Stock Solution Calculators

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A stock solution is a concentrated solution of a reagent prepared in advance, from which working solutions of lower concentration are prepared by dilution. Stock solutions are used in laboratories to avoid weighing small amounts of reagent each time, to ensure consistency between experiments, and to simplify solution preparation. The dilution equation C1V1 = C2V2 is used to calculate the volume of stock solution required to prepare a working solution: V1 = (C2 × V2) / C1. Stock solutions must be properly labeled (reagent name, concentration, solvent, date prepared, preparer's initials) and stored under appropriate conditions (temperature, light sensitivity, shelf life).

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Preparing a Stock Solution

Example: prepare 100 mL of 1 M NaCl stock. MW of NaCl = 58.44 g/mol. Mass = molarity × volume × MW = 1 mol/L × 0.100 L × 58.44 g/mol = 5.844 g. Weigh 5.844 g NaCl → transfer to 100 mL volumetric flask → dissolve in ~80 mL water → bring to 100 mL mark → mix. Verify concentration by titration or conductivity if precision is needed.

Dilution from Stock: C1V1 = C2V2

C1 = stock concentration; V1 = volume of stock to use; C2 = desired working concentration; V2 = total final volume. V1 = (C2 × V2) / C1. Example: prepare 50 mL of 0.01 M NaCl from 1 M stock: V1 = (0.01 × 50) / 1 = 0.5 mL. Add 0.5 mL of 1 M stock to a flask; bring to 50 mL with water.

Labeling Requirements

Every stock solution must be labeled: Reagent name (chemical name + formula). Concentration (with units). Solvent. Date prepared. Preparer's initials. Expiry or review date. Hazard information (GHS pictograms for hazardous chemicals).

Common Stock Solutions

100× PBS (phosphate-buffered saline): dilute 1:100 for working concentration. EDTA (0.5 M, pH 8.0): add to working solutions at 1–10 mM. SDS (10%): dilute to 0.1% for SDS-PAGE sample buffer.

Glossary

Stock Solution
A concentrated solution prepared in advance from which working solutions are diluted; used for convenience, consistency, and to avoid weighing small amounts repeatedly; must be labeled with concentration, date, and preparer.
C1V1 = C2V2
The dilution equation; V1 = (C2 × V2)/C1; used to calculate volume of stock solution needed for any working concentration; based on conservation of moles of solute during dilution.
Working Solution
A diluted solution prepared from a stock for immediate experimental use; prepared fresh or at appropriate intervals; concentration matched to experimental requirements.

Frequently Asked Questions

A stock solution is a prepared, concentrated solution from which diluted working solutions are made by dilution. Why use stock solutions: Convenience: avoids weighing small, hard-to-measure amounts of reagent each time. Consistency: the same stock is used for all experiments, ensuring identical composition. Efficiency: one accurate preparation provides material for many experiments. Stability: some reagents are more stable concentrated than dilute (e.g., antibody stocks at higher concentration in glycerol). Examples: 10× PBS, 1 M Tris-HCl, 10 mg/mL BSA, 100× antibiotic solutions (penicillin/streptomycin), 10 mM primer stocks.

Steps for preparing a stock solution: (1) Determine the target concentration and volume. (2) Calculate mass needed: mass (g) = molarity × volume (L) × molar mass (g/mol). (3) Weigh accurately on an analytical balance. (4) Transfer quantitatively to a volumetric flask of the correct size. (5) Add about 80% of the final volume of solvent; dissolve completely (stir, heat if needed). (6) Cool to room temperature (volume is temperature-dependent). (7) Bring to the mark with solvent precisely. (8) Mix by inversion. (9) Label: reagent, concentration, solvent, date, preparer. (10) Store appropriately (refrigerator, -20°C, away from light as required).

V1 = (C2 × V2) / C1. V1 = volume of stock to take. C2 = desired final concentration. V2 = desired total final volume. C1 = stock concentration. Example 1: prepare 200 mL of 0.05 M NaCl from 1 M stock: V1 = (0.05 × 200)/1 = 10 mL. Pipette 10 mL stock into a 200 mL volumetric flask; add water to the 200 mL mark. Example 2: prepare 500 μL of 100 μM primer from 100 μM × 1,000 = 10 mM stock: V1 = (100 μM × 500 μL) / 10,000 μM = 5 μL. Add 5 μL of 10 mM stock to 495 μL water.

Storage best practices: Temperature: most aqueous stock solutions at 4°C (refrigerator). Freezing: enzyme solutions, antibodies, labile biological reagents → −20°C or −80°C in small aliquots to avoid freeze-thaw cycles. Light-sensitive solutions (ethidium bromide, fluorescent dyes, some drugs): store in amber tubes or wrap in foil. Sterility: filter-sterilize (0.22 μm membrane) solutions for cell culture use. Shelf life: inorganic salts (PBS, NaCl, Tris): months to years at 4°C. Organic reagents (IPTG, antibiotics): weeks to months at −20°C. Enzymes: days to months at −20°C. Biological reagents (antibodies, growth factors): follow manufacturer's recommendations. Labeling: always note preparation date and expected expiry. Never use unlabeled solutions.