STANDARDIZATION OF SOLUTIONS: WHAT IT CONSISTS OF AND EXERCISES - CHEMISTRY - 2025

The standardization of solutions is a process that allows the exact determination of the concentration of a solution. The substances used for this purpose are called primary standards.

A solution is standardized using the volumetric titration (titration) method, either by classical or instrumental techniques (potentiometry, colorimetry, etc.).

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To do this, the dissolved species reacts with a weighed primary standard in advance. Therefore, the use of volumetric balloons is essential for these quantitative analyzes.

For example, sodium carbonate is a primary standard that is used in the standardization of acids, including hydrochloric acid that becomes a titrant, since it can be used in the titration of sodium hydroxide. Thus, the basicity of a sample can be determined.

Volumes of the titrant are added continuously until it has reacted with an equivalent concentration of the analyte. This indicates that the equivalence point of the degree has been reached; in other words, the titrant "neutralizes" the analyte completely by transforming it into another chemical species.

It is known when the addition of the titrant should be finished by using indicators. The instant in which the indicator changes color is called the end point of the titration.

What is standardization?

Standardization is nothing more than obtaining a secondary standard that will serve for quantitative determinations. How? Because if you know its concentration, you can know what the analyte will be once it has been titrated.

When high accuracy is required in the concentration of the solutions, both the titrant solution and the solution to which it is going to titrate are standardized.

Reactions in which the titration method is used include:

-Acid-base reactions. Using the volumetric method, the concentration of many acids and bases can be determined.

-The oxide reduction reactions. Chemical reactions that involve oxidation are widely used in volumetric analysis; such as, for example, iodometric determinations.

-The precipitation reactions. The silver cation precipitates together with an anion from the group of halogens, such as chlorine, obtaining silver chloride, AgCl.

-Reactions of complex formation, for example the reaction of silver with the cyanide ion.

Characteristics of the primary standards

The substances used as primary standards must meet a series of requirements to fulfill their standardization function:

-Have a known composition, since otherwise it will not be known exactly how much of the standard should be weighed (much less calculate its subsequent concentration).

-Be stable at room temperature and withstand the temperatures necessary for drying in the oven, including temperatures equal to or higher than the boiling temperature of water.

-Have great purity. In any case, the impurities should not exceed 0.01 to 0.02%. In addition, impurities can be determined qualitatively, which would facilitate the removal of possible interferents in the analyzes (erroneous volumes of titrant used, for example).

-Be easy to dry and cannot be so hygroscopic, that is, they retain water during drying. Nor should they lose weight when exposed to air.

-Do not absorb gases that can produce interferences, as well as the degeneration of the pattern

-React quickly and stoichiometrically with the titrant reagent.

-Have a high equivalent weight that reduces errors that may be made during the weighing of the substance.

Examples of primary standards

To standardize bases

-Sulfosalicylic acid

-Benzoic acid

-Potassium acid phthalate

-Sulfanilic acid

-Oxalic acid

To standardize acids

-Trishydroxymethyl aminomethane

-Sodium carbonate

-Borax (mixture of boric acid and sodium borate).

-Tri-hydroxymethyl-aminomethane (known as THAM)

To standardize redox reagents

-Arsenous oxide

-Iron

-Potassium dichromate

-Copper

Exercises

Exercise 1

A certain amount of sodium carbonate (primary standard for acids) weighing 0.3542 g is dissolved in the water and titrated with a hydrochloric acid solution.

To reach the turning point of the methyl orange indicator, added to the sodium carbonate solution, 30.23 mL of the hydrochloric acid solution were spent. Calculate the concentration of the HCl.

This is the solution that is going to be standardized, using sodium carbonate as the primary standard.

Na ₂ CO ₃ + 2 HCl => 2 NaCl + H ₂ O + CO ₂

pEq (Na ₂ CO ₃ = pm / 2) (molecular weight of Na ₂ CO ₃ = 106 g / mol)

pEq = (106 g / mol) / (2 Eq / mol)

= 53 g / Eq

At the equivalence point:

mEq HCl = mEq of Na ₂ CO ₃

VHCl x N HCl = mg Na ₂ CO ₃ / pEq N _a CO ₃

30.23 mL x N HCl = 354, mg / (53 mg / mEq)

And then clearing the normality of the HCl, N:

30.23 mL x N HCl = 6.68 mEq

N HCl = 6.68 mEq / 30.23 mL

N HCl = 0.221 mEq / mL

Exercise 2

Potassium phthalate (KHP) is used to standardize a NaOH solution, a primary standard that is an air-stable solid that is easy to weigh.

1.673 grams of potassium phthalate are dissolved in 80 mL of water and 3 drops of a phenolphthalein indicator solution are added, which develops a pink color at the end point of the titration.

Knowing that the KHP titration consumes 34 mL of NaOH, what is its normality?

Equivalent weight of potassium phthalate = 204.22 g / Eq

At the equivalence endpoint:

Equivalents of NaOH = Equivalents of KHP

VNaOH x N = 1.673 g / (204.22 g / Eq)

Equivalents of KHP = 8,192 · 10 ^-3 Eq

So:

V NaOH x N OH = 8.192 · 10 ^-3 Eq

And since 34 mL (0.034L) were used, it is substituted in the equation

N NaOH = (8.192 · 10 ^-3 Eq / 0.034 L)

= 0.241 N

Exercise 3

A sample of pure CaCO ₃ (a primary standard) weighing 0.45 g, is dissolved in a volume of water, and after solubilizing it, is completed with water at 500 mL in a volumetric flask.

Take 100 mL of the calcium carbonate solution and place it in an Erlenmeyer flask. The solution is titrated with 72 mL of ethylenediaminetetraacetic acid (EDTA), using eriochrome black indicator T.

Calculate the molarity of the AEDT solution

At the equivalence point of the degree:

mmol EDED = mmol CaCO ₃

V x molarity of EDTA = mg CaCO ₃ / PM CaCO ₃

From the dissolution of calcium carbonate in 500 mL, 100 mL was taken for the titration, that is, 0.09 g (one-fifth of 0.45g). Thus:

0.072 L x M EDTA = 0.09 g / 100.09 g / mol

M of AEDT = 8.99 10 ^-4 mol / 0.072 L

= 0.0125

References

1. Galano Jiménez A. & Rojas Hernández A. (nd). Standard substances for standardization of acids and bases.. Recovered from: depa.fquim.unam.mx
2. Standardization of titrant solutions.. Recovered from: ciens.ucv.ve:8080
3. Wikipedia. (2018). Standard solution. Recovered from: es.wikipedia.org
4. Chemistry 104: Standardization of Acid and Base Solutions. Recovered from: chem.latech.edu
5. Day, RA and Underwood, AL (1989). Quantitative Analytical Chemistry. 5th Edition. Pearson, Prentice Hall.