A parts per million (ppm) solution is made by dissolving a specific amount of solute into a specific amount of solvent so that the ratio of solute to solution is one part solute to one million parts solution.
Understanding PPM
"ppm" stands for "parts per million." It's a way to express very dilute concentrations of a substance. Think of it as how many units of a substance are present for every million units of the total solution.
Calculating PPM
Before preparing a ppm solution, you need to understand the units involved. For aqueous solutions (where water is the solvent), these relationships are crucial:
- 1 ppm = 1 mg/L (milligram per liter)
- 1 ppm = 1 µg/mL (microgram per milliliter)
These equivalencies are valid because the density of water is approximately 1 g/mL.
Steps to Make a PPM Solution
Here's a step-by-step guide:
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Determine the desired concentration: Decide what ppm concentration you need. For example, let's say you want a 5 ppm solution.
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Choose your solvent: The most common solvent is water, but you can also use other solvents like ethanol.
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Calculate the required mass of solute: Using the equivalencies above, calculate how much solute you need to add to the solvent.
- For an aqueous solution: If you want to make 1 liter of a 5 ppm solution, you'll need 5 mg of solute.
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Weigh the solute accurately: Use a calibrated analytical balance to weigh out the calculated amount of solute.
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Dissolve the solute: Add the weighed solute to a container (like a volumetric flask). Then, add a portion of the solvent to the flask and mix well to dissolve the solute completely.
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Bring to final volume: Once the solute is fully dissolved, carefully add more solvent until the solution reaches the final desired volume indicated on the volumetric flask. Make sure the bottom of the meniscus aligns with the calibration mark on the flask.
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Mix thoroughly: Invert the flask several times to ensure the solution is homogeneous.
Example
Let's make 250 mL of a 20 ppm aqueous solution of copper sulfate (CuSO₄).
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Desired concentration: 20 ppm
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Solvent: Water
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Calculate solute mass:
- 20 ppm = 20 µg/mL
- We need 250 mL, so: 20 µg/mL * 250 mL = 5000 µg
- Convert to mg: 5000 µg = 5 mg
- Therefore, we need 5 mg of CuSO₄.
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Weigh: Accurately weigh out 5 mg of CuSO₄.
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Dissolve: Add the CuSO₄ to a 250 mL volumetric flask, add some water, and mix until dissolved.
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Bring to volume: Add water to the flask until it reaches the 250 mL mark.
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Mix: Invert and mix thoroughly.
Important Considerations
- Accuracy: Use accurate measuring equipment (balances, volumetric flasks, pipettes) to ensure the correct concentration.
- Solubility: Make sure the solute is soluble in the chosen solvent.
- Purity: Use high-purity solutes for accurate results.
- Storage: Store the solution properly to prevent degradation or contamination.
