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Selenous Acid is a chemical compound having the chemical formula H2SeO3. It is a white crystalline compound and the principal oxide of selenium. It is a trace essential element as well as an antioxidant. It is white and hygroscopic in nature. It is analogous to sulfurous acid and can be easily be formed when selenium dioxide is added to water. Selenious acid is the acid form of sodium selenite, a form of selenium. It is a co-factor metabolic element that is found in traces in the human body. The IUPAC names of Selenous Acid is Selenous acid and Selenic(IV) acid. It has a density of 3gm/cm2 along with a molar mass of 128.97 g/mol. Selenous Acid has limited use in the industry. However, it is important for the steel and the medical industry for the treatment of terminal illnesses like cancer. Cold-bluing is a unique process that happens in the steel industry, where selenous acid is used
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Keyterms: Acid, Chemical compound, Crystalline compound, Sodium selenite, Selenic(IV) acid, Co-factor metabolic element
What is Selenous Acid?
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Selenous Acid is referred to the chemical compound which has a chemical formula H2SeO3 but structure-wise it can be written as (HO)2SeO. It is one of the principal oxoacids of selenium that is analogous to sulfurous acid.
Properties of Selenous Acid
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Selenous Acid has a molecular weight of 128.97 g/mol along with a density of 3.0 g/cm3.
- It has a melting point of 70°C.
- The IUPAC name of this compound is Selenic(IV) acid.
- It is primarily used as an oxidizing agent.
- The isotope of this compound is used in the medical field as radiopharmaceuticals, which is used for labelling.
- It is also soluble in ethanol.
- It is also a regent for alkaloids.
Uses of Selenous Acid
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Selenous Acid has the following uses:
- Selenous Acid is used as an organic synthesizer as an oxidizing agent for the synthesis of compounds like 1,2-dicarbonyl compounds. This is the process by which glyoxal is prepared in the laboratory from acetaldehyde.
- It is also a key component in the Mecke reagent used for drug checking.
- One of its major uses is in the steel industry. In this industry, it is used in protecting and changing the colour of the steel, especially steel parts on firearms.
The cold-bluing process uses selenous Acid, copper Nitrate and nitric acid to change the steel colour from silver-grey to blue-grey or black. There is another procedure followed in the industry which uses copper sulfate and phosphoric acid. In this process, a deposition of a coating of copper selenide is seen. Instead of bluing, this process creates a black iron oxide.
- It is used in the chemical darkening and patination of brass, copper and bronze producing a rich dark brown colour which can be more enhanced with mechanical abrasion.
- Selenous Acid is also found in traces in humans as a source of selenic acid. It plays an important role in maintaining tissue health and muscle due to its antioxidant properties.
- It is an active component of the glutathione peroxidase enzyme. It protects the cell components from oxidation due to peroxides produced during the metabolism of body cells.
In Cancer Prevention
It has provided to be useful in preventing cancer in various animal models when it is administered in the cell exceeding the nutritional requirements. It is efficient in preventing malignancy in cells and the result is validated in a larger population.
Effect on Human Health
It is highly toxic in excessive quantities. When ingested in any significant amount, it can prove to be fatal. However, it is still needed by our body in trace amounts.
The symptoms of selenous poisoning can be seen after several hours of exposure. The symptoms mainly consist of nausea, stupor, hypotension which can result in death.
Selenous Acid Structural Formula
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The structural formula of Selenous Acid can be shown using the following figure.
Structural Formula of Selenous Acid
Things To Remember
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- Selenous Acid is a white crystalline compound and is the principal oxide of selenium with a density of 3gm/cm. It is a trace element and acts as an antioxidant.
- It is used in the medical industry ad radiopharmaceuticals for labelling. It is soluble in ethanol and is a reagent of alkaloids.
- Selenous Acid is used as an organic synthesizer in the preparation of glyoxal from acetaldehyde. It is also a key component in Mecke reagent in drug checking.
- One of its major use is in the steel industry where it is used for the Cold-bluing process. It is also used in Chemical darkening and patination of brass copper and bronze.
- Selenous Acid is also found in traces in the human body as a trace element. It is used to maintain human muscles and tissue health due to its antioxidant properties.
- It is also an active component of the glutathione peroxidase enzyme, protecting the cell components from oxidation due to peroxides produced during the metabolism of body cells.
Sample Questions
Ques. What does hygroscopic nature mean? (ICSE SAMPLE PAPER) (3 MARKS)
Ans. A Hygroscopic substance is one that can easily attract water from its surrounding, either through the method of adsorption or absorption.
One example of a high hygroscopic substance is Calcium Chloride which quickly dissolves in the water it absorbs making it also deliquescent in nature.
