# Serial dilution This snip off is from Himedia Widal test teaching kit.

I understand 1/20 1/40 1/80... are the concentrations in each test tube. They are obtained by the following calculation: (2nd testube) 1/2 X 1/20.
But I can't decipher the numbers written at the top, 10-1 , 10-2 etc.

Besides which of of these two set of numbers should be called dilution factor.

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mfb
Mentor
That should be ##2^{-n}## for different n. Maybe it was copied from a different problem where the dilution was a factor 10 each step.

• TytoAlba95
Baluncore
2021 Award
Welcome to PF.
I think they have used old numbers from steps of 0.1 when they should now use steps of 0.5
Following the dilution, the concentration in the tubes will be;
#1, 1:20 = 0.05 = 0.1 × 2-1
#2, 1:40 = 0.025 = 0.1 × 2-2
#3, 1:80 = 0.0125 = 0.1 × 2-3
#4, 1:160 = 0.00625 = 0.1 × 2-4
#5, 1:320 = 0.003125 = 0.1 × 2-5
#6, 1:640 = 0.0015625 = 0.1 × 2-6
#7, 1:1280 = 0.00078125 = 0.1 × 2-7
In each case the 0.1 = 10-1 = initial dilution of 10.

• TytoAlba95
Which number here could be called dilution factor?

Thanks

Baluncore
2021 Award
The dilution factor is equal to the final volume divided by the aliquot volume.
For tube #1; 1:20; The dilution factor is 2.0 mL / 0.1 mL = 20.
The dilution factors for tubes #1 to #7 are 20, 40, 80, 160, 320, 640 and 1280.

• TytoAlba95
The dilution factor is equal to the final volume divided by the aliquot volume.
Thanks

Merlin3189
Homework Helper
Gold Member
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