Qualitative Analysis of Anions: Identifying Chemical Characteristics and Unknown Samples

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Introduction

The objective of this experiment was to use qualitative analysis to determine the chemical characteristics of four known anions by systemic confirmatory testing. The chemical characteristics observed were to be used to identify an unknown sample. Sulphuric acid was to be added to a carbonate solution and an effervescent reaction would confirm the presence of carbonate anions. Another sample of carbonate solution was to be reacted with hydrochloric acid and the gaseous product was to be reacted with a suspended drop of barium carbonate.

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The presence of clouding in the droplet would also confirm the presence of carbonate anions. Silver nitrate was to be added to chloride solution and the formation of a white precipitate, silver chloride, would confirm the presence of chloride anions. Ammonium hydroxide would be added to dissolve part of the precipitate.

The resultant supernatant solution was to be decanted and re-acidified with nitric acid to reform the precipitate and definitively confirm the presence of the chloride anion.

Silver nitrate was to be added to iodide solution and the formation of a yellow precipitate, silver iodide, would confirm the presence of iodide anions. Another sample of iodide solution was to be reacted with drops of acetic acid until sufficiently acidic. Potassium nitrate was to be added to the solution causing a colour change. Methylene chloride was to be added to this sample and shaken to confirm the presence of iodide anions by forming two separate and differently coloured layers. A barium chloride-calcium chloride mixture was to be added to sulphate solution and heated to 90oC.

The sample was to stand for a period of time to allow the formation of a white precipitate, barium sulphate, confirming the presence of the sulphate anion. The addition of hydrochloric acid and subsequent heating were to have no effect on the presence, further confirming the presence of sulphate anions.

Materials and Methods

Reagents and Chemicals:

Sulphuric acid (H2SO4)
Carbonate solution
Hydrochloric acid (HCl)
Barium hydroxide (BaOH)
Silver nitrate (AgNO3)
Ammonium hydroxide (NH4OH)
Nitric acid (HNO3)
Iodide solution
Acetic acid (HC2H3O2)
Potassium nitrate (KNO2)
Methylene chloride (CH2Cl2)
Barium chloride-calcium chloride mixture (BaCl2-CaCl2)
Sulphate solution

Safety Precautions:

Carbonate solution is hazardous in case of skin contact (sensitizer, irritant), of eye contact (irritant), inhalation (lung irritant), and ingestion. It should be kept in a cool, well-ventilated area. In case of spill, use appropriate tools to put the spilled solid in a convenient waste disposal container and neutralize the residue with a dilute solution of acetic acid. Hydrochloric acid is very hazardous in case of skin contact (sensitizer, corrosive, irritant, permeator), eye contact (corrosive, irritant), of inhalation (lung sensitizer, respiratory tract irritant), and ingestion (toxic). It should be kept in a dry container, kept away from oxidizing agents, organic materials, metals, alkalis, and moisture. In case of spill, dilute with water and mop with an inert dry material. Residue should be neutralized with dilute sodium carbonate. Chloride solution is hazardous in case of skin contact (irritant, permeator), of eye contact (irritant), ingestion, and inhalation (irritant).

In case of spill, dilute with water and mop with an inert dry material and spread water on the contaminated surface. Iodide solution is lightly hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, and inhalation. In case of spill, use appropriate tools to put the spilled solid in a convenient waste disposal container. And spread water on the contaminated surface. Silver nitrate is very hazardous in case of skin contact (corrosive, permeator), of eye contact (irritant), of inhalation, and ingestion.. Silver nitrate kept away from heat, sources of ignition, and combustible material. In case of spill, use appropriate tools to put the spilled solid in a convenient waste disposal container. Ammonium hydroxide is very hazardous in case of skin contact (toxic, corrosive, irritant, permeator), eye contact (irritant), inhalation (toxic to upper respiratory tract), and ingestion (toxic). Water should never be added to ammonium hydroxide and it should be kept away from incompatibles such as metals, acids. In case of spill, dilute with water and mop with inert dry material.