Ques. List some Properties of Selenous Acid. (2 marks)
Ans. Selenous Acid has a molecular weight of 128.97 g/mol with a density of 3.0 g/cm3. It has a melting point of 70°C. It is also an oxidizing agent.
Ques. What are radiopharmaceuticals drugs? Give examples. (3 + 1 marks)
Ans. A drug containing a radioactive element and used in the diagnosis and treatment of a disease, including cancer is called a radiopharmaceuticals drug. It acts both as a radioactive and a therapeutic agent.
Some examples are Technetium-99m, Iodine-131 and Cesium-137.
Ques. What is the effect of selenium on Human health? (3 marks)
Ans. When consumed in an excess amount, it can be toxic for our body. Even though it is essential in trace amount, over dose of it can cause various symptoms like nausea, stupor, hypotension when can result in death.
Ques. What is the use of selenous acid in the steel industry? ( 4 marks)
Ans. It is majorly used in the steel industry for protecting and changing the colour of the steel, especially steel parts on firearms. A process called cold-bluing uses the selenous Acid, copper Nitrate and nitric acid to change the steel colour from silver-grey to blue-grey or black.
It is used in the chemical darkening and patination of brass, copper and bronze producing a rich dark brown colour which can be more enhanced with mechanical abrasion.
Ques. What other process is used in the industry instead of cold-bluing? (3 marks)
Ans. In the other procedure which happens in the industry, copper sulfate and phosphoric acid are used. In this process, a deposition of a coating of copper selenide is seen. Instead of bluing, this process creates a black iron oxide.
Ques. Where is it an active component? What are its uses? ( 1+ 2marks)
Ans. Selenous Acid is an active component of the glutathione peroxidase enzyme. It protects the cell components from oxidation due to peroxides produced during the metabolism of body cells.
Ques. Discuss the effectiveness of selenous acid in Cancer prevention. (2 marks)
Ans. Selenous Acid has proved to be useful in various animal models in Cancer prevention. It has proved to be efficient in preventing malignancy in cell and the result is validated in larger population.
Ques. What are the uses of selenous acid as an organic synthesizer? (2 marks)
Ans. Selenous Acid is used as an organic synthesizer an oxidizing agent for the synthesis of compounds like 1,2-dicarbonyl compounds. This is the process by which glyoxal is prepared in the laboratory from acetaldehyde.
Ques. Write the formula for selenic acid. What is the anion name and formula? (2 + 2 marks)
Ans. Selenic acid is a colourless inorganic compound. It is dibasic in nature and acts as an oxidizing agent. It is used in rust removal and fertilizer production.
It has the chemical formula H2SeO4. The anion formed from selenic acid is a selenate ion. It is represented by the chemical formula SeO2−4SeO42−.It acts as a strong oxidizer and at room temperature, it is readily soluble in an aqueous solution.
Ques. Selenious acid ( H2SeO3 ), aKa2 = 5.0×10−8. What is the [OH−] of a 0.30 M solution of a selenious acid? (5 marks)
A) 2.85×10−3
B) 5.0×10−6
C) 3.5×10−12
D) 3.5×10−13
Ans. The dissociation of selenious acid in an aqueous solution is shown below-
H2SeO3 \(\rightleftharpoons\) H+ + HSeO3−
HSeO3− \(\rightleftharpoons\) H+ + SeO3−2
It is a 2 step dissociation because it is diprotic in nature.
The acid dissociation constant Ka for the steps are given as Ka1=3.0×10−3 and Ka2 = 5.0 × 10−8. From the values we can easily say that the value of Ka1>>>>Ka2.
So we can ignore the second dissociation and the H+ ions from the 1st dissociation are considered. The concentration of the acid is 0.30M. The concentrations of the products after dissociation at equilibrium can be considered as ‘x’. The equilibrium can be shown as:
H2SeO3\( \rightleftharpoons\) H+ + HSeO3−
| T=0 | 0.30 | ------ | ----- |
| T=equilibrium | 0.30−x | x | x |
The dissociation constant Ka can be given as: Ka1=(x)(x)(0.30−x)=x20.30−x
The value of x is very small compared to 0.30, hence can be ignored in the denominator. Therefore,
Ka1=x20.30−x=3.0×10−3
Ka1=x20.30=3.0×10−3
X2=(3.0×10−3)(0.30)=9×10−4
X=9×10−4−−−−−−−√=3×10−2
The concentration of H+ will be equal to [H+]=x=3×10−2M The relationship between H+ and [OH−] can be given as: [H+][OH−]=10−14.
Therefore, [OH−]=10−14[H+]=10−143×10−2 [OH−]=3.3×10−13≈3.5×10−13M.
Hence the correct answer is Option D.
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