Analytical Methods

Sample analysis involves the use of one or more analytical methods to determine the concentration of the constituent of interest. The level of detection is a measure of the resolution possible using a given analytical technique. The methods of analysis used to define the physical and chemical characteristics of water samples can be defined as follows (http://water.epa.gov/scitech/drinkingwater/labcert/analyticalmethods.cfm):

  • Gravimetric analysis (i.e. weighting) is used to assess the mass of a given constituent that may be present. The total suspended solids and total dissolved solids content of water are the most common parameters determined gravimetrically.
  • Volumetric analysis is based on the principle of conservation of mass in which the concentration of an unknown constituent or group of constituents is compared volumetrically to a known standard
  • Physicochemical analysis is used to measure physical properties other than mass and volume. Turbidimetry, colorimetry, potentiometry, polarography, adsorption spectrometry, fluorometry, spectroscopy and nuclear radiation are representative of the physicochemical methods.

Instrumental methods of analysis

Analysis should be conducted promptly after sample collection so that the chemical nature of the sample does not change. The methods seen as follows, are suitable for on-site analysis.; they involve the use of apparatus and chemicals. Lists of required materials are provided with each titrimetric, spectrophotometric and colorometric procedure. In some cases, other appropriate equipment may be substituted for the apparatus listed.

Titration Methods

Titration is based on the use of a buret, from which a standard solution is added to the sample until an "end point" is reached. The end point is generally indicated by a color change or detected by potentiometric device (e.g., pH meter).

Several types of burets are available:

  • semimicro burets (2 or 3 mL capacity) are used to titrate low concentrations of species in the sample
  • large burets (25 or 50 mL capacity) are used to titrate species found in higher concentrations
  • automatic burets feature a reservoir for "automatic" filling of the buret and an overflow and reset to 0 mL

Digital titrators provide a more portable approach to titration in the field. These hand-held units are widely accepted because they are rugged and easily carried from one location to another. The digital titrator is equivalent to a buret in the conventional titration methods. The titrator acts as a plunger and forces concentrated titrant from an attached plastic cartridge. Each cartidge can perform the same amount of testing as one quart of titrant in conventional tests.

Photometric Methods

Photometers or spectrophotometers provide the most accurate means of measuring the color of a reacted sample. In field analysis applications, simple filter photometers have been replaced by monochromator-based spectrophotometers. The essential components of a spectrophotometer include the following:

  • a stable source of radiant energy
  • a system of lenses, mirrors and slits that define, collimate (make parallel) and focus the beam
  • a monochromator, to resolve the radiation into component wavelengths or "bands" of wavelengths
  • a transparent container to hold the sample
  • a radiation detector with an associated read-out system

Light from a tungsten bulb is reflected off of a parabolic mirror and dispersed with a double pass through a high-dispersion prism. The selected wavelength is imaged onto a movable slit, ensuring a uniform band width.

Colorimetric Methods

Colorimetric comparator tests are not as accurate as the photometric or spectrophotometric methods. Colorimetric methods have become popular because of their simplicity and relatively low cost. However, tight control of most industrial water systems should not be entrusted to this technique alone.

In a comparator test, a color is developed that is proportional to the concentration of the substance being determined. The concentration present in the sample is determined by comparison with sealed color standards.

Other instrumental methods used in the laboratory

Common methods of water analysis often involve highly sophisticated electronic instrumentation not generally used on-site. Some of them include (http://www.gewater.com):

  • Ion Chromatography is used to measure trace levels of anions in feedwater, steam, condensate, and boiler water.
  • Atomic Absorption Spectroscopy (AA), Inductively Coupled Ion Spectroscopy (ICP), X-ray Fluorescence Spectroscopy, and other laboratory procedures are used routinely to measure many elements at trace levels. Some instruments can provide concurrent read-outs of over 40 elements in ppb measurements.
  • Gas Chromatography (GC), or Gas Chromatography and Mass Spectroscopy (GC/MS), quantitatively separates and detects volatile components (e.g., neutralizing amines) in boiler condensate.
  • High-Pressure Liquid Chromatography (HPLC) permits the separation and detection of trace organic compounds in antimicrobial applications.
  • Total Organic Carbon (TOC) measurements are used to determine the amount of organic compounds present in water as a result of water treatments or process leaks.
  • Nuclear Magnetic Resonance Spectroscopy (NMR) provides an analytical tool to aid in determining the structure of organic polymers and other organic water treatment chemicals.
  • Fourier Transform Infrared Analysis (FT-IR) permits the qualitative and quantitative determination of the composition of organic compounds or the chemical structure of inorganic compounds.

References