37 Experiment Qual: Common Anions Calcium ion and carbonate ion coma ne to form

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37 Experiment Qual: Common Anions Calcium ion and carbonate ion coma ne to form a calcium carbonate precipitate, a preliminary test for the presence of carbonate ion in a solution To observe and utilize some of the chemical and physical properties of anions OBJECTIVES To separate and identify the presence of a single anion in a solution containing a mixture of anions The following techniques are used in the Experimental Procedure: TECHNIQUES Also review Dry Lab 4. A, B, and C for additional techniques. Common anions in aqueous solution are either single atom anions (Cl B poly. INTRODUCTI r,I)or ON atomic anions usually containing oxygen (OH so CO2, PO43). In nature, the Noble most common anions are chloride, silicate, carbonate, phosphate, sulfate, sulfide, ni 7A Gases trate, aluminate, and combinations thereof. Specific anion tests are subject to interference from other anions and cations. 3A 4A 5A Therefore, to characteristically identify an anion in a mixture, preliminary elimination of the interferences is necessary. Only six of the many known inorganic anions will be identified in this experiment: phosphate, PO carbonate. CO chloride, CI odide I; sulfide, S and nitrate. NO3 The chemical properties of several of these anions have been seen in previous experiments in this manual (for example, see Experiments 3, 11, and 24). Many anions can be detected directly in the sample solution by the addition of a single test reagent. However, some anion-detection procedures require a systematic removal of the inter- ferences before the use of the test reagent. For example, a test for the presence of Common anions detected in this PO4 requires the prior removal of Aso.s a test for CO, requires the prior removal experimen of SO The separation and identification of the anions are outlined in the flow diagram Flow diogram: a diagram that Summarizes procedure for following on page 414 (see Dry Lab 4D). Follow the diagram as you read through the Introduc- a rigid sequence of steps tion and follow the Experimental Procedure. For more informction on anion qualitative analysis, go to net/chemistry. Experiment 37 41 3

Explanation / Answer

The following ions are identified by the complex they form.

1. Iodide ion(I-): Fe(III) ion oxidizes iodide ion to yellow-brown colored tri-iodide complex (I3-). Now tri-iodide complex (I3-) reacts with starch to form deep blue complex which conforms the presence of iodide ion

I3- + Starch ----> I3-- starch complex

2. Nitrate ion (NO3-): All of the nitrate salts are soluble and do not form any precipitate. Hence they are identified by brown ring test which is formed due to an iron complex. Nitrate ion is first reduced to nitric oxide(NO) by iron(II) ion in presence of concentrated H2SO4.

In presence of excess Fe(II) ion, NO reacts with Fe(II) ion to form brown colored Fe(NO)2+ comples that forms a brown in the interface of aqueous layer and con. H2SO4.

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