There are certain methods for detection of ions and microbes in water used for industrial, laboratory and drinking purposes for ensuring the quality of water.
These methods are classified as under:
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1. Chemical Methods:
i. Titration.
ii. Spectrophotometry.
iii. Electrochemical techniques.
iv. Ion chromatography.
2. Microbiological Methods:
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For Coliform Group:
i. Multiple tube fermentation method.
ii. Membrane filter method.
1. Chemical Methods:
Some chemical methods for detection of ions in water are discussed here:
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i. Titration:
The well-established titration for alkalinity, hardness and chloride are still widely used. Potentiometric end-point detection with pH or ion selective electrodes has tended to replace indicator colour changes with the greater ease of automation and consistency.
With large volume sample, detection limits can be reduced to ppm levels in some cases. This technique can be applied to chemical addition to water, for i.e. boric acid in the primary water circuits of light water reactors and mixtures of hypochlorite, chloride and chlorate.
ii. Spectrophotometry:
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Spectrophotometry analysis using colour reactions of specific ions are also still popular.
Advantages:
There are some advantages of this method:
a. With automated analysers they can prove quicker and more cost-effective than other techniques like ion chromatography (IC).
b. Most ions can be determined in various types of water at ppm or lower concentrations.
c. In some cases it offers particular advantages over other techniques, e.g. for iron the detection limit is lower than, for example, flame AAS, while for silica the colour reaction is extremely sensitive and also specific to soluble silicates, enabling them to be distinguished from total silicon, as measured by AAS or ICP-OES.
iii. Electrochemical Techniques:
An electrochemical technique enjoys some advantages like modest cost and their suitability for direct measurement in difficult matrices.
Ionic concentration at or above ppm levels can be detected by voltammetry. In this method, potential applied to a working electrode is varied in small steps and the resulting current recorded as a function of potential. The potential can be varied in steps, pulses or both, giving staircase or normal pulse voltammetry or polarography (with dropping mercury electrode) which are suitable for qualitative analysis and differential pulse or square wave voltammetry for quantitative analysis. For greater sensitivity, stripping steps are used. Stripping analysis involves two stages, electrolysis followed by stripping.
During electrolysis a specific potential is applied to the working electrode for a certain period of time during which the electroactive species in the water sample are deposited on the working electrode. The working electrode is glassy carbon covered with a thin film of gold or mercury, a hanging mercury drop electrode or a solid electrode. Stripping during which deposited species are re-dissolved into the solution, is performed electrochemically or by chemical oxidation. Elements are identified by their characterised redox potential.
a. Potentiometric Stripping Analysis (PSA):
PSA uses chemical oxidation by dissolved oxygen or mercury ions to determine metals such as lead, copper, cadmium, and zinc. The electrode potential is measured against the time during stripping. The rest time at each metal’s redox potential is proportional to the amount of the metal in sample.
b. Constant Current Stripping Analysis (CCSA):
CCS A uses the same measuring principle, but applies a constant current to control stripping. Application includes determination of mercury, nickel, cobalt, and arsenic.
c. Voltammetric Stripping Analysis (ASV/CSV):
In this, the electrode potential is gradually changed and resulting current is measured. The stripping current is proportional to the amount of species deposited. Metals which form amalgam such as lead, copper, cadmium, zinc, tin, thallium, manganese, bismuth, gallium can be measured using a mercury film or drop electrode. Voltammetry can differentiate between oxidation states of an element and so can be used for environmentally significant assays. Electrochemical techniques lack the wide elemental range and long linear response of some atomic and mass spectroscopy technologies.
iv. Ion Chromatography:
It is a major technique for ion determination. Determination of common anions such as bromide, chloride, fluoride, nitrates, phosphate and sulphate is often desirable to characterise water and to assess the need for specific treatment.
Ion Chromatography with Chemical Suppression of Eluant Conductivity:
A water sample is injected into a stream of carbonate, bicarbonate eluant and passed through a series of ion exchangers. The anions of interest are separated on the basis of their relative affinities for a low capacity, strongly basic anion exchanger. The separated anions are directed onto a strongly acidic cation exchanger or through a hollow fibre of cation exchanger membrane or micro-membrane suppressor bathed in continuously flowing strongly acid solution.
In the suppressor the separated anions are converted to their highly conductive acid forms and the carbonate bicarbonate eluant is converted to weakly conductive carbonic acid. The separated anions in their acid forms are measured by conductivity. They are identified on basis of retention time as compared to standards. In most ion chromatographic methods, analysis is carried out with conductivity detection; amperometric, UV-visible spectrophotometric and fluorescence detectors are available for selective and more sensitive analysis.
Only ion chromatography provides a single instrumental technique that may be used for their rapid, sequential measurement. Ion chromatography eliminates the need to use hazardous reagent and it effectively distinguishes among the halides, SO3–, SO4–, or NO2–, NO3–. This method is not recommended for the routine determination of fluorine because equivalency studies have indicated positive or negative bias and poor precision in some samples. IC using special techniques such as dilute eluant or gradient elution with fibre suppressor or membrane suppressor can determine fluorine accurately.
2. Microbiological Method:
Following discussion describes the procedures for making microbiological examination of water samples to determine water quality for it’s use in laboratory investigation and also for domestic purpose.
Tests for detection and enumeration of indicator organism, rather than of pathogens are used. Coliform group of bacteria as herein defined, is the principle indicator of suitability of water for domestic industrial purpose. Coliform group density is taken as a criterion of degree of pollution and thus of sanitary quality. Significance of tests and interpretation of results are well authenticated and have been used as a basis for standards of bacteriological quality of water supplies. Two standard methods are presented for the detection of the coliform group.
i. Multiple Tube Fermentation Method:
It is customary to report results of coliform test by the multiple tube fermentation procedure as a most probable number (MPN) index. This is an index of the number of coliform bacteria that, more probably than any other number, would give the results shown by the laboratory examination.
ii. Membrane Filter Method:
It is direct plating method. This procedure permits the direct count of coliform colonies. In both procedures coliform density is reported conventionally as the MPN or membrane filter count per 100 ml.
Use of either procedure permits appraising the sanitary quality of water and the effectiveness of treatment processes. Because it is not necessary to provide a quantitative assessment of coliform bacteria for all samples, a qualitative, presence/absence test is included.
Both the multiple tube dilution technique and membrane filter procedures have been modified to incorporate incubation in confirmatory tests at 44.5°C to provide estimate of density of fecal organism.