1. Experimental

a. Safety

When handling dichloromethane extra caution should be exercised because the compound is a selected carcinogen, which means that it can be carcinogenic to humans. Only handle it in the hood.

Potassium chromate is a regulated carcinogen because it contains Cr(VI). This implies that it is carcinogenic to humans. Special care should be taken when handling the compound.

b. Experiment

Since SnCl₄, SnCl₄* 2 THF and SnCl₄* 2 ACN are extremely moisture sensitive, the entire synthesis has to be carried out, and the final products have to be stored under inert gas.

If the provided SnCl₄ is not fresh, it should be distilled prior use. Why?

Both, the SnCl₄ and the Lewis base (dry THF or dry CH₃CN) have to be diluted. The reaction is exothermic and dichloromethane has a pretty low boiling point. Thus, the addition of the ligand solution has to be added slowly to the SnCl₄ solution.

2. Characterization

a. Iinfrared spectra (YH 1033)

- Acquired as Nujol mull between CsI plates, the adducts hydrolyze pretty fast and attack (oxidize) the plates, so efficiency is important here).

- Read the chapter on how to acquire infrared spectra (Appendix A and B, p. 131-140)


b. Chloride analysis

The first step would be to prepare a standardize AgNO₃ solution. If there is no stocl solution available, a ~0.1 M solution can be prepared from the solid AgNO₃ that has to be dried at 100 ^oC for some time. In order to determine its exact concentration, sodium chloride is used for standardization About 10-20 mg of dry NaCl (previously dried at 130 ^oC for at least two hours) are dissolved in 50 mL deionized water. Two drops of conc. sulfuric acid are added. It is important that the pH is properly adjusted to pH~5-7 using sodium or potassium bicarbonate.

The potassium chromate is added after the pH-adjustment. It serves as an indicator in this titration. Ag₂CrO₄ (K_sp=1.12*10^-12, 25 ^oC) is reddish-brown in color, while AgCl (K_sp=1.77*10^-10, 25 ^oC) is white (Silver dichromate (K_sp=2*10^-7, 25 ^oC) and silver hydroxide (K_sp=1.52*10^-8, 20 ^oC, black)). The solution is originally bright yellow and changes color to orange-brown at the end point. All the solutions (standardization and actual samples) should be titrated to the same end point color, which requires a little bit of practicing.

Since the actual samples are prepared in basic solution, concentrated sulfuric acid (ACS grade) is dropwise first to acidify the solution (pH~2-3) before NaHCO₃ is added. Careful: The mixture foams heavily at this point!


d. Tin analysis (need to make an appointment as group with the instructor three days before planning to acquire!)

In order to determine the quantity of tin in the compounds, ICP-AE will be performed (inductively coupled plasma atomic emission) for the samples. In order to do so, 30-40 mg of the sample have to be dissolved in ~20 mL of 6 M hydrochloric acid. This solution is then placed in a volumetric flask and fill it up to 100 mL using 1% HCl to form the stock solution. This solution has then to be diluted again (10 mL into 100 mL) using 1% HCl. The final concentration of Sn in the sample has to fall into the calibration range!!

In order to calibrate the instrument, several standards in the concentration range of 1 to 20 ppm from a 1000 ppm stock solution have to be prepared. What's the best strategy for that? This part can be done by the group since one set of standards is enough to run the samples.

A 1% nitric acid solution has to be prepared in addition as a rinse solution for the instrument.


3. Hints to questions

ad 2: What type of reaction are you looking at if you react THF with TiCl₄?

ad 3: Generally, complexes are characterized using which number?

ad 4: Which factors play a role in the stability of a complex?


4. Problems to be considered

a. Why is it necessary to acidify the solution first before adding NaHCO₃?

b. A student forgot to dilute the Sn-samples. What would he observe in his ICP-AE measurements? How could he fix the problem other than diluting the samples?

c. Why is it not advisable to use KOH when dissolving the compounds for the Sn-analysis?