Question

Design a serial dilution to achieve a final dilution of 10^-4 such that there will be enough solution remaining in each dilution to use for an assay that requires 100 mL of sample in triplicate. Remember that better accuracy is obtained with relatively large dilutions if the total dilution is made from a series of smaller dilutions rather than one large dilution.

Answer 1

Dilution to be prepared = 10^-4

Sample volume (in triplicates) required for an assay from each dilution = 3 * 100 mL = 300 mL.

We would go about making four 1 / 10 dilutions to achieve 10^-4 . Also, since the sample volume required is 300 mL, we would need to make dilutions in a volume greater than the required volume so that the sample volume is withdrawn comfortably for the assay.

Since 300 mL is the required volume, we would make 400 mL dilutions, which means, for a 1 / 10 or 10^-1 dilution, we need : 400 * 1 / 10 = 400 * 0.1 = 40 mL sample volume and 400 - 40 = 360 mL water or diluent.

Now let us make the following dilutions in 4 bottles/flasks appropraite to hold 400 mL solution:

Bottle #	Sample source	Sample volume mL	Diluent volume mL	Final dilution
1	original sample	40	360	1 / 10 or 10^-1
2	bottle # 1	40	360	1/ 100 or 10^-2
3	bottle # 2	40	360	1 /1000 or 10^-3
4	bottle # 3	40	360	1 / 10000 or 10^-4

While making the dilutions, when we add 40 mL of the original sample to be diluted in 360 mL water, the dilution becomes: 40 / 400 = 1 / 10 or 10^-1

bottle # 2: when we repeat the same pattern of diluton by taking the sample from already diluted bottle#1, the dilution becomes a product of the two dilutions: 1 / 10 * 1 / 10 = 1 / 100 or 10^-2

bottle#3: Here, the final dilution is a product of the previous dilution (bottle#2) and the current dilution:

1 / 100 * 1 / 10 = 1 / 1000 or 10^-3

bottle # 4: here again, the final dilution is the product of existing dilution of the source (bottle#3) and the current dilution: 1 / 1000 * 1 / 10 = 1 / 10000 or 10^-4.