Published on Feb 21, 2020
To Study the Mohr's salt
Mohr's salt or ammonium ferrous sulphate is the inorganic compound with the formula (NH4)2Fe(SO4)2.6H2O containing two different cations Fe2+ and NH4+. It is classified as a double salt of ferrous sulphate and ammonium sulphate.
It is a common laboratory reagent. Like the other ferrous sulphale salts, ferrous ammonium sulphate dissolves in water to give the aquo complex which has octahedral molecular geometry. Mohr's salt has a molar mass of 392.21 g/mol and it appears as a blue green solid
Mohr's salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water; containing a little of sulphuric acid and the crystallization from the solution. On crystallization light green crystals of ferrous ammonium; sulphate separate out.
Ferrous sulphate FeSO4.7H2O
Ammonium sulphate (NH4)2 SO4 \
Mohr's Salt FeSO4-(NH4)2SO4.6HO
Two beakers, china dish, funnel, funnel stand, glass rod, wash bottle, tripode stand, wire gauze, ferrous sulphate crystals, ammonium sulphate crystals, dilute sulphate crystals, dilute sulphuric acid, and ethyl alcohol.
Before coating, steel sheet or rod is passed through sulphuric acid. This treatment produces large quantities of ferrous iron(II) sulphate as a by product. A little more sulphuric acid may be added to ensure that the final solution remains slightly acidic. Since ammonia solution is moderately alkaline, the change from excess sulphuric acid to excess ammonia may be detected either with an indicator eg:- litmus paper, or by measuring the pH of the mixture with a pH meter
Preparation of Standard Solution of Mohr's Salt
100ml of the decinormal solution of Mohr's salt is prepared by weighing accurately accrual 3.92of A.R. quality crystals, dissolving in water to which 40ml of dilute sulphuric acid had been added (to prevent hydrolysis) and making up the solution in a standard measuring flask. The solution is well shaken. Experimentally, it has been found that the mass of Mohr's salt prepared is 8.19
1. Take a clean 250ml beaker, transfer 7g FeSO4 and 3.5g (NH4)2SO4 crystals to it. Add 2.3 ml dilute H2SO4 (to prevent hydrolysis)
2. In another beaker boil 20 ml H2O for 5 minutes (to expand air)
3. Add boiling hot H2O to the content in small quantities. Stir until salts have completely dissolved.
4. Filter it and heat the filtrate in a china dish till it concentrate to crystallization point
5. Place the china dish over the beaker full of cold H20,cool it and collect the crystals.
Colour =pale green
In analytical chemistry, this salt is preferred over other salts of ferrous sulphate for titration purposes as it is much less prove to oxidation by air to ferric or iron (III). The oxidations of solutions of ferrous iron(II) is very much dependent occurring much more readily at high temperature and pH . The ammonium ions make solutions of Mohr's salt slightly acidic, which slows this oxidation process. It is also used to print in print edition coating in order to extend the life of the word version. Also it had been used in medicine, electroplating and so on
Causes irritation to the respiratory tract symptoms may include coughing, shortness of breath
Causes irritation to the gastrointestinal tract. Symptoms may include nausea, vomiting and diarrhea. Low toxicity in small quantities but larger dosages may cause nausea, vomiting, diarrhoea and black stool. Pink urine discoloration is a strong indicator of iron poisoning. Liver damage, coma and death from iron poisoning has been recorded.
• Skin contact
Causes irritation to skin. Symptoms include redness itching and pain
• Eve contact
Causes irritation, redness and pain
Mohr's salt is named after the German chemist, Karl Freidrich Mohr, who made many important advances in the methodology of titration in the 19th century. It is a double salt. A double salt is a substance obtained by the combination of two, different salts which crystallizes together as a single substance but ionize as two distinct salts when dissolved in water.
• Practical chemistry, B.Sc Main By A.Q. Thomas