A stock solution is a concentrated preparation of a chemical compound that you dilute down to a lower “working concentration” each time you run an experiment. Making one correctly means weighing or measuring your compound, dissolving it in the right solvent, and storing it so it stays stable for weeks or months. The process sounds simple, but the details matter: a poorly made stock can introduce errors that ripple through every experiment that draws from it. Getting those details right involves some straightforward math, thoughtful solvent choices, and storage practices that protect your reagent from degradation.
Why Bother with a Stock Solution
The practical reason is consistency and convenience. If you need a compound at, say, 10 micromolar in every assay plate you run this month, you do not want to weigh out milligrams of powder each time. Instead, you dissolve a larger quantity once at a much higher concentration, store it, and pipette a small volume into each experiment. This cuts down on weighing errors, saves material, and means every experiment draws from the same batch. When something goes wrong, you only have one preparation to troubleshoot rather than a dozen independent weigh-outs.
Stock solutions also let you work with compounds that are difficult to handle in tiny amounts. Many bioactive molecules are used at nanomolar or micromolar concentrations in cell culture or biochemical assays, which translates to microgram or sub-microgram quantities per well. Weighing that little powder accurately is nearly impossible on a standard analytical balance. By preparing a millimolar stock, you shift the measurement challenge to a volume that a calibrated pipette handles well.
Calculating the Right Concentration
Before you touch a balance, you need to know three things: what final working concentration your protocol calls for, roughly how much total working solution you will need over the stock’s useful life, and what concentration is realistic for your compound to stay dissolved. Common stock concentrations are 10 mM, 50 mM, or 100 mM for small molecules, though these are conventions rather than rules. The key methods for expressing concentration include molarity, percentage by weight or volume, and mass-per-volume units, and the formulas to move between them are well established in laboratory reference guides.1PubMed. Preparation of Solutions and Reagents
To prepare a molar stock, you need the compound’s molecular weight (listed on the certificate of analysis or the supplier’s website). Multiply the desired molarity by the molecular weight to get grams per liter, then scale to whatever volume you actually want. If you are making 10 mL of a 10 mM stock of a compound with a molecular weight of 300 g/mol, for instance, you need 0.03 grams of powder dissolved in solvent up to a final volume of 10 mL. Always dissolve the compound first in slightly less solvent than your target volume, then top up. Adding powder to an already measured-out volume of solvent will overshoot your final volume and throw off the concentration.
For percentage-based stocks, the math is simpler but the same principle holds: know whether the recipe means weight-per-volume (w/v), weight-per-weight (w/w), or volume-per-volume (v/v), because each one calculates differently. A 10% w/v solution of sodium chloride, for example, is 10 grams of salt per 100 mL of final solution, not per 100 mL of water. The distinction matters whenever your solute takes up meaningful volume.
Choosing the Right Solvent
Water is the default solvent for anything that dissolves readily in it, partly because most downstream experiments happen in aqueous environments. Deionized or ultrapure water is standard; tap water introduces ions and organics that can interfere with sensitive assays. When a compound will not dissolve in water at the concentration you need, organic solvents step in. DMSO (dimethyl sulfoxide) is the workhorse for hydrophobic small molecules in biology labs, because it is miscible with water and tolerated by most cell lines at low concentrations, usually below about 0.1 to 1% of the final culture volume.
Ethanol, methanol, and DMF (dimethylformamide) are other common choices, each with trade-offs in toxicity, evaporation rate, and compatibility with downstream applications. The guiding principle is that the solubility of a hydrophobic compound in a water-cosolvent mixture depends on how well the polarity of the mixture matches the polarity of the solute.2PubMed. Solubility enhancement of hydrophobic compounds by cosolvents: role of solute hydrophobicity on the solubilization effect In practical terms, this means a very hydrophobic drug may dissolve easily in DMSO at 50 mM but crash out of solution the moment you add it to an aqueous buffer, even at low dilution. You can sometimes get around this by adding the stock dropwise to rapidly stirring buffer, but testing for precipitation after dilution is always worth doing.
