Dissolve exactly 40.00 grams of sodium hydroxide in enough distilled water to bring the total volume to one liter, and you have a 1M NaOH solution. The number 40 comes from sodium hydroxide’s molar mass: one mole weighs 40.00 grams. The procedure itself is straightforward, but the details matter more than you might expect. NaOH dissolves with a surprising amount of heat, reacts readily with carbon dioxide in the air, and can cause serious chemical burns on contact with skin. Getting the concentration right and keeping it stable over time takes a bit more care than just dumping pellets into a beaker.
What You Need Before You Start
Gather your materials and equipment before opening any containers. For one liter of 1M NaOH, you need:
- Sodium hydroxide: 40.00 g of NaOH pellets, flakes, or beads (analytical-grade or reagent-grade, depending on your application)
- Distilled or deionized water: roughly 800 mL to start, with more to bring the final volume to 1 L
- A volumetric flask or graduated cylinder: 1 L capacity for the final volume measurement
- A heat-resistant beaker: borosilicate glass (like Pyrex) or polypropylene, at least 1.5 L capacity to allow room for mixing
- A magnetic stir bar or glass stirring rod
- An analytical balance: capable of reading to at least 0.01 g
- Personal protective equipment: chemical splash goggles, nitrile or neoprene gloves, and a lab coat at minimum
One important note about the container: do not use a volumetric flask for the actual dissolving step. The heat generated when NaOH meets water can crack standard glassware and will throw off the calibrated volume markings. Use a sturdy beaker or Erlenmeyer flask for dissolving, then transfer and dilute to volume in the volumetric flask once the solution has cooled.
Step-by-Step Procedure
Weigh 40.00 grams of NaOH on a balance. Work quickly once the container of pellets is open, because sodium hydroxide is hygroscopic and starts absorbing water from the air almost immediately. It also reacts with atmospheric carbon dioxide, forming a thin crust of sodium carbonate on the surface of each pellet. The longer the pellets sit exposed, the less accurate your measurement becomes.
Pour roughly 700 to 800 mL of distilled water into your mixing beaker. Then add the NaOH to the water, not the other way around. This is the single most important procedural rule when working with concentrated bases or acids. Adding water to a pile of dry NaOH can cause localized boiling and spattering of caustic liquid. Adding the solid to a large volume of water disperses the heat far more safely.
Stir continuously as you add the pellets. They dissolve quickly in water, but clumps can form on the bottom of the beaker if you add everything at once. A magnetic stir plate works well here. The solution will heat up noticeably within seconds, and with 40 grams going into less than a liter of water, the temperature can climb by 10 to 20 °C depending on your starting water temperature and how fast you add the pellets.
Once all the NaOH has dissolved and the solution is clear, let it cool to room temperature. This is not optional. Liquids expand when warm, so measuring your final volume while the solution is still hot will give you a concentration that is slightly too high once it cools. After the solution reaches about 20–25 °C, transfer it to a 1 L volumetric flask and add distilled water until the bottom of the meniscus sits exactly at the 1 L graduation mark. Stopper the flask and invert it several times to ensure thorough mixing.
Why the Solution Gets So Hot
Dissolving NaOH in water is strongly exothermic, meaning it releases a substantial amount of heat. This is not a subtle warming. Research on the thermal effects of dissolving various alkalis and salts has confirmed that sodium hydroxide is among the best performers for heat generation, particularly under stirred conditions.1EDP Sciences (E3S Web of Conferences). Experimental Study of the Thermal Effect of the Dissolution Reaction for Some Alkalis and Salts with Natural Mixing and Forced Stirring At higher concentrations the temperature spike gets more dramatic: preparing a 10M or even 5M NaOH solution can bring the mixture close to boiling if you add the pellets too quickly.
For a 1M solution, the heat release is manageable but still enough to crack a cold thin-walled glass container or cause a burn if the solution splashes on bare skin. Using a Pyrex or polypropylene container and adding pellets gradually eliminates most of the risk. If you are making multiple liters, consider placing the beaker in an ice bath to absorb the excess heat, especially in warm ambient conditions.
Safety Precautions That Actually Matter
NaOH is corrosive at virtually any concentration you would prepare in a lab. A 1M solution has a pH of about 14 and can cause chemical burns on skin within seconds of contact. Eye exposure is the most serious risk: even a small splash can cause permanent corneal damage. Splash goggles, not safety glasses, are the correct choice because goggles seal against the face and prevent liquid from reaching your eyes from the sides or below.
