Hello! I am freshman college student and I really need some enlightenment regarding the identification of cations. The experiment is referred to as 'looking for the unknown' and each is given a test solution and is asked to conduct a sequence of tests by adding certain reagents leading to the confirmation of what cation the test solution contains.
I have a lot of questions in connection to the experiment. One of those questions is about the possibility of not getting a precipitate even if one is aware that there should be (like for example, upon the addition of hydrochloric acid to a test solution containing a Group 1 cation) or how do you know the sufficent amount of reagent to add to avoid errors in qualitative analysis?
Please forgive me for this kind of inquiry. I am really trying to learn and would like to understand all of it for future reference. Thank you all in advance!
Identification of cations in one simple salt or in an inorganic mixture is quite methodical. If precautions are not followed, you may not detect cation or you can make wrong reporting due to interference by other cation or even anion. Therefore, due care should be taken in maintaining the experimental conditions and complete separation of cations of a group before proceeding to next group.I have notified two posts ; one dealing with considerations required for identification of cations and another with identification of cations without using H2S gas on my website: profrssindhufansclub.wordpress.com - visit this site. Many things will be clear to you. If there is need of seeking clarification, you can write that on space provided for comments. I will surely communicate to you.
I thank you for giving response, sir. I will surely read the articles in the site.
I was perplexed when our class conducted this experiment in which we are instructed that the test solutions contain Group 1 cations (Hg 2 2+, Pb 2+, and Ag +). We did follow the procedures of our laboratory manual and I followed the procedures in the best way I could (according to my understanding). The first reagent to be added was a 3M hydrochloric acid and we expect that there will be a white precipitate which contains the mercurous chloride, plumbous chloride, and silver chloride. However, some, including me, were not able to get that kind of result and for us, there was no precipitate at all. Probably because we added too much chloride ions, a soluble chloride complex was formed but this is very uncertain because there is no precipitate even in the very introduction of HCl. Moreover, this experience is common to those whose unknown was a mercurous and a plumbous.
It depends on the concentration of the cation. HgCl2 and PbCl2 are relatively more soluble in water than AgCl.
Thank you again for the response. Can it probably mean that, assuming we've done the right procedures, there could have been a mistake in the concentrations of the mercurous and plumbous? In my case, I just added 4 gtts of HCl and after that, I became afraid of adding more. Also, I would like to ask some recommendations on how can we know the right amount of reagent to add. Usually, the lab manual would only instruct like "add 6M NH4OH" or "put crystals of sodium hydroxide" and no body is sure of how much drops to instill in the test solution. Again, thank you very much for all your responses!
You can find the solubility of the respective salts and calculate the amount of HCl to add that results in precipitation.
I see. I'll delve into that. Again, thank you very much! Have a great day!
Please note that if your solution for testing lead cation is hot, PbCl2 precipitate formed on adding HCl will dissolve in that solution. While adding HCl, your solution should not be hot.
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