Step-by-Step Procedure and Key Safety Precautions
We discussed the step-by-step method for performing an experiment to prepare Arsenious Sulphide sol in this article. To understand the objective, procedure, and materials needed to conduct the experiment, carefully read the content on this page. Try to run the experiment and balance this with your outcome.
Aim
The aim of this experiment- Sulphide Colloidal Solution Preparation, Arsenic sulphide -As2S3 sol.
Theory
Arsenic Sulphide-
The inorganic compound with the formula As2S3 is Arsenic trisulfide. It is a dark yellow solid which is water-insoluble. It also occurs as the mineral orpiment, which has been used as a pigment called King's yellow. It is developed in the field of arsenic compound analysis. It is an intrinsic p-type semiconductor of group V/VI and exhibits phase-change properties that are photo-induced. As4S4, a red-orange solid known as the mineral realgar, is the other principal arsenic sulfide.
In both crystalline and amorphous forms, As2S3 occurs. Both types feature polymeric structures consisting of centres of trigonal pyramidal As(III) connected by centres of sulfide. The centres of the sulfide are double-coordinated with two arsenic atoms. The compound adopts a ruffled sheet structure in the crystalline form. Van der Waals forces consist of the bonding between the sheets. In geological samples, the crystalline form is commonly found. Amorphous As2S3 does not have a structure with layers but is more strongly cross-linked. There is no medium or long-range order, like other lenses, but the first co-ordination sphere is well defined. As2S3 is a strong former glass and has a broad glass-forming shape.
As2S3 (Arsenious sulphide) is a colloid that is lyophobic. It is formed in boiled distilled water due to the hydrolysis of As2O3 (Arsenious oxide). Additional H2S gas is transferred through the solution. The reaction takes place as follows:
As2O3+ 3H2O · 2As(OH)3
2As(OH)3 + 3H2S → As2S3 + 6H2O
The particles in the colloidal solution of Arsenious sulphide are surrounded by ions of HS that are formed due to H2S dissociation. H+ ions surround the sulfide ion sheet.
Materials Required
The apparatus and materials required for coagulation of arsenious sulphide sol are as follows:
Beaker 250 mL
Round bottom flask 500 mL
Glass tubing
Conical flask of 250 mL volume
Filter paper
Funnel
Glass rod
Wire gauze
Burner
Tripod stand
Iron stand with clamp
Distilled water
H2S gas
Solid arsenious oxide
Procedure:
Take and clean 250 mL of the conical flask by steaming out the process.
Add 0.2 g of solid arsenic oxide and distilled water for 100 mL.
Boil the solution for 10 minutes.
Separate the hot solution with the help of filter paper.
As2O3 using Kipps apparatus pass H2S through As2O3 as shown below.
The solution's colour shifts to yellow, which is resulting in the As2S3 formation.
By slowly heating, expelling hydrogen sulfide gas from the sol.
Mark the filter as 'arsenic sol sulfide.
Precautions:
Using clean equipment for the experiment affects As2S3 if there are even traces of impurities.
As2S3 is extremely toxic, so treat it carefully.
Did You Know?
Arsenic groundwater contamination is a form of groundwater pollution that is mostly due to high concentrations of arsenic in deeper groundwater levels that occur naturally. Due to the use of deep tube wells for water supply in the Ganges Delta, it is a high-profile epidemic, causing large numbers of people to have extreme arsenic poisoning. A 2007 study found that arsenic toxicity from drinking water is likely to affect over 137 million people in more than 70 countries. After mass water poisoning in Bangladesh, the problem became a major health issue.
A major issue is arsenic pollution of the groundwater in Bangladesh. Bangladesh had one of the highest child mortality rates in the world prior to the 1970s. These problems have been compounded by inadequate water purification and drainage systems, as well as frequent monsoons and flooding. UNICEF and the World Bank have promoted the use of wells to tap into deeper groundwater as a solution. As a result, millions of wells were installed. Infant death and diarrheal disease have been decreased by fifty percent due to this intervention. With over 8 million wells installed, however, approximately one in five of these wells are now polluted with arsenic above the drinking water level of the government.
