apart. Is there any change of color where the wires touch the paper? At which pole does it occur? How do you explain it (R)? Will this reaction serve to detect an electric current?

c. Prepare an electrolyte for the cell by mixing equal volumes of concentrated sulfuric acid and water, making enough of the solution to fill the tube A nearly full. When the solution has cooled, pour it into the tube A and bring the ends of the two wires connected with B and C upon the starch paper. Can you detect any current? Is your cell acting as a battery? Now connect the wires with the bindingposts. What changes in the cell do you note? Which plate turns brown? What is the brown substance? What is formed at the other plate? What gases are evolved? When one plate has become decidedly brown, disconnect the wires from E and F and bring the free ends upon the starch paper, assuring yourself that it is still moist. Is iodine set free now? What is the source of the current (R)?

204. Analysis of solder. What two metals are present in ordinary solder? Devise a method for detecting their presence in it and submit it to the instructor for approval. When your method has been approved, try it.

CHAPTER XXVII

MANGANESE AND CHROMIUM

205. Salts of manganese. To a solution of manganous chloride add a solution of hydrogen sulfide (R); a solution of ammonium sulfide (R). Try the action of sodium hydroxide and of ammonium hydroxide (R). Is the precipitate soluble in excess of the reagents? Does it undergo any change on exposure to the air (R)?

206. Manganates and permanganates. a. In a mortar grind 5 g. each of manganese dioxide and potassium hydroxide and 2.5 g. of potassium chlorate. Transfer the mixture to a small iron dish and heat to redness until the mass fuses and becomes deep green. The green material has the composition KMnO (R). When the mass has cooled, add cold water and dissolve as much of it as possible, decanting the solution from the residue.

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b. The manganate is stable only in the presence of an excess of alkali. What would you argue as to the strength of manganic acid? In water it decomposes according to the equation

3 KMnO ̧ + 4 H2O = 2 KMnO ̧ + Mn(OH), + 4 KOH

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Add dilute nitric acid to the solution. What change in color do you note? The compound KMnO is deep purple in color. Does a precipitate form? What is it? Remembering that the reaction is reversible, what is the reason that nitric acid promotes the reaction from left to right? How would you promote the reaction from right to left? Try it.

c. In the presence of sulfuric acid, how does potassium permanganate act as an oxidizing agent (R)? What becomes

of the manganese? Are these compounds notably colored? Add a little acidified solution of permanganate to a solution of ferrous sulfate. How do you account for the disappearance of the color (R)? In a similar way try the action of permanganate upon oxalic acid and sulfurous acid (R). Pour a little hydrochloric acid upon two or three crystals of potassium permanganate (R).

d. Make a solution of potassium permanganate by adding 1 g. of the salt to 100 cc. of water, and fill a burette with the solution. Weigh accurately 0.5 g. of pure iron wire and dissolve it in an excess of dilute sulfuric acid. What is present in the solution? Slowly add the permanganate from the burette, constantly stirring the solution. When the color of a drop added fades rather slowly, add the solution more carefully, stopping the instant a permanent pink color is produced (R). The continuance of the color shows that all of the ferrous sulfate has been oxidized. From the weight of the iron oxidized, calculate the weight of permanganate used up. From the volume of the permanganate, calculate the quantity of the permanganate in a liter of the solution. Knowing the strength of the solution, could you determine the amount of iron present in a sample of unknown composition? This is a favorite method of analysis.

207. Salts of chromium. Try the effect of the following reagents upon a solution of a chromium salt, writing all the equations: ammonium sulfide, sodium carbonate, ammonium hydroxide. Does sodium hydroxide occasion a precipitate (R)? Is this soluble in excess of the reagent (R)? Try boiling the solution.

208. Chromates and dichromates. a. In an iron dish melt equal weights of potassium hydroxide and potassium nitrate (about 5 g. of each), and gradually stir into the mixture about 5 g. of finely powdered chrome iron ore or 3 g. of chromic oxide (R). The nitrate is used as an oxidizing agent. Allow the fused mass to cool, and extract the soluble portion with hot water. What is the color? To what is it due? To 3 or 4 cc. of the solution add nitric or sulfuric acid.