If you are working with acids or bases, pH-adjusted water or dilute acid/base solutions can improve solubility dramatically. Many amino acids and pharmaceutical compounds have ionizable groups that become charged at the right pH, pulling the molecule into aqueous solution. Just make sure the pH of your final diluted solution is still compatible with your experiment.
Getting the Compound Fully Dissolved
Dropping powder into solvent and hoping for the best is a recipe for a cloudy stock with an uncertain actual concentration. Vortexing is the first line of attack for most small-molecule stocks: a few seconds of vigorous mixing will dissolve many compounds instantly. For stubborn solutes, gentle warming in a water bath (30 to 37°C for most compounds, never higher unless you know the molecule is heat-stable) speeds dissolution by increasing molecular motion and solvent capacity.
Sonication is a powerful alternative for compounds that resist simple mixing. Plate-based sonication can dissolve compounds even in high-throughput 384-well plate formats, and it can re-dissolve precipitates that form in DMSO stocks after water uptake or repeated freeze-thaw cycling.3PubMed. High throughput sonication: evaluation for compound solubilization A standard ultrasonic bath works well for individual tubes. Keep the water level in the bath high enough that the tube sits submerged to the level of the liquid inside, and limit sonication time to avoid heating the solution excessively.
After dissolution, hold the tube up to a light source and look for particles, cloudiness, or a Tyndall effect (a visible beam of light scattering through the liquid). If you see any, your compound is not fully in solution. Either increase solvent volume (lowering your stock concentration), switch solvents, or filter out the undissolved material and note that your actual concentration is now lower than planned.
Diluting Stocks to Working Concentrations
The classic relationship for dilution is straightforward: the concentration of your stock multiplied by the volume you pipette out equals the concentration of your working solution multiplied by its final volume. If you have a 10 mM stock and need 100 µM in 10 mL of buffer, you add 100 µL of stock to 9,900 µL of buffer. This relationship scales cleanly for single dilutions.
Serial dilutions, where you make a series of progressively more dilute solutions by repeatedly diluting from the previous tube, require a bit more planning. The basic dilution relationship on its own does not specify the delivery volume at each step, which becomes important when you are automating the process or programming a liquid handler to execute a dilution series.4Cambridge Open Engage. Modification of the Basic Dilution Equation for the Programming of Serial Dilutions For manual serial dilutions, a common approach is to decide on a fixed dilution factor (for example, 1:3 or 1:10), calculate the volume of solution to carry forward, and add enough diluent to reach the next step’s total volume.
One practical pitfall: always change pipette tips between dilution steps. Carrying even a tiny residual droplet on the outside of a tip into the next tube adds unintended compound, and the error compounds with each step. Another: mix thoroughly at every stage. Under-mixing is the most common reason serial dilutions produce inconsistent dose-response curves.
Storing Stocks So They Last
Temperature is the single biggest factor in stock solution shelf life. Aqueous stocks of small molecules generally keep for days to weeks at 4°C and months at −20°C. DMSO stocks can be stored at −20°C or at room temperature depending on the compound; DMSO freezes at about 19°C, so a −20°C freezer will solidify it, which is fine as long as you thaw it completely and mix before pipetting. For long-term storage of valuable compounds, −80°C is common.
Freeze-thaw cycling is a real concern, not just a theoretical one. When researchers subjected a diverse set of 320 compounds in DMSO to 25 freeze-thaw cycles while exposing them to ambient air after each thaw, measurable degradation occurred for some compounds within just a handful of cycles.5Journal of Biomolecular Screening. The effect of freeze/thaw cycles on the stability of compounds in DMSO The practical lesson is to aliquot your stock into single-use portions before freezing. Aliquoting takes a few extra minutes up front but eliminates the problem entirely.
Proteins and other biological macromolecules are especially vulnerable to thermal damage. Interleukin-1β, for example, degrades through oxidation and aggregation at temperatures at or above 39°C, while at or below 30°C the degradation pathway shifts to a different chemical modification.6PubMed. Stability of interleukin 1 beta (IL-1 beta) in aqueous solution: analytical methods, kinetics, products, and solution formulation implications This is a good reminder that “cold” storage is not one-size-fits-all. The optimal temperature depends on the specific molecule and the degradation pathways it is prone to.
