Cation Analysis Formal

Cation Analysis Faith Bittner Jimmy O'Keefe C126 Experimental Chemistry II Section 25032 April 10, 2023 Introduction The world of chemistry is heavily dependent on qualitative analyses, or numerical data and its interpretation. This includes volumes taken to reach an endpoint in a titration, or the amount of heat, in degrees, needed to reach a boiling point. However, there is another side of chemistry where numerical data is not present. This is known as a qualitative analysis. Rather than determining quantities of substances, the goal is to identify what substances are in an unknown sample. This can be achieved through a variety of tests and methods, which can include flame tests, spot tests, and separations. Separations usually involve precipitation reactions. Flame tests and spot tests help to confirm the presence of the unknown. A flame test involves a molecule containing a metal ion to be placed in a flame. Different metals will flicker different colors and can be used as an identification method. However, there is a lot of room for contamination with this method. Spot tests are when the unknown compound is reacted with a known reagent. A precipitate, color change, or nothing will result. The problem with spot tests is if more than one ion in the unknown reacts with the known reagent. Therefore, separations are the best method to use when there are more at least two unknown cations present. Various precipitating reagents can be used to precipitate certain ions out of a solution. Then, a centrifuge can separate the precipitate. Furthermore, precipitate-dissolving reagents are used to dissolve the precipitate or the supernatant. There are times where an unknown substance will be encountered, and it will need to be examined to discover its contents. A prime example of this is in forensic science. There are always questioned substances that are found at crime scenes and breaking these into their components can help gain understanding of what the scientists are dealing with. In addition, it can also lead to solving the case by identifying a crucial piece of evidence and connecting it with a suspect.

The purpose of this lab was to develop a scheme that could separate and identify a compound composed of unknown metal ions, which could have included silver (Ag + ), iron (Fe 2+ ), nickel (Ni 2+ ), lead (Pb 2+ ), and potassium (K + ). Experimental Procedure/Data and Calculations The procedure for this experiment consisted of two parts: testing the known metal cations and determining the metal cations present in an unknown solution. The first test conducted was the spot test. One drop of each of the metal ion solutions was added to one drop of each of the spot test reagents. The spot test reagents were K 2 CrO 4 , K 4 [Fe(CN) 6 ], NH 3 , and dimethylglyoxime. After initial observations were recorded, a stirring rod was used, and additional observations were made. The data from this test can be found in Table 1 of the Results section. The second test performed was the separation method using precipitation reactions. A few drops of each metal ion solution were added to a test tube with a few drops of each precipitating reagent. The precipitating reagents were 6M HCl and 6 M NH 3 . If a precipitate formed, then two additional samples were created. Each precipitate sample was centrifuged and decanted. Then, the solubility of the precipitate was tested with each precipitate-dissolving reagent. The precipitate-dissolving reagents included 6 M NH 3 , 6 M HNO 3 , and hot DI water. Table 2 of the Results section contains the precipitating reagents data, while Table 3 displays the results with the precipitate-dissolving reagents. The third test was the flame test. A wire loop was dipped into the metal ion solution. The wire was then placed into a Bunsen burner flame. The color change of the flame was observed for each solution. The observations of this part can be found in Table 4 of the Results section. From the observations and data of the known metal cations in each test, a flow chart scheme was developed. This is labeled as Figure 1. Part 2 of this experiment involved using this analysis scheme on an unknown solution that contained any combination of the five metal cations. The procedure in the aforementioned paragraph, along with the flow chart, was tested using the unknown. Based on observations of the unknown with the different tests, a conclusion was able to be made about which cations were present. The unknown used in the experiment was #74.

Fe 3+ yellow, light ppt very dark green, almost black after stirring: spread out orange ppt after stirring: broken up more super pale yellow, no ppt Ag + brown ppt after stirring: more definitive ppt yellow with orange ppt after stirring: orange ppt gone, just yellow NR NR Ni 2+ light yellow, no ppt dark yellow, dark orange ppt after stirring: spread out ppt light blue/purple blood-reddish ppt after stirring: very viscous ppt, sticks to rod K + yellow, no ppt yellow, no ppt NR NR Like the spot test, the precipitating reagents were used to see which metal ions would form a precipitate with them. This was helpful for the unknown solution because if the solution formed a precipitate or not, certain cations could automatically be ruled out. For example, no reaction (NR) with the HCl would automatically indicate Ni 2+ or K + . However, if a precipitate formed, the extra step of using the precipitate-dissolving reagent was applied, and further observations were made. Table 2: Precipitating Reagents Pb 2+ Fe 3+ Ag + Ni 2+ K + 6 M HCl spotty white ppt light green, no ppt chalky white ppt NR NR 6 M NH 3 milky white ppt rust color, ppt NR NR NR Table 3: Precipitate-Dissolving Reagents Pb 2+ /HCl Pb 2+ /NH 3 Fe 3+ /NH 3 Ag + /HCl 6 M NH 3 ppt breaks up, floating chunks filmy, ppt still on bottom no change no change 6 M HNO 3 no change no change gas evolution ppt layer on top

Hot DI H 2 O no change foggier and film separated ppt slightly slightly cloudier Lastly, a flame test is used as a method of confirming the identity of the unknown, like the spot test. Table 4: Flame Test Pb 2+ turned orange after about 2-3 seconds Fe 3+ orange sparks after 2-3 seconds Ag + 4-5 seconds, flickering orange, not super obvious Ni 2+ 2-3 seconds to start burning orange, heavier color K + 2-3 seconds lighter orange also with blue flame inside of the orange The analysis of the unknown began with adding NH 3 , as noted in the flow chart. After the addition of 6 M NH 3 to the unknown, there was no reaction. This meant that the sample did not need to be centrifuged and was a combination of Ag + , Ni 2+ , or K + based on Table 2. A spot test was performed to help distinguish which cations were present. When the unknown reacted with K 2 CrO 4 , a dark reddish maroon precipitate formed. The K 4 [Fe(CN) 6 ] with the unknown created yellow solution with an orange precipitate. NH 3 with the unknown formed a light blue/purple color. Lastly, the dimethylglyoxime resulted in a pink/blood red color when added to the unknown. As the results in Table 1 display, a dark maroon color resulting from K 2 CrO 4 occurs when Ag + is in solution. The K 4 [Fe(CN) 6 ] produced a yellow solution with an orange precipitate for both Ag + and Ni 2+ . From the known experiments, NH 3 formed a light blue, purple solution when reacted with Ni 2+ . Lastly, dimethylglyoxime formed a blood-reddish precipitate with Ni 2+ . The similarities with the two spot tests confirmed the presence of Ag + and Ni 2+ . The spot tests were the most indisputable because of how obvious the precipitate and color changes were. Only one drop of unknown and reagent were needed for the reaction, so it was simple to complete. Also, there were many wells on the spot plate. This meant the same reaction could be ran multiple times, and each reaction could be visualized along with the others. Like with any experiment, qualitative or quantitative, there is always room for error. A huge one for this experiment was the possibility of contamination. For example, the unknown

References

Anliker, K., Breen, N., and Nguyen, M., Experimental Chemistry II, C126 Laboratory Manual, Hayden-McNeil, 2007