What is Secondary Standard Substance?

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Grade Level:

Class 12

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Definition

What is it?

A secondary standard substance is a chemical substance whose exact concentration is not precisely known and must be determined by reacting it with a primary standard substance. It is less stable and may react with air or moisture, making its concentration change over time.

Simple Example

Quick Example

Imagine you have a bottle of 'chai masala' powder, but you don't know its exact strength (how much spice is in it). You can use a 'known' strong tea (primary standard) to taste and figure out the exact strength of your masala. The 'chai masala' is like the secondary standard because its strength needs to be checked.

Worked Example

Step-by-Step

Let's say we want to find the exact concentration of a NaOH (sodium hydroxide) solution (our secondary standard).

Step 1: We take a known primary standard, Oxalic Acid, with a precise concentration of 0.1 M (moles per litre).
---Step 2: We accurately measure 25 mL of this 0.1 M Oxalic Acid solution into a flask.
---Step 3: We fill a burette with our unknown NaOH solution.
---Step 4: We slowly add the NaOH solution from the burette to the Oxalic Acid in the flask, using an indicator like phenolphthalein. The indicator changes colour when the reaction is complete (neutralization point).
---Step 5: We note down the volume of NaOH solution used. Let's say we used 20 mL of NaOH.
---Step 6: Using the formula M1V1 = M2V2 (for 1:1 reactions, or adjusted for stoichiometry), we can calculate the NaOH concentration. For Oxalic Acid (H2C2O4) and NaOH, the reaction is H2C2O4 + 2NaOH -> Na2C2O4 + 2H2O. So, (M_acid * V_acid) / n_acid = (M_base * V_base) / n_base, where n is the stoichiometric coefficient.
---Step 7: (0.1 M * 25 mL) / 1 = (M_NaOH * 20 mL) / 2.
---Step 8: M_NaOH = (0.1 * 25 * 2) / 20 = 5 / 20 = 0.25 M. So, the concentration of the NaOH secondary standard is 0.25 M.

Why It Matters

Understanding secondary standards is crucial in quality control for medicines and food, ensuring they have the correct strength. Chemists in labs, doctors testing blood samples, and environmental scientists monitoring water quality all use this concept. It helps ensure products are safe and effective, and measurements are accurate.

Common Mistakes

MISTAKE: Thinking secondary standards are always 'less pure' than primary standards. | CORRECTION: While purity is a factor, the main difference is their stability and ability to be weighed accurately to prepare a solution of known concentration. Secondary standards are less stable and react with the environment.

MISTAKE: Assuming a secondary standard solution's concentration remains constant over a long time. | CORRECTION: Secondary standards, like NaOH, can absorb moisture or CO2 from the air, changing their concentration. They need to be standardized (checked) regularly.

MISTAKE: Using a secondary standard directly without standardizing it first against a primary standard. | CORRECTION: You must always standardize a secondary standard against a primary standard to accurately determine its concentration before using it for other experiments.

Practice Questions

Try It Yourself

QUESTION: Why is NaOH considered a secondary standard substance? | ANSWER: NaOH is considered a secondary standard because it readily absorbs moisture and carbon dioxide from the atmosphere, causing its concentration to change over time, and it is difficult to weigh accurately due to its hygroscopic nature.

QUESTION: A student wants to determine the concentration of an HCl solution. Which type of standard (primary or secondary) would the HCl solution most likely be? Explain why. | ANSWER: The HCl solution would most likely be a secondary standard. This is because concentrated HCl is volatile and its exact concentration can change when diluted, requiring standardization against a primary standard like sodium carbonate.

QUESTION: If you are given a solid substance and asked to prepare a solution of known concentration, what properties would you look for to decide if it can be used as a primary standard or if it would be a secondary standard that needs standardization? List at least three properties. | ANSWER: For a primary standard: 1. High purity (99.9% or more). 2. Stable in air and non-hygroscopic (doesn't absorb water). 3. High molecular weight (reduces weighing errors). For a secondary standard, it would lack these properties, being less pure, less stable, or hygroscopic.

MCQ

Quick Quiz

Which of the following is a key characteristic of a secondary standard substance?

Its concentration is precisely known from its preparation.

It is highly stable and does not react with atmospheric components.

Its concentration must be determined by titration against a primary standard.

It has a very high molecular weight, making weighing accurate.

The Correct Answer Is:

Secondary standards are substances whose exact concentration is not precisely known and must be found out by reacting them with a primary standard. Options A, B, and D describe properties of a primary standard.

Real World Connection

In the Real World

In an Indian pharmaceutical company, chemists routinely use secondary standards to check the strength of acidic or basic ingredients in medicines. For example, a batch of diluted sulfuric acid (a secondary standard) might be used to check the purity of a drug. Before using it, the acid's exact concentration is verified against a primary standard to ensure the medicine's dosage is correct and safe, just like how a 'halwai' might check the sweetness of his 'chashni' (sugar syrup) before making sweets.

Key Vocabulary

Key Terms

PRIMARY STANDARD: A highly pure, stable substance of precisely known concentration used to standardize other solutions. | TITRATION: A laboratory method to determine the concentration of an unknown solution by reacting it with a solution of known concentration. | STOICHIOMETRY: The relationship between the amounts of reactants and products in a chemical reaction. | HYGROSCOPIC: A substance that readily absorbs moisture from the atmosphere. | CONCENTRATION: The amount of a substance dissolved in a given volume of solution.

What's Next

What to Learn Next

Now that you understand secondary standards, you should learn about 'Primary Standard Substances'. Knowing about primary standards will help you fully grasp how we accurately determine concentrations in chemistry and why some chemicals are chosen over others for specific tasks.