01What Is Serial Dilution?
A serial dilution is a sequence of dilutions where each new tube is made from the previous tube rather than from the original stock.
Serial dilution is useful when a single direct dilution would require a very small volume that is hard to pipette accurately.
02How It Works
In a 1:10 serial dilution, each tube contains one tenth of the concentration of the previous tube. After two steps the total dilution is 1:100; after three steps it is 1:1000.
The same pattern works for 1:2, 1:5, 1:20, or any other repeated dilution factor.
03The Math
The concentration after n steps equals the starting concentration divided by the dilution factor raised to n.
If C0 = 1.0 M and each step is 1:10, then C3 = 1.0 / 10^3 = 0.001 M.
04Common Dilution Factors
A 1:2 dilution is useful for gentle two-fold series. A 1:5 dilution covers range faster. A 1:10 dilution is common in microbiology because it is easy to interpret on a log scale.
- 1:2: one part sample in two parts total
- 1:5: one part sample in five parts total
- 1:10: one part sample in ten parts total
05Step-by-Step Protocol
Label all tubes before starting. Add the correct solvent volume to each tube first, then transfer the sample volume from the previous tube.
Mix thoroughly at every step before transferring to the next tube. Poor mixing is one of the easiest ways to ruin a dilution series.
- Prepare tubes and solvent.
- Transfer sample from stock or previous tube.
- Mix completely.
- Change tips between transfers.
- Repeat for the required number of steps.
06Worked Example
Start with 1.0 M stock and prepare a 1:10 series over 6 steps, with 10 mL final volume in each tube.
For each tube, transfer 1.0 mL from the previous solution and add 9.0 mL solvent. The concentrations are 0.1 M, 0.01 M, 0.001 M, 0.0001 M, 0.00001 M, and 0.000001 M.
07Total Dilution Factor
Total dilution factor multiplies across steps. A 1:10 dilution repeated three times gives 1:1000 overall.
For mixed factors, multiply them together. A 1:2 followed by 1:5 followed by 1:10 gives an overall 1:100 dilution.
08Sources of Error
Serial dilution error compounds because each tube depends on the previous one. A pipetting error early in the series affects every later tube.
Use calibrated pipettes, change tips, avoid bubbles, and mix thoroughly to reduce systematic and random errors.
09Frequently Asked Questions
Q1Why use serial dilution instead of one dilution?
Serial dilution avoids extremely small pipette volumes and creates a usable concentration range. It is often more practical than making a very large dilution directly.
Q2What is a 10-fold dilution?
A 10-fold dilution is the same as a 1:10 dilution. The final concentration is one tenth of the previous concentration.
Q3How do I calculate concentration at step n?
Divide the starting concentration by the dilution factor raised to n. For a 1:10 series, step n has concentration C0/10^n.
Q4Do I use stock for every tube?
No. In a serial dilution, each tube is prepared from the previous diluted tube.
Q5How much solvent do I add for 1:10 in 10 mL?
Use 1 mL of sample and 9 mL of solvent to reach 10 mL total.
Q6Why is mixing important?
If a tube is not mixed evenly, the next transfer may not represent the intended concentration. The error then propagates through the rest of the series.
Q7What is the total dilution after four 1:10 steps?
It is 1:10000. Multiply the factor by itself four times: 10^4.
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