Neutralize residue with dilute acetic acid. Nitric acid is very hazardous in case of skin contact (corrosive, irritant, permeator), of eye contact (irritant, corrosive), and ingestion. It is slightly hazardous in case of inhalation (lung sensitizer, respiratory tract and mucus membrane irritant). Nitric acid is explosive in the presence of reducing materials, of organic materials, of metals, of alkalis. In case of spill, dilute with water, mop with an inert dry, and neutralize the residue with a dilute solution of sodium carbonate. Acetic acid is very hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, of inhalation. It is hazardous in case of skin contact (corrosive, permeator), of eye contact (corrosive). It should be kept away from sources of heat, ignition, and oxidizing material. In case of spill, Dilute with water, mop with an inert dry, and neutralize the residue with a dilute solution of sodium carbonate.

Potassium nitrite is extremely hazardous in case of skin contact (corrosive, irritant), of eye contact (irritant), of ingestion, and inhalation. In case of spill, use appropriate tools to put the spilled solid in a convenient waste disposal container. Potassium nitrite should be kept dry, away from sources of ignition, heat, and combustible materials. Methylene chloride is very hazardous in case of eye contact (irritant), of ingestion, of inhalation, and skin contact (irritant, permeator). In case of spill dilute with water and mop with an inert dry material. Sulphate solution is hazardous in case of eye contact (irritant), skin contact (irritant), and ingestion. In case of skin contact, wash with soap and water, and cover area with an emollient. In case of spill, Dilute with water, mop with an inert dry material spread water on the contaminated surface. (ScienceLab, 2005)

Procedure

Testing for Carbonate, CO32- Anions:
- Place 3mL of Carbonate Solution in a test tube.
- Add 2 drops of 6M HCl to the solution and observe any effervescence.
- Suspend 1 drop of BaOH over the test tube and note any changes in the suspended droplet.
- Record your observations.
Testing for Chloride, Cl- Anions:
- Take 5mL of Chloride Solution in another test tube.
- Add 5 drops of 0.1M AgNO3 to the solution and look for the formation of a white precipitate.
- Slowly add NH4OH drop-wise to the solution and observe any changes.
- Decant the supernatant solution and add 6M HNO3 to the remaining precipitate.
- Record your observations.
Testing for Iodide, I- Anions:
- Place 5mL of Iodide Solution in a test tube.
- Add 5 drops of 0.1M AgNO3 to the solution and check for the formation of a cloudy pale yellow precipitate.
- Record your observations.
- In a separate test tube, add 2 drops of acetic acid drop-wise to the Iodide Solution.
- Add 2 drops of KNO2 to the solution and observe any color changes.
- Shake the test tube after adding 15 drops of Methylene chloride and look for the formation of two distinct layers.
- Record your observations.
Testing for Sulphate, SO42- Anions:
- Take 3mL of Sulphate Solution in a test tube.
- Add 0.5mL of the BaCl2-CaCl2 mixture to the solution.
- Heat the test tube in a boiling water bath at 90°C for 10 minutes.
- Observe the formation of a white precipitate.
- Add 5 drops of 6M HCl to the solution and heat it again for approximately 5 minutes at 90°C.
- Record your observations.
Testing Unknown Sample for Anions:
- Follow the procedure outlined above for each anion test (Carbonate, Chloride, Iodide, and Sulphate) using the Unknown Sample 7 instead of known solutions.
- Record your observations for each test.

Observations

Table 1: Testing and confirming for the presence of Carbonate, CO32- anions

Test Conditions	Chemicals Used	Observations
2 drops 6M HCl	1 drop BaOH suspended over test tube Carbonate Solution, 3mL	Small amount of effervescence noted about 5 seconds after being suspended over test tube. The base of the droplet clouded with white precipitate. The addition of HCl to the sample resulted in some rising gas bubbles, indicating the formation of a gas. The clouding of the barium hydroxide droplet indicated it was reacting with carbon dioxide escaping the tube, confirming the presence of carbonate anions in the sample.

