CHAPTER VII. ANALYSIS BY OXIDATION OR REDUCTION. In this chapter the student is required to perform the following estimations, some of which will again be considered under "Mixed Analysis.' (a) The Estimation of Iron by Standard K,Mn,O Solution, (b) Copper by I and Standard Na,S,O (a). THE ESTIMATION OF IRON BY STANDARD SOLUTION OF POTASSIUM PERMANGANATE (Marguerite's Method). For Apparatus, Reagents.-The usual volumetric apparatus, For standardising the solution soft iron wire such as is used for binding flowers is best suited. This wire contains 99.6% of iron. Pure K,MnO is required for the standard solution. If in doubt concerning its purity it may be purified by crystallising twice from hot distilled water and drying at 100° C. analysis, the student may take a sample of pure FeSO47H,O or pure FeSO4, (NH4)2SO4,6H2O (ferrous ammonia sulphate), and a ferric salt such as Fe,(SO4)3 The application of this and the Bichromate method to ores will be considered later under "Mixed Analysis," Method, Reactions.-When K,Mn,O, is added to a solution of FeSO, the following reaction takes place in the presence of HSO4 4 10FeSO4 + KMn,O + 8H2SO1 =5Fe2(SO4)3 + Ê2SO + 2MnSO4 +8H2O, or 316 2 parts by weight of KMn,O will oxidise 560 parts by weight of Fe. The free H2SO, is always necessary, as it prevents the formation of a brown precipitate of manganous oxide. If the free acid be HCl the solution to be titrated must be kept very dilute and at a low temperature, or chlorine may be liberated and spoil the analysis. It should be noted that organic matter of any kind will decompose the permanganate, hence it is preferable to use burettes with glass cocks instead of the rubber fitting. When titration with the permanganate is complete, a permanent pink colour results. Knowing the volume and value of the permanganate, the percentage of iron is easily calculated. Preparation of the Standard Solution ().—Carefully weigh out 3·156 gm. of the pure dry K,Mn,Og. Transfer to the litre flask and add about 500 c.cs. distilled water, and when dissolved, make the solution up to the mark at about 16° C. Store in a glass stoppered bottle, and when not in use keep in the dark. This solution will keep for months, but it is advisable that the student should restandardise a solution when required again after standing some time. The length of time will depend upon the nature of the solution; a few days in some cases, or a few weeks in others. N. Checking the Standard Solution.-If carefully prepared from the pure salt as directed, the solution should be almost exactly 10; but the student, to be absolutely sure, must check the standard. Only one method is given. here. For others he may consult Sutton or Blair*. Thoroughly clean a piece of the iron wire about 25 cm. long by drawing it through a piece of fine sandpaper doubled between the fingers. Continue till perfectly bright, and then finish by drawing through a piece of clean dry linen rag. Avoid handling the cleaned wire with the fingers. Weigh out two pieces, each of about 1 gm., using the assay balance if available. Note the exact weights of the two portions. 4 Take a 300 c.c. flask and clamp it at an angle on a retort stand. Fit it with a cork and delivery tube bent to lead down into a beaker. In the flask pour 100 c.cs. E. H2SO, and in the beaker a solution of pure Na,CO. To the solution in the flask add about half a gram of Na,CO,,1ŐHÖ (crystal). This fills the flask with CO2. Quickly insert the iron wire, then the cork and tube, and set the beaker so that the tip of the tube is under the surface of the solution. Apply a low bunsen flame to the flask till the iron is all dissolved. Remove the flame, and, on cooling, the solution in the beaker rises and enters the flask and at once effervesces, forming CO2 which drives the liquid down again. Remove the cork and tube. The flask now contains the solution and CO, (these precautions are taken to prevent the FeSO, being oxidised by the air to Fe2(SO4)3 Quickly cool the flask 4 N. under the tap. Cork it and fill the burette with K2Mn2Og. Place the flask underneath the burette; remove the cork and proceed to titrate with the permanganate, holding the flask by the neck with the left hand and manipulating the tap with the right. Agitate the contents of the flask by swirling after (or during) each addition, running the solution in rapidly so long