FAQs on Arsenious Sulphide: Complete Chemistry Guide
1. What is Arsenious Sulphide and what is its chemical formula?
Arsenious Sulphide is an inorganic compound with the chemical formula As₂S₃. It is also known as arsenic(III) sulfide or arsenic trisulfide. In chemistry, it is well-known for forming a stable lyophobic colloid (a type of solution) which appears as a distinct bright yellow sol when prepared in water.
2. How is the colloidal sol of Arsenious Sulphide prepared in a laboratory?
A colloidal sol of Arsenious Sulphide is typically prepared using a double decomposition chemical method. The process involves:
- Passing hydrogen sulphide (H₂S) gas through a cold, dilute aqueous solution of arsenious oxide (As₂O₃).
- The resulting reaction forms the insoluble Arsenious Sulphide particles of colloidal size.
The chemical equation for this preparation is: As₂O₃ + 3H₂S → As₂S₃(sol) + 3H₂O.
3. What are the key characteristics of an Arsenious Sulphide sol?
An Arsenious Sulphide (As₂S₃) sol has several important characteristics based on the CBSE syllabus:
- Nature: It is a classic example of a lyophobic ('solvent-hating') sol, meaning the dispersed particles have low affinity for the dispersion medium (water).
- Charge: The colloidal particles are negatively charged.
- Appearance: It is a bright yellow coloured solution.
- Stability: It is only stable due to the charge on its particles and can be easily coagulated by adding an electrolyte.
- Optical Property: It exhibits a prominent Tyndall effect, scattering light that passes through it.
4. Why do the particles in an Arsenious Sulphide sol carry a negative charge?
The negative charge on Arsenious Sulphide particles is due to a phenomenon called preferential adsorption. During its preparation from arsenious oxide and hydrogen sulphide, the As₂S₃ particles adsorb sulphide ions (S²⁻) from the solution onto their surface. This creates a negatively charged layer around each particle, represented as [As₂S₃]S²⁻. This uniform negative charge causes the particles to repel each other, preventing them from aggregating and settling, which stabilises the sol.
5. How does the Hardy-Schulze rule apply to the coagulation of Arsenious Sulphide sol?
The Hardy-Schulze rule explains how electrolytes cause coagulation. Since Arsenious Sulphide sol is negatively charged, it is coagulated by positive ions (cations). The rule states that the coagulating power of an ion is directly proportional to the magnitude of its charge. Therefore:
- A trivalent cation like Aluminium (Al³⁺) is the most effective coagulating agent.
- A divalent cation like Barium (Ba²⁺) is less effective than Al³⁺ but more effective than a monovalent cation.
- A monovalent cation like Sodium (Na⁺) has the least coagulating power for As₂S₃ sol.
6. What makes Arsenious Sulphide sol fundamentally different from a starch sol?
The fundamental difference is their interaction with the solvent (water):
- Arsenious Sulphide Sol: This is a lyophobic ('solvent-hating') sol. The particles are inorganic and have no affinity for water. They are stabilised only by their electrical charge and are irreversible.
- Starch Sol: This is a lyophilic ('solvent-loving') sol. Starch molecules readily interact with and are solvated by water molecules. These sols are much more stable and are reversible; the solid can be reconstituted into a sol by adding water after evaporation.
7. What would be the observable difference if you add a solution of MgCl₂ versus NaCl to an Arsenious Sulphide sol?
If you add these electrolytes, both will cause coagulation, but at different efficiencies. According to the Hardy-Schulze rule, the divalent Mg²⁺ ion from MgCl₂ has a much greater coagulating power than the monovalent Na⁺ ion from NaCl. This means a much smaller concentration of MgCl₂ solution would be needed to cause the Arsenious Sulphide sol to precipitate compared to the NaCl solution. The coagulation process would appear significantly faster and more effective with the addition of MgCl₂.