Table 2: Testing and confirming for the presence of Chloride, Cl- anions

Test Conditions	Chemicals Used	Observations
5 drops 0.1M AgNO3 added	NH4OH is added drop-wise Supernatant solution is decanted, 6M HNO3 is added Chloride Solution, 5mL	Cloudy white precipitate formed. Precipitate partially dissolved upon addition, distinct clear supernatant formed above remaining precipitate. Cloudy white precipitate reformed, small white granulations settled at the bottom of the test tube. 6 drops of ammonium hydroxide, NH4OH, were added to the sample. 6 drops of nitric acid, HNO3, were added to acidify the sample. The formation of a cloudy white precipitate in the presence of silver nitrate indicated the presence of chloride anions. The subsequent addition of ammonium and re-acidification of the sample resulted in the reformation of a cloudy white precipitate, confirming the presence of chloride anions.

Table 3a: Testing for the presence of Iodide, I- anions

Test Conditions	Chemicals Used	Observations
5 drops 0.1M AgNO3 added	Iodide Solution, 5mL	Cloudy pale yellow precipitate formed. The presence of a cloudy yellow precipitate with silver nitrate indicates the presence of iodide anions.

Table 3b: Testing and confirming the presence of Iodide, I- anions

Test Conditions	Chemicals Used	Observations
Acetic acid is added drop-wise 2 drops KNO2 added 15 drops Methylene chloride added, test tube shaken	Iodide Solution, 5mL	Blue Litmus Paper confirmed the solution was acidic by turning red. Solution turned a mustard-brown color. Two distinct layers formed in the test tube. The bottom layer was reddish magenta, and the top layer was a burnt-orange color. 2 drops of acetic acid, HC2H3O2, were added to acidify the sample. Upon confirming the sample was indeed acidic, it reacted with the potassium nitrate to change to a mustard-brown color. The addition of the methylene chloride formed two distinctly colored layers, thus confirming that the solution had iodide anions present.

Table 4: Testing and confirming the presence of Sulphate SO42- anions

Test Conditions	Chemicals Used	Observations
0.5mL of BaCl2-CaCl2 mixture added Test tube heated, stood for 10 minutes 5 drops 6M HCl, test tube heated	Sulphate Solution, 3mL	Very faint wisps of cloudy white precipitate appeared. Noticeable cloudy white precipitate distributed throughout the solution. Addition of HCl made the test tube feel warm. After heating the test tube, the precipitate remained dispersed in the solution. In each instance of heating, the test tube was placed in a boiling water bath at 90oC. The final heating of the test tube was for approximately 5 minutes at 90oC. The reaction of the sulphate solution with the barium chloride-calcium chloride solution, when heated, resulted in the appearance of a fine white precipitate, indicating the sample had sulphate anions present. The subsequent addition of HCl and reheating resulted in the solution remaining relatively the same, further confirming that sulphate anions were present.

Table 5a: Testing Unknown Sample 7 for the presence of Carbonate, CO32- anions

Test Conditions	Chemicals Used	Observations
2 drops concentrated H2SO4 added, Test tube shaken	Unknown Sample 7, 3mL	No visible effervescence. No noticeable change to the test tube’s temperature. The lack of effervescence and heat produced by the sample indicated the absence of carbonate anions in the sample.

Table 5b: Testing Unknown Sample for presence of Carbon Dioxide, CO2 to confirm the presence of CO32- anions

Test Conditions	Chemicals Used	Observations
2 drops 6M HCl 1 drop BaOH suspended over test tube	Unknown Sample 7, 3mL	No noticeable effervescence. Suspended droplet remained clear, despite being held over the test tube for 15 seconds. The sample did not react with the hydrochloric acid to produce a gas that reacted with the barium hydroxide, thus the suspended droplet remained clear. The results further confirmed the absence of carbonate anions.

Table 6: Testing and confirming Unknown Sample for the presence of Chloride, Cl- anions

Test Conditions	Chemicals Used	Observations
5 drops 0.1M AgNO3 added NH4OH is added drop-wise Supernatant solution is decanted, 6M HNO3 is added	Unknown Sample 7, 5mL	Pale yellow precipitate formed. No change to solution. No change to solution. 6 drops of ammonium hydroxide, NH4OH, were added to the sample. Nitric acid, HNO3, was unable to be added to the precipitate as no clear supernatant was formed. The lack of formation of a cloudy white precipitate in the presence of silver nitrate indicated that the sample did not have chloride anions present. The subsequent addition of ammonium and re-acidification of the sample were rendered purposeless.

