Prepare a 0.048 mol/L BaCl2 (1.175% w/v BaCl2 • 2H2O) stock solution.
2. Prepare a 0.18 mol/L (0.36 N) H2SO4 (1% v/v) stock solution.
3. With constant stirring to maintain a suspension, add 0.5 mL of the BaCl2 solution to 99.5 mL of the
H2SO4 stock solution.
4. Verify the correct density of the turbidity standard by measuring absorbance using a
spectrophotometer with a 1-cm light path and matched cuvettes. The absorbance at 625 nm should be
0.08 to 0.13 for the 0.5 McFarland standard.
5. Transfer the barium sulfate suspension in 4- to 6-mL aliquots into screw-cap tubes of the same size as
those used for standardizing the bacterial inoculum.