as the colour is quickly destroyed, but when the colour becomes somewhat persistent proceed more slowly till the final adjustment is made by single drops. Note the number of c.cs. used, reading as far as possible to the second decimal place, and repeat the operation with the second piece of wire. Assume, for example, that the two pieces of wire weigh 1024 gm. and N. 0984 gm., and that the volume of 10 K,Mn,O, required for each was 18.28 c.cs. and 17.50 c.cs. respectively. The actual amount of iron taken is 1024 × 996·1020 gm. and 0984 × ·996·0980 gm. N. 10 8 If the solution is exactly KMn2O, one c.c. should contain 003156 gm. K,Mn,Og, which is equivalent to 0056 gm. Fe. With the first portion 1 c.c. K,Mn,Os=18-28 •1020 = 0056017 gm. Fe. These results should not differ by more than '00002 gm., and the standardisation must be proceeded with till such results are obtained. N. 10 Label the storage bottle K,Mn2O. 1 c.c. = 00560 gm. Fe *The Chemical Analysis of Iron. The Actual Analysis. 1. The determination of Iron in Ferrous Ammonium Sulphate.-Weigh out 1 gm. of the salt. Transfer to a 200 c.c. flask and dissolve in a little recently boiled distilled water. Add 100 c.cs. E. H2SO4. Titrate with N. 10 K,Mn2O. Note the volume used, and repeat the operation on a fresh portion of the salt. Calculation.-Suppose 10124 gms. of the salt were taken and that N. 25.75 c.cs. K,Mn2O were required for titration. 10 N. 8 25.75 c.cs. KMn2O 0056 × 25·75 1442 gm. Fe Theoretically, 362 parts FeSO4, (NH)2SO4, 6H,O contain 56 parts Fe by weight, or 14.28%. 2. The determination of Iron in Ferric Sulphate, Fe(SO4)3--Weigh out 3 gm. of the white anhydrous sulphate and transfer it to a 200 c.c. flask. Dissolve the salt in a little distilled water. Make up to 100 c.cs. with distilled water and cautiously add 10 c.cs. 36E. H2SO. Introduce about 5 gm. pure zinc; insert a cork with a long, thin, leading tube, the end of which dips into distilled water. The hydrogen thus evolved reduces the ferric to ferrous sulphate. Continue the action, aided by heat if necessary, till the Zn is completely dissolved. The ferric salt should now be completely reduced. Test by removing a drop on a glass rod and bringing it in contact with a drop of KCNS on a porcelain plate. If completely reduced no red tint should appear immediately on mixing the two drops. If not quite reduced, add more zinc and a little acid if necessary. When thoroughly reduced and the zine all dissolved, insert a cork after removing the delivery tube, and cork and cool rapidly under the tap. N. Remove the cork and titrate as usual with the K2Mn2Og result. Repeat the operation on a fresh portion. -- Note the Calculation. The necessary calculations are similar to those last given, and need not be repeated here. The student must, however, perform them as usual. Note. If the zinc used is not pure, a blank must be run to determine the amount of iron contained. N. (b) THE ESTIMATION OF CALCIUM BY K,Mn,O, SOLUTION. N. Apparatus, Reagents.-The usual volumetric and gravimetric apparatus. The student may take for analysis pure precipitated CaCO3. The 10 KMnO solution used for iron will serve again here. Method, Reactions. The calcium is precipitated as the oxalate, CaCO according to the equation, That is, 3156 parts K,Mn2O, oxidise 56 × 5 = 280 parts CaO, exactly half 8 N. the quantity of Fe oxidised. Therefore 1 c.c. 10 K,Mn,O= '0028 gm. CaO. Knowing the volume and value of the K,Mn,O,, the quantity of CaO is easily calculated. Preparation and Checking of Standard Solution. As the solution is already prepared and checked the student may proceed to the analysis. The Actual Analysis.—Weigh out 2 gm. of the pure carbonate of lime and transfer to a 200 c.c. beaker. Add 50 c.cs. distilled water and cover with a clock glass. Push the clock glass slightly aside and pour in a little at a time E. HCl. Continue adding the acid till the carbonate is all dissolved. Rinse the cover into the beaker by means of the wash bottle. Bring the bulk of the solution up to about 100 c.cs. Add 5E. NH HO till distinctly 3E. alkaline (check by smell). Heat to boiling and add hot (NH,),C,O, till in slight excess (about 10 c.cs. should do). Boil for a few minutes and allow to settle. Wash by decantation several times and finally bring the precipitate on to the filter. Wash the paper and precipitate with boiling water till no reaction is obtained for chlorine. 