A stock solution is a concentrated mixture prepared to be diluted later into multiple working solutions for various experiments. This concentrated form is foundational in many scientific fields, including biology, chemistry, and environmental science. Preparing one large, accurately measured stock solution provides efficiency, saving time that would otherwise be spent weighing out tiny amounts of solid material for every experiment. It also improves consistency and accuracy, ensuring all subsequent working solutions originate from the same precise starting concentration.
Understanding Concentration Terminology
The language used to describe the strength of a stock solution relies on specific terminology that quantifies the amount of solute dissolved in the solvent. The most common unit in scientific practice is Molarity, symbolized by an uppercase M. Molarity defines the concentration as the number of moles of solute present in one liter of the final solution.
The concept of a mole represents a specific, very large number of molecules, making Molarity a measurement based on the amount of substance rather than its mass. Other concentration expressions are also used, such as Weight/Volume Percent (W/V%), which expresses the mass of solute in grams per 100 milliliters of total solution. For extremely dilute mixtures, particularly in environmental monitoring, scientists use parts per million (ppm) or parts per billion (ppb).
Required Calculations for Preparation
Preparing a stock solution from a solid material requires calculating the precise mass of solute needed to achieve the desired Molarity. This calculation first involves finding the substance’s molar mass, often called formula weight, which is the sum of the atomic masses of all atoms in its chemical formula. The required mass in grams is then determined by multiplying the desired concentration (Molarity in moles per liter) by the final volume (in liters) and the substance’s molar mass (in grams per mole). For example, a 1-liter solution of 0.5 M sodium chloride requires 29.22 grams of salt, calculated using sodium chloride’s molar mass of 58.44 grams per mole.
Stock solutions are prepared with the intention of later dilution to create working solutions of lower concentration. This process relies on a straightforward dilution formula: C1V1 = C2V2. This equation relates the initial concentration (C1) and volume (V1) of the stock solution to the final concentration (C2) and final volume (V2) of the diluted working solution. Using this calculation ensures that the number of moles of solute transferred from the stock remains the same in the final, diluted working solution. This mathematical consistency is fundamental to maintaining the accuracy that stock solutions provide across multiple experiments.
The Standard Preparation Procedure
The physical preparation of a stock solution begins with the accurate measurement of the solute using an analytical balance. The calculated mass of the solid must be weighed precisely, often on weighing paper or a watch glass, ensuring the balance is tared, or zeroed, before measurement. Once weighed, the solid material is transferred to a beaker or flask where it will be dissolved.
A small amount of the solvent, typically distilled or deionized water to prevent impurities, is added to the beaker to completely dissolve the solute. It is important to stir gently, often with a magnetic stir bar, until the solid is fully in solution. After complete dissolution, the liquid is carefully transferred into a volumetric flask, which is laboratory glassware specifically designed to hold one exact, precise volume.
The beaker and any stirring implements must be rinsed multiple times with small portions of the solvent to ensure all dissolved solute is quantitatively transferred to the volumetric flask. The flask is then filled with solvent until the liquid level is just below the calibration mark etched on the neck. The final, delicate step, known as “bringing to volume,” involves adding the last few drops of solvent slowly using a transfer pipette until the bottom of the curved liquid surface, or meniscus, aligns exactly with the mark.
The flask is then stoppered tightly, and the solution must be thoroughly mixed by inverting and turning the flask repeatedly, usually ten times or more. This inversion ensures the homogeneous distribution of the solute throughout the entire volume of the solvent, completing the stock solution preparation.
Labeling and Safe Storage Practices
Once the stock solution is prepared and mixed, it must be immediately labeled to prevent confusion and misuse. A comprehensive label should include:
- The chemical name of the solute
- The exact concentration (e.g., 1.0 M)
- The date of preparation
- The initials of the person who prepared it
For certain chemicals, the label should also include specific storage requirements and hazard warnings, providing a safety reference for future users.
Proper storage is necessary to maintain the solution’s integrity and prolong its effective shelf life. Many stock solutions, particularly those used in biological experiments, are temperature-sensitive and must be stored in a refrigerator at a specific temperature to prevent degradation. Solutions containing photosensitive compounds, which break down when exposed to light, require storage in dark or amber-colored containers to shield them.
Airtight containers are necessary to protect solutions from air exposure, which could cause oxidation or contamination. The stability of a stock solution should be monitored, and it should be discarded if there are visible signs of precipitation or cloudiness. Following these practices ensures the concentrated stock solution remains a reliable and accurate source for all future dilutions.