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 mixing specified amounts of barium chloride and sulfuric acid together. Mixing the two compounds 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 lengthens the shelf life and stability of the suspensions. The standard can be compared visually to a suspension of bacteria in sterile saline or nutrient broth. If the bacterial suspension is too turbid, it can be diluted with more diluent. If the suspension is not turbid enough, more bacteria can be added.
McFarland Nephelometer Standards:
| McFarland Standard No. | 0.5 | 1 | 2 | 3 | 4 |
| 1.0% Barium chloride (ml) | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 |
| 1.0% Sulfuric acid (ml) | 9.95 | 9.9 | 9.8 | 9.7 | 9.6 |
| Approx. cell density (1X10^8 CFU/mL) | 1.5 | 3.0 | 6.0 | 9.0 | 12.0 |
| % Transmittance* | 74.3 | 55.6 | 35.6 | 26.4 | 21.5 |
| Absorbance* | 0.132 | 0.257 | 0.451 | 0.582 | 0.669 |
*at wavelength of 600 nm
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“With his brows knit, his mind made up, his will resolved and resistless, he advances, crashing his way through the host of weak, half-formed, dilettante opinions, honest and dishonest ways of thinking, with their standards raised, sentimentalities and conjectures, and tramples them all into dust. See how he prevails; you don’t even hear the groans of the wounded and dying.”
—Henry David Thoreau (1817–1862)