Dilution is a fundamental process in chemistry and biology that involves reducing the concentration of a substance in a mixture, typically by adding more solvent. This technique is often used to make a concentrated substance weaker, safer, or more palatable, such as when mixing a cleaning solution or preparing a beverage concentrate. In scientific settings, dilution is a precise, mathematical procedure that enables accurate measurements and safe experimentation. The core principle is that the total amount of the original substance remains unchanged; only the volume of the entire mixture increases.
Defining the Components of a Solution
To understand dilution, one must clarify the components of a liquid mixture. The mixture itself is called the solution, a uniform blend where one substance is evenly dispersed within another. The substance present in the lesser amount, which is being dissolved, is known as the solute (e.g., salt or a concentrated chemical).
The component present in the greater amount, which does the dissolving, is called the solvent, often water in biological and everyday contexts. The relationship between the solute and solvent defines the solution’s concentration—a quantitative measure of the amount of solute per unit volume of the total solution. Dilution is the action of changing this ratio by increasing the volume of the solvent.
The Mathematics of Dilution
The precise relationship between the initial, concentrated solution and the final, diluted one is governed by the dilution formula: C1V1 = C2V2. Here, C1 and V1 represent the concentration and volume of the starting solution, while C2 and V2 represent the concentration and volume of the final, diluted solution. The formula holds true because the total quantity of the solute (C × V) remains constant before and after the solvent is added.
This equation allows scientists to calculate how much of the concentrated starting material, often called the stock solution, is needed to prepare a desired volume of a weaker solution. For example, if a laboratory has a 10 Molar (M) stock solution and needs 500 milliliters of a 0.5 M solution, the formula solves for the volume of the stock solution, V1. Calculating V1 = (C2V2) / C1 shows that only 25 milliliters of the stock solution are needed, which is then mixed with enough solvent to reach the final 500-milliliter volume.
Comparing Simple and Serial Dilution Techniques
Dilution is performed using two primary techniques: simple dilution and serial dilution.
A simple dilution is a one-step process where a measured amount of stock solution is combined with a precise volume of solvent to achieve the final desired concentration in a single vessel. This method is common when the required final concentration is not significantly lower than the stock concentration.
Serial dilution involves a sequence of successive dilutions, where the dilution from the previous step serves as the starting material for the next. This technique is necessary when a solution must be diluted across a vast range of concentrations, often by factors of 10 or 100 at each step. For example, a 1:10 serial dilution is performed by taking one milliliter of stock solution and adding it to nine milliliters of solvent, then repeating the process with the resulting mixture.
The purpose of serial dilution is to achieve extremely low concentrations that would be highly inaccurate to measure in a single step. In microbiology, this technique is indispensable for estimating the number of microorganisms in a sample, such as bacteria in a liquid culture. Since the starting sample often contains billions of cells per milliliter, direct counting is impossible. A series of 1:10 dilutions systematically reduces the cell density until a plate grown from the final dilution contains a number of colonies small enough to be counted accurately.
Why Dilutions Matter in Science and Health
Dilution is a foundational technique across the fields of science and health.
In clinical testing, patient samples like blood or urine often contain analytes, such as glucose or specific proteins, at concentrations too high for measuring instruments to read accurately. Technicians must dilute these samples to bring the concentration into the linear range of the testing equipment, ensuring the final result is reliable and clinically meaningful. The measured concentration is then multiplied by the dilution factor to determine the true value in the original sample.
In pharmacology and medicine, dilution is fundamental to creating safe and effective drug dosages. Many medications are manufactured as highly concentrated stock solutions to ensure stability and reduce storage volume. Before administration, a pharmacist or nurse precisely dilutes the drug to achieve the exact concentration required for intravenous delivery or injection. This precision is vital for drugs with a narrow therapeutic window, such as epinephrine, where a small error in concentration could have severe consequences for the patient.
Environmental science also relies on dilution for monitoring and safety. When assessing water quality, researchers dilute samples to measure trace contaminants like heavy metals or pesticides. Dilution is also a factor in industrial discharge and wastewater treatment, reducing pollutant concentrations to comply with regulatory standards before release into the environment.