUES ARE COVERED IN ANALYTICAL II (SCH 220)
5) Calibrating and Measuring Concentration
All analytical results depend on a final measurement X of a physical or chemical property of the
analyte. This property must vary in a known and reproducible way with the concentration CA of
the analyte.
Ideally, the measurement of the property is directly proportional to the concentration. That is.
CA = kX
Where k is the proportionality constant
With two exceptions, analytical methods require the empirical determination of k with chemical
standards for which CA is known. The process of determining k is thus an important step in most
analyses: this step is called a calibration.
Calibration determines the relationship between the analytical response and the analyte
concentration. Usually this is accomplished by the use of chemical standards.
Almost all analytical methods require some type of calibration with chemical standards.
However, gravimetric methods and some coulometric methods are among the few absolute
methods that do not rely on calibration with chemical standards. Several types of calibration
procedures are described below:
2
Under certain circumstances, no sample processing is required prior to the measurement step. For
example, once a water sample is withdrawn from a stream, a lake, or an ocean, its pH can be
measured directly.
However, under most circumstances, we must process the sample in any of a variety of different
ways. The first step is often the preparation of a laboratory sample.
Preparing Laboratory Samples
A solid laboratory sample is ground to decrease particle size, mixed to ensure homogeneity, and
stored for various lengths of time before analysis begins.
Absorption or desorption of water may occur during each step, depending on the humidity of the
environment. Because any loss or gain of water changes the chemical composition of solids, it is
a good idea to dry samples just before starting an analysis. Alternatively, the moisture content of
the sample can be determined at the time of the analysis in a separate analytical procedure.
Liquid samples present a slightly different but related set of problems during the preparation step.
If such samples are allowed to stand in open containers, the solvent may evaporate and change the
concentration of the analyte.
If the analyte is a gas dissolved in a liquid, the sample container must be kept inside a second
sealed container, perhaps during the entire analytical procedure, to prevent contamination by
atmospheric gases.
Extraordinary measures, including sample manipulation and measurement in an inert
atmosphere, may be required to preserve the integrity of the sample.
Defining Replicate Samples
Most chemical analyses are performed on replicate samples whose masses or volumes have been
determined by careful measurements with an analytical balance or with a precise volumetric
device. Replication improves the quality of the results and provides a measure of reliability.
Quantitative measurements on replicates are usually averaged and various statistical tests are
performed on the results to establish reliability.
Preparing Solutions: Physical and Chemical Changes
Most analyses are performed on solutions of the sample made with a suitable solvent. Ideally, the
solvent should dissolve the entire sample, including the analyte rapidly and completely. The
conditions of dissolution should be sufficiently mild so that loss of the analyte cannot occur.
Unfortunately, many materials that must be analyzed are insoluble in common solvents.
Examples include silicate minerals, high-molecular-weight polymers and specimens of animal
tissue. Under this circumstance, conversion of the analyte in such materials into a soluble form is
often the most difficult and time-consuming task in the analytical process.
The sample may require heating with aqueous solutions of strong acids, strong bases, oxidizing
agents and reducing agents, or some combination of such reagents. It may be necessary to ignite
the sample in air or oxygen or to perform a high-temperature fusion of the sample in the presence
of various fluxes.
1
, Once the analyte is made soluble, we then ask whether the sample has a property that is
proportional to analyte concentration and that we can measure. If it does not, other chemical steps
may be necessary to convert the analyte to a form suitable for the measurement step. For
example, in the determination of manganese in steel, manganese must be oxidized to MnO4
before the absorbance of the colored solution is measured.
At this point in the analysis, it may be possible to proceed directly to the measurement step, but
more often than not, we must eliminate interferences in the sample before making
measurements.
4) Eliminating Interferences
Once we have the sample in solution and have converted the analyte to an appropriate form for
measurement, the next step is to eliminate substances from the sample that may interfere with
measurement.
Few chemical or physical properties of importance in chemical analysis are unique to a single
chemical species. Instead, the reactions used and the properties measured are characteristic of a
group of elements of compounds.
Species other than the analyte that affect the final measurement are called interferences, or
interferents. A scheme must be devised to isolate the analytes from interferences before the final
measurement is made.
No hard and fast rules can be given for eliminating interferences; indeed, resolution of this
problem can be the most demanding aspect of an analysis.
NOTE: SEPARATION TECHNIQUES ARE COVERED IN ANALYTICAL II (SCH 220)
5) Calibrating and Measuring Concentration
All analytical results depend on a final measurement X of a physical or chemical property of the
analyte. This property must vary in a known and reproducible way with the concentration CA of
the analyte.
Ideally, the measurement of the property is directly proportional to the concentration. That is.
CA = kX
Where k is the proportionality constant
With two exceptions, analytical methods require the empirical determination of k with chemical
standards for which CA is known. The process of determining k is thus an important step in most
analyses: this step is called a calibration.
Calibration determines the relationship between the analytical response and the analyte
concentration. Usually this is accomplished by the use of chemical standards.
Almost all analytical methods require some type of calibration with chemical standards.
However, gravimetric methods and some coulometric methods are among the few absolute
methods that do not rely on calibration with chemical standards. Several types of calibration
procedures are described below:
2
