Tested Overhead Projection Series

a-- TOPS Compiled by HUBERT N. ALYEA Princeton Univerriy

21. METALS OF GROUPS IV-VIII. C. Group VI. (Chromium) continued

Dem. 638-Dichmmaie Volcano

T o show: A miniature volcano from the thermal de- composition of ammonium dichromate.

Materials: H-4, (NH4)2Cr207 (HANDLE WITH CARE, WASH HANDS AFTERWARDS); filter paper, ethanol.

Procedure: Insert a 3 X 1 cm strip of filter paper in a coke-cap full of ammonium dichromate on the asbestos stage of H 4 . Moisten the paper with a few drops of ethanol. Project. Ignite.

Observations: A mountain of green CrzOa forms; also fires flash from the erupting mass, simulating a volcano.

Note: Place a small lump of Dry Ice in a coke cap, surround it with a pile of ammonium dichromate, insert the ethanol-soaked filter paper and ignite. When the volcano begins to erupt, put a few drops of warm water on the Dry Ice to simulate "smoke."

Dem. 639-Electro-lorming PbCrOr

To show: The formation of PbCrOa by electro-form- ing.

Materials: E-l with Pb strips for electrodes; I<2Cr207- aq, 6-v dc source.

Procedure: Project with Pb plates immersed in I<zCr207-aq and apply dc current.

Observations: A heavy coating of PbCr04 forms on the positive pole.

Dem. 640--Chmmate and Dichmmate

To show: The relation between Cr04- and Crp07=.

Materials: G2, K2Cr207, NaOH, and HCI.

Procedure: Project C-2 with cells half-full of dichro- mate solution. (a) To cell 2 add NaOH dropwise until it turns yellow. (b) Now add HC1 to cell 2 and NaOH to cell 1, reversing colors.

Observations: 2Cr04= + H + e Cr207= + OH- (yellow) (orange)

Dem. 64 1-Dichromak + SO1 or H2S

To show: The reducing action of SO, or H,S on di- chromate.

Materials: C-3, KpCrzOraq, NaHS03 or Na8, H2S04- dil.

Procedure: Project C-3 quarter-full of dil. H2S04. Add varying volumes of a dilute solution of dichromate, followed by drops of NaHSOs-aq, or of Na,S-aq.

Observations: The Cr+6 in dichrornate is reduced to Cr+3, in the form of green CrSOa+.

Dem. 642-Perchmmate Test

T o show: The perchromate test for chromium com- pounds.

Materials: KzCr207-aq, 3% H202, ether, dil. H2SOn, H-3.

Procedure: Project culture tube half-full of water. To it add a dropperful of dil. H2S01, then a dropperful of KpCr207-aq. Now add half a dropperful of H202 solution and quickly dump in 5 ml of ether, shake and project.

Observations: A transient blue perchromate is formed in the upper, ether, layer. This can he used either as a test for chromates or for a peroxide.

Reactions: SIDGWICK, N. Y., Chemical Elements and Their Compounds, Oxford Press, 1950, pages 1006,and 1007 states that KCrOs.HpO and Cr06 form under varying conditions.

Dem. 643-Chmmyl Chloride Test

Experiment developed by August J. Colo.

To show: Test for chromium using micro techniques.

Materials: Conc. H2S04, unknown such as K2Cr207, NaC1-aq, Ph(NO&aq; 2-02. wide-mouth square glass bottle bearing a dropper reaching down into a 13 X 70 mm test-tube or culture tube.

Procedure: Place conc. HZS04 in the dropper, 10 drops of the unknown solid (e.g., ILCr207) in the cul- ture tube, and 3 ml of the test solution in the bottle, e.g., water, Pb(NO&aq, or AgNOa-aq. Project. Run the conc. acid into the unknown in the culture tube.

Observations: If chromium is present, yellowish gas, chromyl chloride (CrOCl,) is given off, and colors the water white, precipitates yellow PhCr04, or red AgCr04. This is also used as a specific test for chlorides.

Reference: HEATH, F. H., J. CHEM. EDUC., 25, 73 (1948).

D. Group V11. (Manganese) Dem. 644-Manganere Compounds fiom the Ore

Experiment developed by Edwin H. Cooper.

To show: Preparation of manganese compounds from manganite ore.

Materials: Pyrolusite (MnOz; if not available, use C.P. product). NaOH, KNOa, dry ice, dil. HNOa, funnel and filter paper, H-3.

Volume 46, Number 7, July 1969 / A495