Protecting Stocks from Light
Many reagents are photosensitive. Fluorescent dyes, certain antibiotics (tetracyclines, for instance), retinoids, and light-sensitive ions like silver nitrate all degrade when exposed to UV or visible light. Amber glass or amber plastic tubes block the wavelengths responsible for photodegradation, extending shelf life and maintaining reagent potency. If amber containers are not available, wrapping clear tubes in aluminum foil achieves a similar effect, though it is less convenient for daily use.
The easiest habit to develop is to default to amber or foil-wrapped storage for any stock whose photosensitivity you are unsure about. The cost is zero, and the downside of not protecting a photosensitive reagent is silent degradation that you may not detect until your experiment fails.
Picking the Right Container
This seems trivial but it is not. The material your tube or bottle is made of can interact with your stock solution in ways that change its concentration or introduce contaminants. PVC-based plastics are a well-known offender: the plasticizer DEHP can leach from PVC containers into stored solutions, and the leaching can be the limiting factor for how long a solution remains usable rather than the stability of the compound itself.7Journal of Pharmaceutical and Biomedical Analysis. Stability, compatibility and plasticizer extraction of miconazole injection added to infusion solutions and stored in PVC containers The migration of phthalate-based plasticizers from PVC depends on the concentration gradient between the plastic and the liquid, meaning it can go in either direction depending on conditions.8Journal of the Brazilian Chemical Society. Migration of phthalate-based plasticizers from PVC and non-PVC containers and medical devices
For most lab purposes, polypropylene microcentrifuge tubes and conical tubes are the default, and they work well for aqueous and DMSO stocks of small molecules. Glass is preferred when working with organic solvents that attack plastics (chloroform, acetone, and some concentrated acids will dissolve or warp polypropylene). Borosilicate glass is chemically inert to nearly everything except hydrofluoric acid and strong hot alkali. For protein stocks, low-binding polypropylene tubes reduce the amount of protein that adsorbs to the tube walls, which matters when you are working at low concentrations where even a thin adsorbed layer represents a meaningful fraction of your total material.
Special Considerations for Protein and Enzyme Stocks
Proteins are not small molecules, and making a protein stock solution brings a different set of challenges. Solubility is often not the main concern; stability is. Many proteins denature (unfold and lose activity) at interfaces: the air-liquid surface, the plastic-liquid surface, even the boundary of a tiny air bubble introduced by pipetting. Adding a low concentration of a non-ionic surfactant like Tween-20 or Tween-80 (often 0.01 to 0.05%) helps coat these interfaces and shield the protein.
Viscosity is another factor you will not encounter with small molecules. At high protein concentrations, solutions become noticeably viscous, which makes accurate pipetting harder. Researchers studying concentrated antibody formulations have found that the viscosity of protein mixtures can reach hundreds or even over a thousand centipoise at total protein concentrations above 200 mg/mL.9Journal of Pharmaceutical Sciences. Viscosity behavior of high-concentration protein mixtures For practical lab work, if your protein stock is viscous enough that you can see it clinging to the pipette tip, slow down your pipetting, use positive-displacement pipettes if available, and always check your delivered volume gravimetrically (by weighing) rather than trusting the pipette setting.
Enzyme stocks often need stabilizing additives. Glycerol at 50% is a common storage additive because it depresses the freezing point, preventing ice crystal formation that damages protein structure, and it stabilizes many enzymes against thermal denaturation. BSA (bovine serum albumin) at low concentrations acts as a carrier protein that reduces adsorptive losses to tube walls. These additives become part of your stock’s recipe and need to be accounted for when you dilute to a working concentration, since glycerol in particular can affect some downstream reactions.
Detergent Stocks and Micelle Behavior
If your experiment involves membrane proteins, lipid bilayers, or cell lysis, you will likely be making stock solutions of detergents. These come with a unique wrinkle: above a certain concentration called the critical micelle concentration, detergent molecules spontaneously form spherical aggregates (micelles) in solution. A stock well above this threshold is stable and consistent, but diluting near or below it changes the physical form of the detergent in solution, which can affect how it interacts with your target.
