Procedure.—­Grind the chrome iron ore (Note 1) in an agate mortar until no grit is perceptible under the pestle.  Weigh out two portions of 0.5 gram each into iron crucibles which have been scoured inside until bright (Note 2).  Weigh out on a watch-glass (Note 3), using the rough balances, 5 grams of dry sodium peroxide for each portion, and pour about three quarters of the peroxide upon the ore.  Mix ore and flux by thorough stirring with a dry glass rod.  Then cover the mixture with the remainder of the peroxide.  Place the crucible on a triangle and raise the temperature !slowly! to the melting point of the flux, using a low flame, and holding the lamp in the hand (Note 4).  Maintain the fusion for five minutes, and stir constantly with a stout iron wire, but do not raise the temperature above moderate redness (Notes 5 and 6).

Allow the crucible to cool until it can be comfortably handled (Note 7) and then place it in a 300 cc. beaker, and cover it with distilled water (Note 8).  The beaker must be carefully covered to avoid loss during the disintegration of the fused mass.  When the evolution of gas ceases, rinse off and remove the crucible; then heat the solution !while still alkaline! to boiling for fifteen minutes.  Allow the liquid to cool for a few minutes; then acidify with dilute sulphuric acid (1:5), adding 10 cc. in excess of the amount necessary to dissolve the ferric hydroxide (Note 9).  Dilute to 200 cc., cool, add from a burette an excess of a standard ferrous solution, and titrate for the excess with a standard solution of potassium bichromate, using the outside indicator (Note 10).

From the corrected volumes of the two standard solutions, and their relations to normal solutions, calculate the percentage of chromium in the ore.

[Note 1:  Chrome iron ore is essentially a ferrous chromite, or combination of FeO and Cr_{2}O_{3}.  It must be reduced to a state of fine subdivision to ensure a prompt reaction with the flux.]

[Note 2:  The scouring of the iron crucible is rendered much easier if it is first heated to bright redness and plunged into cold water.  In this process oily matter is burned off and adhering scale is caused to chip off when the hot crucible contracts rapidly in the cold water.]

[Note 3:  Sodium peroxide must be kept off of balance pans and should not be weighed out on paper, as is the usual practice in the rough weighing of chemicals.  If paper to which the peroxide is adhering is exposed to moist air it is likely to take fire as a result of the absorption of moisture, and consequent evolution of heat and liberation of oxygen.]

[Note 4:  The lamp should never be allowed to remain under the crucible, as this will raise the temperature to a point at which the crucible itself is rapidly attacked by the flux and burned through.]

[Note 5:  The sodium peroxide acts as both a flux and an oxidizing agent.  The chromic oxide is dissolved by the flux and oxidized to chromic anhydride (CrO_{3}) which combines with the alkali to form sodium chromate.  The iron is oxidized to ferric oxide.]