Question

Hi, i'm working on the lab report for Iodometric determination of Cu in Brass and i need help with these questions. I greatly appreciate your help.

Info: This experiment has two parts: standardization of the thiosulfate solution, and assay of the copper. In the standardization of the thiosulfate solution, a known amount of iodine is created from KIO3 and excess iodide, then titrated with the thiosulfate. In the copper assay, iodine is formed from the reaction between copper and excess iodide, then titrated with the thiosulfate.

1. How is the sample prepared for titration? how is the sample titrated?

2. what is the reaction between the titrant and iodine? how is the titrant concentration calculated?

Answer 1

1)SOLUTION PREPARATION =The brass sample which you will receive does not have to be dried before use. Accurately weight brass & directly into separate 250 mL Erlenmeyer flasks. In the fume hood add 5 mL of 6 M HNO₃. Warm the solution on a hot plate in the fume hood until dissolution is complete. Add 10 mL of concentrated (not 3 M) H₂SO₄ and continue heating until white SO₃ fumes appear. It is important that you do not mistake ordinary water vapor for SO₃ fumes. It is also important at this point that the flask not be removed from the hood. SO₃ fumes are dangerous and ought not to be inhaled. Cool the flask in air for one or two minutes and then in an ice bath, then carefully add 20 mL of distilled H₂O. Boil for one or two minutes then again cool in an ice bath. Continue to keep the flask in the ice bath and using your medicine dropper add concentrated NH₃(aq) dropwise, and with adequate mixing, until the light-blue color of the solution is completely changed to the dark-blue color of the copper tetraammine complex. As many as 400 drops (20 mL) may be required. The solution must be kept cool in an ice bath since the reaction between the concentrated H₂SO₄ and concentrated NH₃ is highly exothermic. Now add 3 M H₂SO₄ dropwise until the dark-blue color just disappears

TITRATION =

If you have let the dissolved samples stand overnight, be sure to warm the sample on a hot plate (this can be done at your desk) in order to dissolve all larger crystals of copper sulfate that might have formed. Be sure to cool the samples to room temperature, or below, with the aid of an ice bath. The solutions will still contain a fine, white precipitate at this point; however, this will not interfere with the rest of the procedure. From this point on work with only one sample at a time. Add 4.0 g of KI or KIO3 to one of your samples and titrate immediately with the standard thiosulfate solution. The sample contains white CuI precipitate and the color of I₃^- must be observed against that precipitate. The slurry will at first appear brown or dark yellow-brown. Continue adding thiosulfate until the slurry is a light mustard color. At this point add 5 mL of starch indicator and titrate until the mixture in the flask takes on a milky pink or lavender hue. Now add 2 g of KSCN and mix well; the solution will darken somewhat. After the addition of thiocyanate, continue to add more thiosulfate dropwise. You should observe a sudden change to a white or cream color. That is the endpoint of the titration.

TITRANT CONCENTRATION CALCULATION( another method)-

assume u take 25 ml of Cu salt salution, and in the titration V ml of F(N/10) concentration thiosalphate solution required.

so the concentration of cupper salt solution is = (V*F/25) N/10

from V1S1 = V2S2