Investigating the Determining Characteristics of Cations and Anions

Introduction: The purpose of this lab was to study the specific characteristics of cations and anions, and ultimately to be able to identify an unknown substance based on our studies and tests using the logic trees developed through the experiment. A logic tree is a graphical display of the findings from this lab which, through a series of yes/no questions, elimination tests, will help us to identify and unknown sample. This lab will result in 2 logic trees, one for identifying cations and the other, anions. The lab was broken into three parts: Part 1 dealt with identifying a cation, Part 2 an anion, and finally Part 3 an unknown salt mixture of anions and cations was identified using the same tests in Part 1 and 2.

Materials and Methods: Part 1: Cation Analysis: The experiment was performed on 4 cations: K+, Zn2+, Cu2+, and Co2. To begin the elimination and confirmation tests, 4 centrifuge tubes were prepared by adding 10 drops of each known cation in metal solution; the tubes were labeled accordingly. The first test performed was the Metal Hydroxide Test, wherein drops of 6 M NaOH were added dropwise to each tube until a precipitate was formed, or a total of 20 drops were administered. This process was repeated for each tube, with drops administered until a precipitate was visible. Drops were administered as follows:K+: 20 drops, Zn2+: 3 drops, Cu2+: 2 drops, Co2+: 2 drops 6 M NaOH added. The results were recorded following each cation, noting the color, clarity and appearance (texture). Next, for each solution in which a precipitate formed (Zn2+, Cu2+, Co2+) an additional 10 drops of 6M NaOH were added to each centrifuge tube individually and the results recorded.

The contents of the centrifuge tubes were then disposed of in the waste container. The next Cation Elimination Test was the Ammonium Hydroxide Test. For this test, centrifuge tubes containing each of the 4 cation metal solutions (K+, Zn2+, Cu2+, Co2+) were again  made by adding 10 drops of solution into the appropriately labeled tube. Drops of 15 M NH4OH were administered to each tube individually until a precipitate was visible, or until 20 drops had been administered. The experiment was performed on each tube individually, as follows: K+: 20 drops 15 M NH4OH added, Zn2+: 3 drops 15 M NaOH added, Cu2+: 1 drop 15 M NaOH added, Co2+: 2 drops 15 M NH4OH added. Following each experiment, the results were recorded, noting color, clarity, and appearance (texture). For all the experiments which yielded a precipitate (Zn2+, Cu2+, Co2+), 10 additional drops of 15 M NH4OH were added to the solution and the resulting changes and products were recorded. The contents of each tube were then disposed of in the appropriate waste container. The final Cation Test was the Confirmation Test­ the Flame Test. A solution of each cation was prepared (K+, Zn2+, Cu2+, Co2+) by pouring a small sample of the solution into the correspondingly

marked centrifuge tube, additionally a sample of 6 M HCl was prepared in a 50mL beaker for purifying the nichrome loop. A Bunsen Burner was assembled and clamped to the ring stand. The nichrome loop was dipped into the prepared 6 M HCl and then placed in the flame of the Bunsen Burner to purify the loop. This process was repeated until the loop could be placed in the flame with no color change. The loop was then placed into the K+ sample and then returned to the flame. The color emitted was recorded. The loop was again purified by dipping it into 6 M HCl and then returned to the flame until no color change was detected. This process was repeated for the remaining 3 cations (Zn2+, Cu2+, Co2+) and the results recorded. Using the already prepared samples of cations, the entire process was repeated again but the resulting flame emissions were observed through a square of cobalt glass held to the eye. The resulting color flashes were recorded. The cation solutions were then disposed of in the appropriate waste containers.

From the information in each of these tests, a Cation logic tree was formed for easy identification of unknown cations.  Part 2: Anion Analysis: The experiment was performed on 4 anions: chloride (Cl­), sulfate (SO42­), nitrate (NO3­), and carbonate (CO32­). A sample of each anion was prepared by placing 10 drops of each anion solution into a centrifuge tube. The tubes were labeled accordingly. The original color of the solution was recorded. The first test performed was an Anion Elimination Test, the Silver Nitrate Test. 0.1 M AgNO3 was added dropwise to each tube until a precipitate was formed or 5 drops of 0.1 M AgNO3 were administered. First, 2 drops of 0.1 M AgNO3 was added to the tube containing Cl­ and the results recorded, noting the appearance (texture), color, and shape. This process was repeated for all the anions as follows: SO42­ : 20 drops, NO3­: 20 drops, CO32­: 1 drop of 0.1 M AgNO3 was added.