Gloves should be nitrile or neoprene. Latex gloves degrade in concentrated NaOH and can give you a false sense of security. If solution contacts your skin, flush with large amounts of water for at least 15 to 20 minutes. Unlike acid burns, base burns tend to feel slippery rather than immediately painful, which means people sometimes do not notice exposure right away. The damage can be deeper by the time it starts to hurt.
Work in a well-ventilated area or near a fume hood if you are dissolving large quantities. NaOH itself does not produce dangerous vapors at room temperature, but the heat of dissolution can generate steam that carries fine caustic droplets. When making concentrated solutions above about 6M, fume hood use becomes genuinely important rather than just good practice.
Keep a clearly marked spill kit nearby. Solid NaOH spills can be swept up and neutralized with a weak acid like dilute citric acid or vinegar. Liquid spills should be contained, then neutralized slowly. Do not pour a strong acid directly onto a large NaOH spill: the resulting exothermic neutralization reaction can cause spattering.
Why Water Quality Matters More Than You Think
For routine work like cleaning, pH adjustment, or general chemistry labs, distilled water is perfectly fine. But for analytical chemistry, electrochemistry, or any application where trace metal contamination matters, the purity of both the water and the NaOH itself becomes a real concern.
Even the highest-purity commercially available NaOH pellets, rated at around 99.99% purity, contain trace amounts of dissolved metals. Electrochemical experiments using platinum electrodes in 0.1M NaOH have demonstrated that metals like copper, zinc, and lead from the NaOH pellets deposit onto electrode surfaces during reactions, blocking catalytic sites and skewing results.2Journal of The Electrochemical Society. Fundamental Aspects of Contamination during the Hydrogen Evolution/Oxidation Reaction in Alkaline Media The contamination comes from the electrolyte itself, not from the electrode material, and similar issues appear with other alkali hydroxides like KOH and LiOH.
If your work requires extremely clean NaOH solutions, several options exist. One approach is to prepare a saturated NaOH solution (around 50% by weight) and let it settle for days. Sodium carbonate, the most common impurity from air exposure, is much less soluble in concentrated NaOH than in dilute solution, so it precipitates out. You then carefully pipette the clear supernatant and dilute it to 1M. Another approach is to purchase semiconductor-grade or trace-metal-grade NaOH, though the cost is substantially higher. For electrochemistry, some researchers pre-electrolyze their NaOH solutions to strip out trace metals before running experiments.
Scaling Up or Down
The math for different volumes is simple: 40 grams per liter. If you only need 500 mL, weigh 20.00 g. For 250 mL, weigh 10.00 g. For 100 mL, weigh 4.00 g. Always dissolve in roughly 70 to 80% of your target volume first, then top up after cooling.
If you need a different molarity, adjust the mass proportionally. For a 0.1M solution, you need 4.00 g per liter. For 2M, you need 80.00 g per liter. Above about 6M (240 g/L), the dissolution becomes aggressive enough that you should add pellets in small batches, stirring continuously, and seriously consider an ice bath. A 10M NaOH solution (400 g/L) is thick, viscous, and generates enough heat to be hazardous if prepared carelessly.
A common school-lab scenario involves making NaOH solutions from a stock concentrate rather than from pellets. If you have a 10M stock solution and need 1M, dilute 100 mL of stock to a final volume of 1 L. Again, add the concentrated solution to the water, stir, cool, and adjust volume. The heat release when diluting a concentrated NaOH solution is still significant, so the same caution applies.
Storage and Shelf Life
Store 1M NaOH in a tightly sealed high-density polyethylene (HDPE) bottle. NaOH solutions slowly etch glass, and over weeks to months a glass-stored solution will absorb silicates from the container walls, changing both the concentration and the impurity profile. Polypropylene and HDPE are resistant to NaOH at this concentration and work well for long-term storage.
The bigger enemy is carbon dioxide. NaOH reacts with CO₂ from the air to form sodium carbonate, and this reaction is not negligible. Research on NaOH-based carbon capture systems has studied this process extensively, and it is well established that even a 1M NaOH solution exposed to ambient air absorbs CO₂ at a meaningful rate.3Elsevier. Low-energy sodium hydroxide recovery for CO2 capture from atmospheric air—Thermodynamic analysis Every molecule of CO₂ that reacts with your solution converts two NaOH molecules into sodium carbonate, reducing your effective NaOH concentration. A solution left with the cap off for days can shift measurably from its original 1M value.
