Problem 24 - Chemistry in 30 days

Question

The Rubin number which was proposed by Ostwald as an alternative to the Gold number in order to measure the protective efficiency of a lyophilic colloid may be defined as the (a) mass in milligrams of a colloid per \(100 \mathrm{c.c}\). of solution which just prevents the colour change of standard sol of dye Congo-Rubin from red to violet when \(0.16 \mathrm{~g}\) eq. \(\mathrm{KCl}\) is added to it. (b) mass in grams of a colloid per \(100 \mathrm{c.c}\). of solution which just prevents the colour change of standard sol of dye Congo-Rubin from red to violet when \(0.1 \mathrm{M}\) is added to it. (c) mass in grams of a colloid per \(100 \mathrm{c.c}\), of solution which just prevents the color change of standard sol of dye Congo-Rubin from red to violet when \(0.2 \mathrm{M} \mathrm{KCl}\) is added to it. (d) mass in grams of a colloid per 100 c.c. of solution which just prevents the color change of standard sol of dye Congo-Rubin from red to violet when \(1 \mathrm{M} \mathrm{KCl}\) is added to it.

Step-by-Step Solution

Answer

(c) mass in grams per 100 c.c. with 0.2 M KCl.

Step 1: Understand the Problem Statement

The Rubin number measures the protective efficiency of lyophilic colloids in preventing color change in a dye solution. We need to identify the condition where a specific concentration of KCl (potassium chloride) is added, and the mass in grams of colloid per 100 c.c. of solution just prevents a color change from red to violet.

Step 2: Analyze the Options Given

Four options are given with varying concentrations of KCl and units of colloid mass. Each option involves a different molarity of KCl or mass of colloid to prevent the color change upon addition of KCl to the dye solution. We need to find the correct definition of the Rubin number based on these as proposed by Ostwald.

Step 3: Identify the Correct Definition

For substances like Congo-Rubin dye, KCl is used to initiate a color change due to its ionic strength. The Rubin number reflects the protective efficiency necessary to prevent this change. Option (a) describes mass in milligrams, while (b), (c), and (d) use mass in grams per 100 c.c of solution. The inclusion of specific molarities and "mass in grams" as part of the definition indicates a standardized condition similar to the definition provided by Ostwald.

Step 4: Select the Correct Option

The correct definition typically uses a molarity close to standard laboratory concentrations for consistency in measurement. Option (c) states the mass in grams of colloid per 100 c.c. needed to prevent this color change using a KCl concentration of 0.2 M, which is a typical experimental concentration.

Key Concepts

lyophilic colloids

Lyophilic colloids, also known as hydrophilic colloids, are a type of colloidal system where the particles of the dispersed phase exhibit a strong affinity towards the solvent. This affinity often results in the formation of a stable colloidal dispersion. Lyophilic colloids are important in many industrial and biological processes due to their stability and ability to be redispersed after drying.
In a lyophilic colloid, the particles are often surrounded by a layer of adsorbed solvent molecules. This creates a strong interaction between the solvent and the colloidal particles, resulting in low interfacial tension and high stability.
Examples of lyophilic colloids include:

• Gelatin in water
• Starch in water
• Rubber particles in benzene

These colloids can be easily prepared by simply mixing the disperse phase in the dispersion medium, and their stability can help in preventing phenomena like sedimentation or aggregation, which are common in other types of colloidal systems.

color change prevention

Preventing color change in chemical solutions is crucial for maintaining the integrity and reliability of experiments, especially when specific visual indicators are used to measure reactions and interactions. In cases such as the Rubin number test, preventing a color change from red to violet is essential to measure the protective efficiency of lyophilic colloids.
Color change can occur due to several factors:

• Changes in pH
• Addition of salts like potassium chloride (KCl)
• Oxidation or reduction reactions

To effectively prevent undesirable color changes, scientists measure how much of a protecting colloid is needed when introducing substances like KCl. This ensures the colloid shields the dye molecules from interacting with ions that could trigger a visual transformation, thereby maintaining the solution’s original color.

Congo-Rubin dye

Congo-Rubin dye is a synthetic dye often used as an indicator in chemical reactions involving colloid stability. It exhibits a visible color change from red to violet under certain conditions, such as when ionic substances are added. This characteristic makes it useful for studying the protective efficiency of various colloids.
When Congo-Rubin dye is mixed with potassium chloride (KCl), the ionic strength can cause the dye to undergo a color change. This transformation serves as a clear visual cue for the determination of a colloid's ability to stabilize a solution.
The use of Congo-Rubin dye is essential because:

• The color change is easily observed, providing straightforward feedback.
• It is sensitive to changes in ionic concentrations, making it ideal for experiments measuring protective efficiency.
• It helps in quantifying how much colloid is necessary to maintain stability in the presence of disruptive ions.

potassium chloride (KCl)

Potassium chloride (KCl) is a common salt used in chemical experiments as a source of potassium ions. It plays a significant role in experiments regarding colloid stability, such as the determination of the Rubin number.
KCl impacts solutions by changing their ionic strength, which can lead to phenomena like aggregation or color changes in a colloidal system. In the context of the Rubin number measurement, adding KCl to a solution with Congo-Rubin dye can induce a visible color change.
This ability to change the ionic environment makes KCl a useful tool in:

• Exploring the protective efficiency of lyophilic colloids
• Inducing color changes for measurement purposes
• Standardizing experiments by using a known molarity to create consistent results

In these experiments, the objective is to find the minimum concentration of lyophilic colloid needed to prevent the color change when a specified amount of KCl is added.

Ostwald method

The Ostwald method is a traditional approach used to quantify the protective effects of colloids on colored dyes. Named after the chemist Friedrich Wilhelm Ostwald, it focuses on measuring how effectively a colloid can inhibit color changes when challenged with certain agents like salts.
This method is particularly applicable in the Rubin number test, which gauges the efficiency of lyophilic colloids protecting a dye, such as Congo-Rubin, from a KCl-induced color change.
Ostwald's approach is beneficial since:

• It provides a standardized procedure for assessing colloid protection.
• It utilizes visual color changes as straightforward indicators.
• It aids in quantifying the precise amount of colloid necessary to maintain solution stability.

By leveraging this method, scientists can ensure consistent and reproducible results, allowing for the clear comparison of different colloids and their protective capacities.