The sodium hydroxide is cannot be used as a primary standard, mainly because NaOH is hygroscopic and absorbs carbon dioxide (CO2) from the air.
Titration is a process in which a small amount of reagent is added to a solution until a chemical reaction (endpoint or equivalence point) occurs. The reaction confirms that the sample solution is at a certain concentration.
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We utilize standards that contain known amounts of analyte to standardize an analytical method. Standardization is a titration technique that uses a standard solution as a reference to determine the exact concentration of a prepared solution. In chemistry, standardization is a titration technique that uses a standard solution as a reference to determine the precise concentration of a prepared solution. Standard solutions are prepared using standard compounds and have concentrations that are accurately calculated.
What is primary and secondary standard?
The two most common forms of standard solutions are primary and secondary standard solutions. Primary standards are used to standardize secondary standard solutions, while secondary standards are used for certain types of analytical experiments.
Primary standards are very pure (99.9% accurate), and stable, with specific chemical and physical properties. As a result, we can make pure solutions using these compounds. Titration and other analytical procedures often use primary standards to determine unknown concentrations of solutes. Common examples of the secondary standard are sodium carbonate (Na2CO3), potassium dichromate (K2Cr2O7), and potassium hydrogen phthalate (KHP), etc.
The purity of the secondary standard solution is low, and the reactivity is high when compared to primary standards. Because of their high reactivity, these solutions are easily contaminated. A most common example of the secondary standard is sodium hydroxide (NaOH).
A primary standard should have the following properties:
- It must be very stable and non-toxic.
- It should not decompose, absorb or react with moisture or air.
- It must have a high degree of accuracy when weighing in the air or ambient temperature.
- It should dissolve easily in water or any other solvent.
- It must be available in a high purity state.
Why is NaOH not a primary standard?
Here are some most important reasons why solid NaOH is not used as a primary standard are mentioned.
- Because NaOH does not fulfill all of the preconditions/ criteria/properties for a good primary standard that was described above, it is not a suitable primary standard.
- When exposed to air for a few minutes, NaOH becomes very delinquent, meaning it gets watery.
- Because specific components in ambient air, such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), and some other acid gases are reacted quickly with it and it also continues to attract moisture, becomes increasingly diluted over time.
- NaOH solutions do not last long unless maintained under nitrogen since they absorb CO2 from the air very quickly, reducing the concentration.
Why KMnO4 is not used as a primary standard?
Potassium permanganate is not used as the primary standard, because it is not free of MnO2, the pure form of KMnO4 is difficult to obtain, it decomposes in presence of sunlight, It reacts readily with any trace of organic matter or any other reducing substance in water, and its color is intense that it acts as its self indicator.
Frequently Asked Questions (FAQ):
Why is NaOH not used as a primary standard for titration analysis?
The principle reason that NaoH is not used as the primary standard for titration is that the sodium hydroxide is highly hygroscopic and not available in high purity, it cannot be used to prepare primary standard titration solutions.
Is H2SO4 a primary standard?
No, sulfuric acid is not used as a primary standard because H2SO4 is hygroscopic, its concentration in air changes rapidly. Strong acids or bases, such as H2SO4, HCl, HNO3, NH4OH, and NaOH are not considered to be primary standard solutions.
Why does sodium hydroxide have to be standardized?
Before starting the titration, it is necessary to standardize the prepared molar/normal solution to determine the exact concentration.
References:
- Wikipedia contributors. "Sodium hydroxide." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Nov. 2021.
- Helmenstine, Anne. ‘What Is a Primary Standard in Chemistry?’ Science Notes and Projects, 29 Mar. 2021, https://sciencenotes.org/what-is-a-primary-standard-in-chemistry/.
- Study.Com, https://study.com/academy/answer/1-sodium-hydroxide-cannot-be-used-to-prepare-a-primary-standard-titration-solution-because-the-compound-is-not-available-in-high-purity-the-most-common-impurity-is-water-because-sodium-hydroxide-is-h.html.
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