THE ESTIMATION OF IRON BY STANDARD SOLUTION OF POTASSIUM PERMANGANATE (Marguerite’s Method).
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 K2Mn2O8 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. For analysis, the student may take a sample of pure FeSO4,7H2O or pure FeSO4,(NH4)2SO4,6H2O (ferrous ammonia sulphate), and a ferric salt such as Fe2(SO4)3. The application of this and the Bichromate method to ores will be considered later under “Mixed Analysis.”
Method, Reactions.—When K2Mn2O8 is added to a solution of FeSO4 the following reaction takes place in the presence of H2SO4
10FeSO4 + K2Mn2O8 + 8H2SO4 = 5Fe2(SO4)3 + K2SO4 + 2MnSO4 + 8H2O, or 316.2 parts by weight of K2Mn2O8 will oxidise 560 parts by weight of Fe. The free H2SO4 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 (N/10)—Carefully weigh out 3.156 gm. of the pure dry K2Mn2O8. 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.
Checking the Standard Solution.—If carefully prepared from the pure salt as directed, the solution should be almost exactly N/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 lingers. 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.
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. H2SO4 and in the beaker a solution of pure Na2CO3. To the solution , in the flask add about half a gram of Na2CO3 10H2O (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 CO2 (these precautions are taken to prevent the FeSO4 being oxidised by the air to Fe2(SO4)3. Quickly cool the flask under the tap. Cork it and fill the burette with N/10 K2Mn2O8, 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 .0984 gm., and that the volume of N/10 K2Mn2O8 required for each was 18.28 c.cs. and 17.50 c.cs. respectively. The actual amount of iron taken is .1024 x .996 = .1020 gm. and .0984 x .996 = .0980 gm.
If the solution is exactly N/10 K2Mn2O8, one c.c. should contain .003156 gm. K2Mn2O8, which is equivalent to .0056 gm. Fe.
With the first portion 1 c.c. K2Mn2O8 = .1020/18.28 = .0056017 gm. Fe.
With the second portion 1 c.c. K2Mn2O8 = .005600 gm. Fe.
These results should not differ by more than .00002.gm., and the standardisation must be proceeded with till such results are obtained.
Label the storage bottle N/10 K2Mn2O8. 1 c.c. = .00560 gm. Fe