For each solution which formed a precipitate (Cl­,CO32­) the solution was centrifuged for 5 minutes and the supernatant was decanted; 5 drops of distilled water were added to both samples. 10 drops of 6 M HNO3 was added dropwise to both the Cl­ and CO32­. The chloride and carbonate tests were next performed to confirm the presence of those anions. To start the chloride test, 10 drops of the chloride anion solution were added to the centrifuge tube. 1 drop of 6 M HNO3 was added and solution was tested on blue litmus paper. The solution turned the litmus paper red. 5 drops of 0.1 AgNO3 were added to the solution and the results recorded. The centrifuge tube was centrifuged at 4950 rpm for 2 minutes. Supernatant was poured off and 4 drops of NH4OH were added. 2 drops of HNO3 were added and precipitate formed. To start the carbonate test, 30mg of Na2CO3 were added to a centrifuge tube. 3 drops of 6 M H2SO4 were added to the solution. Next, the sulfate test was performed to determine whether nitrate anion solution or sulfate anion solution formed precipitate in the presence of BaCl2. 10 drops of each anion

solution were placed in separate centrifuge tubes and 1 drop of 6 M HNO3 were added to each sample. 2 drops of BaCl2 were added to both samples. The sulfate test was repeated identically on a sample of sulfate anion solution. Finally, the nitrate test was performed to confirm the presence of nitrate anion by reducing it to form ammonia gas. 10 drops of nitrate anion solution and 10 drops of NaOH were added to a centrifuge tube. A pipette was used to transfer the sample into a tube with dry walls. Hot water bath was started on a hot plate at 85°C. 0.195 grams of zinc were collected and 0.076 grams of zinc were added to the solution. The sample tube was placed in the hot water bath and a cotton ball was placed in the tube above the solution. A strip of red litmus paper was folded over the top of the sample tube and sample was left to heat. After about 15 minutes, litmus paper turned blue. Part 3: Unknown Ionic Compound Analysis: For the final section of this lab, an unidentified substance was assigned.

Following the Logic Trees formulated in Parts 1 and 2, identify the compound. 0.5003 grams of the unknown substance dissolved completely in 5 mL of distilled water. 1.3mL of the substance was measured out and 5 drops of 6 M NaOH was added to the solution and the were results recorded. 10 additional drops of 6 M NaOH were added. Following the logic tree, a Bunsen Burner was assembled and attached to the ring stand. A small part of the solution was poured into a beaker and the flame test was performed following the procedure in Part 1. The results were observed through cobalt glass and recorded, from this the cation identity was confirmed. The remaining stock solution was disposed of in the appropriate waste container. For the Anion Analysis, 0.1009 g of the unknown compound was measured. 0.4999 grams of Na2CO3 was measured out. Both powders were poured together into a 50mL beaker and 5mL of distilled water and a boiling chip were added. The solute’s starting volume was marked on the side of the beaker.

The  solution was placed on the heating plate at 100 C and allowed to boil for 10 minutes, while continually adding water to maintain the solute’s volume at the mark. Observations were recorded. The resulting mixture was divided equally into 2 centrifuge tubes and placed into the centrifuge. It was allowed to centrifuge until separated. The precipitate was fished out and discarded. The remaining solution was used as the stock solution. The solution was then analyzed following the cation tree. The Silver Nitrate test was first performed. 3 drops of 0.1 AgnO3 were added and the results recorded. Following the tree, 4 drops of 6 M HNO3 were added with 5 mL of distilled water. The results were recorded. Finally, in accordance with the tree, the Chloride Confirmation test was performed following the procedures in Part 2. The results were recorded, and the identity of the anion confirmed.

Results and Discussion:Part 1 Cation Analysis: In part 1, Cation Analysis, the Metal Hydroxide test was used to find the solubility of hydroxide formed by metal cations. When drops of 6 M NaOH were added to each Cation, the following was observed: The K+ solution had 20 drops added but no reaction was observed; the original solution was clear and after the drops were added, there had been no visible reaction. This result determined that the potassium cation formed an amphoteric metal hydroxide, i.e. a metal hydroxide that is insoluble but dissolves in a basic solution. The Zn2+ solution had 3 drops added; the original solution was described as clear, after the drops were added, a cloudy, white precipitate formed.

Because a precipitate was formed, 10 more drops of 6 M NaOH were added and the white solid dissolved completely. In the Cu2+ solution, originally the solution was light clear blue, after 2 drops were added, a dark blue solid precipitate formed. When 10 additional drops of NaOH were  added, the entire solution solidified into a light blue gel­like looking precipitate. For the Co2+ solution, initially pale red, when 2 drops were added, a dark blue, hard precipitate formed. Following the 10 additional drops of NaOH, a blue­green solid formed, and the solution turned a pale, cloudy pink.

The Ammonium Hydroxide Test was used to determine whether metal hydroxide or ammine complexes were formed by each cation in the presence of ammonia. When 20 drops of 15 M NH4OH was added to the K+ solution, the innitially clear solution had no visible reaction. After 3 drops of NH4OH were added to the zinc cation (originally a clear solution), a thin, white solid formed; when 10 more drops were added, there was no change to the solute or precipitate. When just 1 drop was added to the Cu2+ solution, the solution seperated and a light, pale, cloudy blue solid formed while the remaining liquid turned dark blue. Following 10 additional drops, the precipitate dissolved completely into the dark blue liquid. Finally, after 2 drops of 15 M NH4OH were added to the Co2+ cation, the solution to a brown and a light blue solid formed at the bottom. When 10 more drops were added, the solution turned to a grey­black color.

Finally, for the flame test, each cation was first observed with the naked eye and then again through cobalt glass. From the naked eye, K+: produced an orange­lilac flame; Zn2+: produced an orange flame Cu2+: produced a light, clear, pale blue burst; Co2: produced a pale yellow flame with sparks. When the experiment was repeated and the results observed through the cobalt glass, the following resulted: K+: a magenta flame; Zn2+: a barely visible orange flame was observed; Cu2+: there was no visible color; Co2: a white flame with sparks was observed.