How to Calculate Concentration From Stock & Diluent

Written by michael judge
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How to Calculate Concentration From Stock & Diluent
Labs often use serial dilution to prepare calibration standards. (Hemera Technologies/ Images)

Labs often make up "stock" chemical solutions. These are highly concentrated solutions where the amount of chemical per unit volume is precisely known. Technicians then dilute these by varying amounts to produce less concentrated mixtures to use, for example, in calibrating instruments. If you know the amount of stock solution and the amount of diluent added to it, you can find the concentration of the resulting dilute solution. To do so, use the standard formula C1V1 = C2V2, where C1 and V1 are the stock concentration and volume and C2 and V2 are the dilute concentration and volume.

Skill level:
Moderately Easy

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  1. 1

    Add the volume of the stock solution which was used and the volume of diluent added to that stock solution. This procedure gives you the value of V2, the total final volume of the dilute solution. For example, if you had combined 25ml of stock solution with 225ml of diluent, you would add 25 and 225 to obtain 250ml. Note that the units must be the same for both these volumes.

  2. 2

    Multiply the concentration of the stock solution by the volume of stock solution which was used. This calculation is equivalent to multiplying C1 by V1. If the stock solution concentration in the example was 100 parts per million (ppm), the calculation would be 100 multiplied by 25 to give 2,500.

  3. 3

    Divide the result of the previous calculation by the combined volume of stock and diluent which you found earlier. You are now dividing the product (C1)(V1) by V2. The value you obtain is the concentration of the dilute solution, C2, in the same units as the stock solution concentration. In the example, you would divide 2,500 by 250 and obtain an answer of 10 ppm.

Tips and warnings

  • You can use any type of units you like for concentration in this procedure. The answer for the dilute solution concentration will always be in the same units you use for the stock solution concentration.

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