Table 7a: Testing Unknown Sample for the presence of Iodide, I- anions

Test Conditions	Chemicals Used	Observations
5 drops 0.1M AgNO3 added	Unknown Sample 7, 5mL	Cloudy pale yellow precipitate formed. The presence of a cloudy yellow precipitate with silver nitrate indicates the presence of iodide anion.

Table 7b: Testing and confirming Unknown Sample for the presence of Iodide, I- anions

Test Conditions	Chemicals Used	Observations
Acetic acid is added drop-wise 2 drops KNO2 added 15 drops Methylene chloride added, test tube shaken	Unknown Sample, 5mL	Blue Litmus Paper confirmed the solution was acidic by turning red. Solution turned a mustard-orange color. Two distinct layers formed in the test tube. The bottom layer was reddish-purple, and the top layer was an orange-brown. 2 drops of acetic acid, HC2H3O2, were added to acidify the sample. Upon confirming the sample was indeed acidic, it reacted with the potassium nitrate to change to a mustard-orange color. The addition of the methylene chloride formed two distinctly colored layers, thus confirming that the solution had iodide anions present.

Table 8: Testing and confirming Unknown Sample for the presence of Sulphate SO42- anions

Test Conditions	Chemicals Used	Observations
0.5mL of BaCl2-CaCl2 mixture added Test tube heated, stood for 10 minutes 5 drops 6M HCl, test tube heated	Unknown Sample, 3mL	No precipitate formed. No noticeable precipitate formed. No change to solution. In each instance of heating, the test tube was placed in a boiling water bath at 90oC. The final heating of the test tube was for approximately 5 minutes at 90oC. The lack of precipitate formation in the test indicated that the sample was absent of sulphate anions. This result was further confirmed by the subsequent addition of hydrochloric acid and reheating failing to produce a precipitate.

Discussion

A strong acid such as H2SO4, sulphuric acid, combined with carbonate produces an effervescent reaction due to the formation of carbon dioxide. The heat felt from the test tube was a result of the exothermic reaction that the carbonate solution underwent in the presence of a strong acid. Sulfuric acid is a strong acid, but only for the first H, so it dissociates into H+ ion and the bisulfate ion, HSO4-. HSO4- is a weak acid and does not dissociate to a great extent, but it also reacts with carbonate (Dartmouth College, 2003). The presence of sulphur may have accounted for the mildly unpleasant scent wafted during the experiment. As indicated by the results of Table 1b, the formation of CO2 acted as a positive indicator for the presence of CO32- in a solution, as expressed by: 2H+(aq) + CO32-(aq) → H2O(l) + CO2(g).

The exposure of the produced gas, carbon dioxide, to barium hydroxide resulted in the a cloudy white precipitate due to the formation of barium carbonate as expressed by: H2O (l) + CO2 (aq) + Ba2+(aq) → BaCO2 (s) + 2 H+(aq). The addition of 01M AgNO3, silver nitrate, to the chloride solution resulted in the formation of a white precipitate due to the formation of silver chloride as expressed by: AgNO3 (aq) + Cl-(aq) → AgCl(s) + NO3 (g). It is interesting to note that chlorides are generally soluble, with the exception of lead(II) and silver chloride. When concentrated NH4OH was added to the silver chloride precipitate dissolved due to the formation of an ammine complex, [Ag(NH3)2]+. Silver nitrate is generally acidified with dilute nitric acid to prevent the precipitation of other non-halide silver salts (Brown, 2012).

Nitric acid acidified the solution due to the addition of H+ ions to the solution, resulting in re-precipitation, thus definitively indicating the presence of chloride anions. This is expressed as AgCl(s) + 2NH3(aq) → Ag(NH3)2+(aq) + Cl-(aq) Ag(NH3)2+(aq) + Cl-(aq) + 2H+(aq) → AgCl(s) + 2NH4+(aq) The addition of 0.1M AgNO3 to the iodide solution resulted in the formation of a cloudy yellow precipitate as expressed by: AgNO3(aq) + I- → AgI(s), which as a precipitate is insoluble in concentrated ammonia. The addition of 6M acetic acid, HC2H3O2, acidified the solution, as confirmed by the litmus test. The newly acidic solution reacted with potassium nitrite, KNO2, reducing the nitrite to nitrogen dioxide as expressed by: I-(aq) + KNO2 → KI(aq) + NO2(g). The liberation of iodine in the reaction resulted in a brownish change in colouration of the solution (SUNY Chemisty, 2005). The failure of the sample to turn a reddish-brown may have been a source of error due to a failure to sufficiently rinse the cleaned test tube with DI water.