To minimize COâ‚‚ absorption, fill containers as full as practical to reduce the headspace above the liquid. Some labs use bottles with COâ‚‚-absorbing traps (tubes filled with a drying agent and soda lime) attached to the bottle cap, allowing air exchange without COâ‚‚ ingress. For most purposes, simply keeping the cap tight and not leaving the bottle open longer than necessary will keep the solution usable for weeks to months.
Label every container with the concentration, the date of preparation, and who made it. NaOH solutions do not change color or develop an obvious smell as they degrade, so you cannot tell by looking at a bottle whether it is still at its target concentration. If accuracy matters for your work, re-standardize the solution by titrating it against a primary standard acid like potassium hydrogen phthalate (KHP) before use.
Common Mistakes and How to Avoid Them
The most frequent error, especially in teaching labs, is weighing NaOH too slowly. Students place a weighing boat on the balance, open the bottle of pellets, and carefully pick them out one by one. During that time, the pellets absorb water from the air and gain mass, and the weighed amount ends up containing less actual NaOH than the balance reads. Work briskly: open the bottle, pour out approximately the right amount, close the bottle, then fine-tune on the balance. The whole operation should take under a minute.
A related mistake is using NaOH from a bottle that has been sitting open or poorly sealed for months. If the pellets look glassy and wet or have a chalky white crust, they have already absorbed water and COâ‚‚. The crust is sodium carbonate. You can still use these pellets, but your actual NaOH content per gram is lower than expected, and your solution will be below 1M unless you account for the degradation. For critical work, start with a freshly opened, well-sealed container.
Measuring the final volume while the solution is still warm is another common pitfall. Water at 40 °C occupies about 1% more volume than at 20 °C. That might sound trivial, but it means your “1.00 M” solution is actually about 1.01 M once it cools, which matters for analytical titrations and standardized procedures. Always bring the solution to room temperature before making the final volume adjustment.
Finally, some protocols call for adding NaOH pellets directly to the volumetric flask and filling with water. This is bad practice for two reasons. The heat can damage the flask, especially if it was calibrated at 20 °C. And the rapid local heating as pellets dissolve on the bottom creates convection currents that make it hard to see the meniscus clearly. Dissolve first in a beaker, cool, then transfer.
When Pellets Are Not the Best Starting Material
NaOH pellets are the most common form sold, but they are not the only option. Flakes dissolve faster because of their higher surface area, and they are easier to weigh in small increments. Beads (sometimes called prills) are small spheres that flow freely and are less prone to clumping, making them popular in industrial settings.
For applications requiring very precise concentrations, some labs skip solid NaOH entirely and buy certified standard solutions. These come in sealed glass ampoules at a precise molarity, verified by the manufacturer against a primary standard. You break the ampoule, pour the contents into a volumetric flask, and dilute to volume. The cost per liter is much higher than making your own, but the accuracy is guaranteed. This approach is common in quality control labs, pharmaceutical testing, and environmental analysis where regulatory standards require documented traceability of reagent concentrations.
Another alternative gaining popularity in teaching settings is pre-measured NaOH capsules or sachets designed to produce a specific volume and concentration when dissolved. These reduce the risk of handling loose caustic pellets and remove the weighing step entirely, though they trade convenience for flexibility: you get exactly the concentration printed on the packet and nothing else.
Checking Your Work by Titration
If you need to know the exact concentration of your NaOH solution rather than trusting that your weighing was perfect, you standardize it. The standard method uses potassium hydrogen phthalate (KHP), a solid acid that can be dried and weighed with high accuracy. You dissolve a known mass of KHP in water, add a few drops of phenolphthalein indicator, and titrate with your NaOH solution until the solution turns pink. From the volume of NaOH required to neutralize the known amount of KHP, you can calculate the true molarity to three or four significant figures.
This step is essential for any quantitative analytical work. The small errors from hygroscopic absorption during weighing, COâ‚‚ contamination during storage, and volume changes from temperature can add up. A solution you prepared as “1.000 M” might actually be 0.985 M or 1.012 M. Titration tells you exactly where you stand. In a research or quality-control setting, recording the standardization date and result on the bottle label is standard practice, and re-standardizing every few weeks catches any drift from COâ‚‚ absorption over time.