In microbiology, McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range.
A McFarland standard can be made by mixing specified amounts of barium chloride and sulfuric acid together. Mixing the two compounds together forms a barium sulfate precipitate, which causes turbidity in the solution. A 0.5 McFarland standard is prepared by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl2•2H2O), with 9.95 mL of 1% sulfuric acid (H2SO4).
The standard can be compared visually to a suspension of bacteria in sterile saline. If the bacterial suspension is too turbid, it can be diluted with more saline. If the suspension is not turbid enough, more bacteria can be added.
Before selecting standards for an application, review these basics of primary standards and polymer-based standards from APS Analytical Standards Inc. Polymer-based standards were originally created for turbidimeters — instruments that measure relative sample clarity — but can now be used in many other applications that involve scattering and absorbence optical instrumentation.
Polymer-based standards are an alternative to standards made of traditional materials, as the polymer materials mimic or exceed the properties of many existing standards.
Companies are now replacing standards such as McFarlandstandards (barium sulphate) and potassium dichromate, as well as haze standards for the beer industry and formazin for turbidity calibrations with styrene divinylbenzene polymer standards.
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