Methylene chloride, CH2Cl2, which is a non-polar organic compound was added to the solution reacted with the iodide in the solution to form a violet coloured layer of denser solution, which settled at the bottom of the test tube. An alternative means of testing for the presence of iodide anions would have been to use starch, which forma a characteristic blue-black complex. The addition of the barium chloride-calcium chloride mixture, BaCl2-CaCl2, to the sulfate solution resulted in the formation of an insoluble white sulphate, barium sulphate as expressed by: Ba2+(aq) + SO42-(aq) → BaSO4(s). Other insoluble barium salts contain anions of weak acids (CO32-, SO32-and PO43-). Precipitation of these anions is generally prevented by acidifying the solution (Yoder, 2014). The products of the reaction were made more prominent by the catalyzing the reaction with heat, resulting in a more noticeably fine white precipitate distributed throughout the solution.

The addition of 6M HCl, hydrochloric acid, served to acidify the solution, further illustrating the insolubility of the barium sulfate precipitate, thereby confirming the presence of sulphate anions. The unknown sample was placed through all confirmatory tests. It failed to react with Sulphuric acid and hydrochloric acid to produce an effervescent reaction, indicating it was carbonate absent. Unknown Sample 7 did not react with the BaCl2-CaCl2 mixture to form a precipitate regardless of heat, indicating the absence of sulphate anions. Unknown Sample 7 formed a pale cloudy yellow precipitate upon the addition of silver nitrate, immediately indicating the presence of iodide anions, disqualifying the need to test further for chloride. The addition of acetic acid to Unknown Sample 7 provide a sufficient acidic environment for the potassium nitrate to release iodine, thus the solution appeared as a dark mustard orange. The methylene chloride confirmed the presence of iodide anion when a distinct reddish-purple layer settled at the bottom of another distinctly coloured layer of solution.

Conclusion

Based on the results of the experiment, Unknown Sample 7 demonstrated the formation of a precipitate in the presence silver nitrate, a change in colouration when mixed with potassium nitrate, and the formation of 2 distinctly coloured layers when methylene chloride was added to the solution and shaken. In accordance with the chemical properties exhibited by the 4 known anions during the qualitative testing, it can be concluded the Unknown Sample 4 clearly demonstrated the characteristic chemical responses of a solution with I- ions present.

References:

Birk, J. P. (2001, December 4). General Chemistry With Qualitative Analysis. Retrieved from Arizona State University: http://www.public.asu.edu/~jpbirk/qual/qual.html
Brown, W. (2012). Chemical Tests. Retrieved from Doc Brown's Chemistry: http://www.docbrown.info/page13/ChemicalTests/ChemicalTestsa.htm
Dartmouth College. (2003, May). ChemLab Chapter 5. Retrieved from Qualitative Analysis of Cations: http://www.dartmouth.edu/~chemlab/chem3-5/qual_an/overview/procedure.html
ScienceLab. (2005, October 10). Material Safety Data Sheet (MSDS) List. Retrieved from Science Lab: http://www.sciencelab.com/msdsList.php
SUNY Chemisty. (2005, December). Anion Analysis. Retrieved from Chemistry 112: http://employees.oneonta.edu/kotzjc/LAB/Anion.pdf
Tro, N. J. (2011). Essential Introductory Chemistry (4th ed.). Toronto: Prentice Hall.
White, R. (2013-2014). Qualitative Analysis Part 2- Anions. Chemistry 1 Laboratory Manual, 7-9.
Yoder, C. (2014, November). Qualitative Analysis of CAans and Anions. Retrieved from Wired Chemist: http://www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/qualitative-analysis

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