Question

It is confirmed by forming a strawberry red precipitate, NicaH,N2O2), with an organic reagent, dimethylglyoxime. 5c Detection of Magnesium Magnesium is confirmed by dissolving the hydroxide with acid and then re-precipitating it in the presence of an organic compound called Magnesium reagent. The presence of Mg is indicated by formation of "blue Mg (aa) 20H (ag) Mg reagent Mg (OH)2 (s) First, you will analyze a known that contains all 10 cations. Record on your report PROCEDURE sheet the reagents used in each step, your observations, and the equations for each precipitation reaction. After completing this practice analysis, obtain an unknown. Follow the same procedures as with the known, again recording reagents and observations. Also record your conclusions regarding the presence or absence of all cations. Before beginning this experiment, review the techniques used in quali- tative analysis found in Appendix "Qualitative Analysis Techniques": heating so- lutions, precipitation, centrifugation, washing precipitates, and testing acidiy. Waste Disposal Instructions All waste from this experiment should be placed in appropriate containers in the laboratory. 1 Initial Observations and Tests for Sodium and Ammonium Note the color of your sample and on your report sheet, record any conclusions about what cations are present or absent. GIVE IT SOME THOUGHT What information can you obtain from the color of your sample? The flame test for sodium is very sensitive, and traces of sodium ion will impart a characteristic yellow color to the flame. Just about every solution has a trace of sodium and thus will give a positive result. On the basis of the intensity and duration of the yellow color, you can decide whether Na is merely a contaminant or is present in substantial quantity. To perform the flame test, obtain a piece of platinum or Nichrome wire that has been sealed in a piece of glass tubing. Clean the wire by dipping it in 12 M HCI that is contained in a small test tube and heat the wire in the hottest part of your Bunsen burner flame. Repeat this operation until you see no color when you place the wire in the flame. Several cleanings will be required before this is achieved. Then place 10 drops of the solution to be analyzed in a clean test tube and perform a flame test on it, If the sample being tested is your unknown, run a flame test on distilled water and then another flame test on a

Answer 1

Qualitative analysis of cations

a) Mg2+

add Mg-reagent forms blue lake

Mg2+ + 4-(p-nitrophenylazo) resorcinol + 2OH- ----> Mg(OH)2-absorbed on-4-(p-nitrophenylazo) resorcinol (blue lake)

b) Na+

Flame test : deep yellow

c) NH4+

heat with NaOH and test with red litmus paper : turns blue, with neutral litmus turns brown

NH4+ + OH- ---> NH3 + H2O

d) Ca2+

add Na2C2O4 forms white precipitate dissolve in HCl and flame test gave brick red color

Ca2+ + C2O4^2- --> CaC2O4 (s)

CaC2O4 + 2H+ --> Ca2+ + H2C2O4

e) Ag+

add 6 M HCl forms white precipitate. Add NH3, dissolve precipitate and add HNO3 to reprecipitate AgCl

Ag+ + Cl- ---> AgCl (s)

AgCl + NH3 ---> Ag(NH3)Cl

Ag(NH3)Cl + H+ --> AgCl (s) + HCl

f) Ni2+

With dimethyl glyoxime in basic medium forms bright red Ni(DMG)2 complex

Ni2+ + 2C4H7N2O2- --> Ni(C4H7NO2)2 (bright red)

g) Zn2+

add K4[Fe(CN)6] in acidic medium, forms gray-blue precipitate or solution

3Zn2+ + K4[Fe(CN)6] ---> Zn3K2[Fe(CN)6]2 (ppt) + 2K+

h) Al3+

in NH3 and aluminon reagent forms red coloration

Al3+ + 3OH- + aluminon --> Al(OH)3-absorbed-aluminon (red)