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Sodium hydroxide solutions are some of the most widely used in different types of laboratories ranging from chemistry to biology and biochemistry, just to name a few. Solutions of this solute at different concentrations are used in a wide variety of applications ranging from chemical reagents for organic synthesis, titrating agents in acid-base titrations, to the preparation and pH adjustment of different buffer solutions.
Each application requires a different concentration level. Likewise, even in those cases in which solutions with similar concentrations are used, there are cases in which it is important to know this concentration very accurately in advance, while in others it is only necessary to know an approximate concentration.
This means that preparing a sodium hydroxide solution is not a simple matter of weighing and dissolving a solute. Depending on the application, different care will be required. Also, in some cases sodium hydroxide solutions can be prepared directly from the solid solute, but in other cases it is more convenient to prepare them by diluting a more concentrated starting solution.
Let’s start by exploring how to make a sodium hydroxide solution from the solid reagent.
Preparation of sodium hydroxide solutions from the solid reagent
The most common way to prepare a sodium hydroxide solution is by dissolving the reagent in the solid state and then diluting to the desired final volume. There are two different situations that require different procedures at the time of preparing the solution.
Approximate Concentration Solutions
In many situations, we only need to prepare solutions with an approximate concentration. For example, when we need a concentrated solution to use it as a catalyst in a hydrolysis reaction, or when we need a solution to neutralize a weak acid in the preparation of a buffer solution.
In these cases, it is not necessary to take excessive precautions or care when weighing or preparing the solution. The materials required for preparation are:
- An appropriately sized beaker (should be of a volume comparable to the amount of solution to be prepared).
- Volumetric flask.
- Analytical balance.
- Spatula.
- A weighing substance or, failing that, paper to weigh.
- Piseta (wash flask).
- stirring rod.
- Funnel.
Preparation of the solution
- Step 1: Weigh the sodium hydroxide.
Using the weighing scale or a paper to weigh and the analytical balance, the appropriate amount of sodium hydroxide is weighed. This is usually found in the form of white pearls with different degrees of purity. If obtaining an exact concentration is not important, then no special care is necessary when weighing the reagent.
Likewise, even in those cases in which it is necessary to know the exact concentration of the resulting solution, it is not useful to weigh sodium hydroxide with special care, since this reagent tends to react with carbon dioxide in the air and become contaminated. with sodium carbonate, so the mass that we weigh of this reagent will always be an approximation.
The mass of sodium hydroxide to be weighed depends both on the concentration of the solution to be prepared and on the total volume of the final solution. The following table shows the masses of sodium hydroxide that must be weighed to prepare different volumes of NaOH solution at different concentrations expressed both in molarity, normality and percentage m/V.
| Concentration (M or N) | %m/V | Solution Volume (mL) | NaOH mass (g) |
| 0.1 | 0.4 | 100 | 0.40 |
| 0.1 | 0.4 | 200 | 0.80 |
| 0.1 | 0.4 | 250 | 1.00 |
| 0.1 | 0.4 | 400 | 1.60 |
| 0.1 | 0.4 | 500 | 2.00 |
| 0.1 | 0.4 | 1,000 | 4.00 |
| 0.1 | 0.4 | 2,000 | 8.00 |
| 0.2 | 0.8 | 100 | 0.80 |
| 0.2 | 0.8 | 200 | 1.60 |
| 0.2 | 0.8 | 250 | 2.00 |
| 0.2 | 0.8 | 400 | 3.20 |
| 0.2 | 0.8 | 500 | 4.00 |
| 0.2 | 0.8 | 1,000 | 8.00 |
| 0.2 | 0.8 | 2,000 | 16.00 |
| 0.5 | 2.0 | 100 | 2.00 |
| 0.5 | 2.0 | 200 | 4.00 |
| 0.5 | 2.0 | 250 | 5.00 |
| 0.5 | 2.0 | 400 | 8.00 |
| 0.5 | 2.0 | 500 | 10.00 |
| 0.5 | 2.0 | 1,000 | 20.00 |
| 0.5 | 2.0 | 2,000 | 40.00 |
| 1.0 | 4.0 | 100 | 4.00 |
| 1.0 | 4.0 | 200 | 8.00 |
| 1.0 | 4.0 | 250 | 10.00 |
| 1.0 | 4.0 | 400 | 16.00 |
| 1.0 | 4.0 | 500 | 20.00 |
| 1.0 | 4.0 | 1,000 | 40.00 |
| 1.0 | 4.0 | 2,000 | 80.00 |
- Step 2: Transfer the reagent to a beaker and dissolve in distilled water.
Depending on the amount of solution to be made and the total mass of NaOH to be weighed, it is convenient to dissolve the reagent in a beaker before transferring the solution to the volumetric flask in which the final solution will be made.
Whether using a weighing scale or weighing paper, the reagent is transferred to a beaker containing a volume of water approximately half the volume of solution to be prepared. Any remains that remain adhered to the weight or paper are dragged with the help of the sink.
Then, using the stirring bar, the solution is stirred until all the solid is completely dissolved.
NOTE: The NaOH dissolution reaction is very exothermic, so it is common for it to heat up considerably during the dissolution process. It is advisable to carry out this process in an ice bath to absorb the heat released and thus facilitate the dissolution process.