5 Push a hole through the bottom Under the beaker place a 200 c.c. flask. of the filter paper with a glass rod, and with a strong fine jet from the wash bottle wash through as much as possible of the precipitate. Wash the paper well with hot E. H2SO4, finishing with a few drops of E. HCl. Make up the contents of the flask to about 100 c.cs. Add 10 c.cs. 36E. H2SO4 and heat the contents of the flask to 70° C. and titrate rapidly with the KMn2O. Note the result and run a duplicate as usual. N. 10 Calculations.—Assume that 2124 gm. CaCO, were taken for analysis, and that 42.2 c.cs. of K,Mn2O were required for the titration. N. 8 The quantity of CaO present = 42.2 × 002811816 gm. = 55.91%. Pure CaCO should contain 56% CaO. Further notes on this method will be given under "Mixed Analysis." (c) THE ESTIMATION OF IRON BY STANDARD POTASSIUM Apparatus, Reagents. For the preparation of the standard solution pure KCгO, is required, also the iron wire as before, E. SnCl, E. HCl and E. K.Fe,C12N12, freshly prepared and containing no ferrocyanide. A white porcelain plate is used for the indicator tests. For analysis the student may take another sample of the ferric sulphate previously used. Method, Reactions. If a solution of K,Cr2O, be added to a solution of a ferrous salt in presence of a strong free acid, oxidation takes place according to the equation, 6FeCl + K,Cr2O, +14HC1 The end point of this reaction is determined by an "external" indicator. A drop of the solution is brought in contact on a white porcelain plate with A a drop of K.Fe2C12N12 freshly prepared and containing no K1FeC ̧NË. rich blue results if the oxidation has not proceeded far. On adding to the solution more K,Cr2O, and testing a drop after each addition, the blue changes to a turbid greenish blue, to a grey, and finally to a brown. When the greenish blue has just disappeared the reaction is complete. When estimating the iron in a ferric salt the reduction is generally brought about, not by zinc, but by stannous chloride, any slight excess of this salt being removed by a few drops of mercuric chloride. Knowing the volume and value of K,Cr2O, solution used, the quantity of iron present is easily calculated. N. Preparation of the Standard Solution K2Cr2O7. 10 As this salt yields 3 atoms of oxygen, which are equivalent to 6 atoms 78+104.8+112 of hydrogen, the normal solution will contain 49.13 gms. 6 = per litre, therefore a solution will contain 4.913 gms. per litre. N. Weigh out this quantity of the salt; dissolve it in distilled water, and make up to 1 litre at 16° C. One c.c. of this solution should be equal to 0056 gm. of iron. Checking the Standard.-Weigh out two portions, each of about 1 gm. clean soft iron wire. Place each portion in a 200 c.c. beaker. To each add 50 c.cs. 5E. HCl. Cover each beaker with a clock glass, and dissolve the wire with the aid of heat. When dissolved remove and rinse the covers. To the hot solution add drop by drop E. SnCl2 with constant stirring. Continue till the yellow colour of the solution has completely disappeared, but avoid excess as far as possible. Dilute to 100 c.cs. Cool quickly and add drop by drop E. HgCl2. If no precipitate forms, insufficient SnCl, was added, and, on the other hand, too copious a precipitate will interfere with the titration. As soon as the precipitate, which should be white, has ceased forming, quickly fill the burette with the K2Cr2O7 2 N. 10 and on a porcelain plate place with a glass rod about a dozen drops of uniform size of K Fe,CN12 solution. Proceed with the titration, constantly stirring, and removing now and then a drop of the solution on the rod, and bringing it in contact with a drop of the indicator on the porcelain plate. As soon as the bluish green has disappeared, read the burette. Time will be saved by calculating in advance the probable quantity of solution required, and running in rapidly to within a few c.cs. of this quantity. N. 10 Assume that 1022 gm. iron wire is taken and that it requires 18.2 c.cs. K2Cr2O7. Now 1022 gm. of the wire contain 1022 × ·996 =·1018 gm. iron. Duplicates should agree within 00002 gm. iron. The Actual Analysis.-Weigh out 3 gm. anhydrous Fe,(SO). Transfer to a 200 c.c. beaker. Treat exactly the same as when dissolving and reducing the iron wire, and titrate with the N. K2Cr2O, as before. |