For membrane protein work, the choice of detergent is not just about dissolving the protein but about mimicking the hydrophobic environment of a cell membrane. Research on G-protein coupled receptors has shown that the headgroup chemistry and hydrocarbon chain length of the detergent directly influence the morphology and composition of mixed micelles, and that features like the hydrophobic thickness of the micelle need to approximate the thickness of a native cell membrane to preserve protein activity.10PubMed Central. Toward rational design of protein detergent complexes: determinants of mixed micelles that are critical for the in vitro stabilization of a G-protein coupled receptor The take-home message is that detergent stocks are straightforward to prepare, but the downstream dilution and the specific detergent you choose can make or break the experiment.
When Automation Helps
If you are making the same stocks repeatedly, or preparing dozens of stocks for a screening campaign, automated liquid handlers can take over the repetitive pipetting. Pharmaceutical, petrochemical, and clinical research labs increasingly use robotic sample preparation to improve throughput and reduce human variability.11PubMed Central. Pitfalls and Inherent Biases in Liquid Handling Robotics: Investigations in Automation for SI Traceable Measurements But automation is not a magic fix. Liquid handlers have their own accuracy and precision profiles that vary by the type of liquid (viscous solutions behave differently than water), the volume being dispensed, and the specific hardware involved.
Calibrating a liquid handler is itself a non-trivial task. Automated gravimetric calibration procedures, in which the robot dispenses liquid onto a balance and adjusts its settings based on the measured weight, have been developed to optimize accuracy for specific reagents and volume ranges.12PubMed Central. Automated Gravimetric Calibration to Optimize the Accuracy and Precision of TECAN Freedom EVO Liquid Handler If you are working in a lab that uses robotic systems, it is worth asking when the instrument was last calibrated and with what liquid. A robot calibrated for water may under-deliver a viscous DMSO stock or over-deliver a volatile organic solvent.
Common Mistakes and How to Avoid Them
Having walked through the full process, here are the errors that trip up even experienced researchers:
- Ignoring purity: The molecular weight on the bottle assumes pure compound. If your reagent is a hydrate or a salt form, the effective molecular weight is higher. A monohydrate of a 200 g/mol compound has a molecular weight of 218 g/mol. Use the form you actually have in hand, not the anhydrous weight listed in a textbook.
- Topping up to volume incorrectly: Dissolve the compound in most of the solvent first, then bring the total volume up to the target mark using a volumetric flask or a graduated cylinder. Adding all the solvent first and then the compound will overshoot.
- Assuming solubility equals stability: A compound may dissolve completely in DMSO at 50 mM, giving you a clear solution, then degrade over days due to oxidation or hydrolysis from trace water. Solubility at the moment of preparation does not guarantee the stock will still be active next month.
- Skipping filtration: For any stock that will be added to sterile cell culture, filter-sterilize through a 0.2 µm syringe filter. This removes both microbial contaminants and undissolved particles. Be aware that some compounds bind to certain filter membranes, particularly nylon filters with hydrophobic compounds. PTFE or PVDF membranes are more universally compatible.
- Not labeling thoroughly: Every stock tube should show the compound name, concentration, solvent, date of preparation, and your initials at minimum. A freezer full of unlabeled tubes is functionally the same as an empty freezer.
Checking That Your Stock Is What You Think It Is
Preparing a stock carefully does not guarantee it is correct. Verification is worthwhile for any stock that will underpin a large experiment or a long campaign of work. The simplest check is a UV-visible spectrophotometry reading: many compounds have a known extinction coefficient at a specific wavelength, so measuring absorbance and back-calculating concentration takes only a minute. For protein stocks, a Bradford or BCA assay gives you actual protein concentration, which may differ from what you calculated if the protein partially precipitated or adsorbed to the tube.
For small-molecule stocks in screening libraries, mass spectrometry coupled with liquid chromatography can confirm both the identity and concentration of the compound. This level of quality control is routine in pharmaceutical high-throughput screening facilities but less common in academic labs, where the cost per sample is harder to justify. At minimum, if an experiment fails unexpectedly, re-making the stock from fresh powder is a faster diagnostic step than most people realize and should be early in your troubleshooting checklist rather than a last resort.