McFarland standards
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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.
Original McFarland standards were 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). Now there are McFarland standards prepared from suspensions of latex particles, which lenthens the shelf life and stability of the suspensions.
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.