- Step 3: Transfer to the volumetric flask.
Once all the solid is dissolved, the contents of the beaker are transferred to the flask through a funnel, dragging any remaining solution with the help of a spout.
- Step 4: Dilute with distilled water
Once all the contents of the beaker have been transferred, continue adding water until the flask is filled one or two centimeters below the gauging mark. It is advisable to let the flask stand for a few minutes to ensure that both the flask itself and its contents reach thermal equilibrium. Finally, it is finished filling up to the capacity mark using the piseta.
- Step 5: Cover and shake.
Once the flask has been reached, the stopper is placed and then shaken gently, inverting and straightening it in such a way that all the contents are mixed well and a homogeneous solution is obtained.
solutions of exact concentration
No matter how much care is taken in preparing a solution using the above procedure, and no matter how accurately the sodium hydroxide is weighed, the actual concentration of the solution will never exactly equal the nominal concentration being prepared. However, in many cases it is necessary to know the exact concentration of the freshly prepared solution, such as when said solution is to be used as a titrating agent in an acid-base titration.
If this is the case, the freshly prepared solution should be standardized or normalized by means of an acid-base titration of a suitable primary standard. A primary standard is a substance of high purity and stability that reacts quickly and quantitatively with another and that can be used as a standard for the determination of the real concentration of a solution.
There are several primary standards that can be used to standardize NaOH solutions, but the most common is potassium hydrogen phthalate or potassium biphthalate.
Standardization with potassium biphthalate
The procedure described here consists in the standardization of a sodium hydroxide solution whose concentration is approximately 0.1 N (or molar). To standardize any other solution, either the weighted primary standard mass can be adjusted, or the sodium hydroxide can be first diluted to a concentration of 0.1 N.
The procedure consists of accurately weighing a mass of about 0.5 g of potassium biphthalate and then dissolving it in a vial or Erlenmeyer flask in 25 mL of distilled water. A few drops of phenolphthalein indicator are added and the solution is then titrated using the NaOH solution.
Once the end point is reached, the volume of titrant is noted and the following formula is applied to determine the actual concentration of sodium hydroxide:
In the above equation, the mass of potassium biphthalate should be placed in grams and the volume of titrant (from NaOH) in milliliters.
For example, if 0.4958 g of potassium biphthalate were weighed and the titration consumed 24.35 mL of NaOH, it means that the actual NaOH concentration is 0.0997 N.
Standardization with benzoic acid
Standardization with benzoic acid is carried out in the same way as with potassium biphthalate, with the only difference that about 0.3 g of benzoic acid is accurately weighed instead of 0.5 g as in the standardization. last case. In this case, the formula to determine the actual NaOH concentration is:
Preparation of sodium hydroxide solutions by dilution
The other most common way to prepare NaOH solutions is by diluting more concentrated solutions. In a laboratory, it is very common to prepare a concentrated stock solution from which different dilutions are made as needed. The original solution usually has a concentration of 1 molar or 1 normal, or a similar concentration.
In those cases in which it is required to prepare much more dilute solutions such as 0.01 N or 0.001 N (which is very frequent), serial dilutions are usually prepared (that is, a first dilution is prepared, then this is diluted more and then the other is further diluted, and so on).
To carry out the dilutions you need:
- A volumetric ball of adequate capacity.
- A beaker.
- A volumetric pipet of the appropriate capacity.
- One foot.
The procedure is very simple:
- Step 1: Pour some of the concentrated solution into a clean, dry beaker.
It is important to never aliquot directly from the container containing the stock solution, as this could contaminate and ruin the entire batch.
- Step 2: With the volumetric pipet, measure the required volume of stock solution.
Care should be taken to fill the pipette to the graduation mark by holding it upright and ensuring that no air bubbles remain.
The amount of solution to be measured and the volumetric pipet to be used depend both on the amount of solution to be prepared and on the initial and final concertations.
- Step 3: Transfer the concentrated solution to a clean, dry volumetric flask and make to the mark.
This step simply consists of transferring the concentrated solution to the balloon and then diluting the solution with pure water using a bottle.
- Step 5: Cover and shake.
This step is the same as step 5 of the previous procedure.
References
Castro S., JM (2013). Procedure manual for the preparation of solutions in water characterization and quality laboratories and operation of water treatment plants. Retrieved from https://www.repositoriodigital.ipn.mx/bitstream/123456789/26030/1/PREPARACION%20DE%20SOLUCIONES%20%28bueno%29.pdf
Pico L., X. (nd). Standardization of titrating solutions. Retrieved from https://xavierpicolozano.files.wordpress.com/2017/03/estandarizacion.pdf
Ramirez S., MT, & Guzmán H., DS (2017). Sodium Hydroxide (NaOH) 1 M standardization. Retrieved from https://www.studocu.com/es-mx/document/universidad-autonoma-metropolitana/laboratorio-de-quimica-analitica/estandarizacion-de-hidroxido-de -sodium-naoh-1-m/2991623
Skoog, DA, West, DM, Holler, J., & Crouch, SR (2021). Fundamentals of Analytical Chemistry (9th edition). Boston, Massachusetts: Cengage Learning.
