How to prepare the stock solutions of exact concentrations of metal ions with the following composition XCl3. nH2O? Is it right if I consider the molecular weight of anhydrous salt for all my calculations? Or do I need to have the anhydrous salt always?
You can use either anhydrous or hydrated versions of the MCl3 salts as long as you know how much water is present in your hydrated compound. If you're unsure you should dry the salt as much as possible and then use the anhydrous molecular mass. Remember though that many anhydrous salts have significant hydration enthalpies and so the solution may get hot as you prepare it.
Actual problem was the unknown amount of water i.e, nH2O. As you both said, there is no alternative for this I guess. Because I need to measure the concentrations based on their absorbance in UV. Unknown concentration can be obtained by the graphical method.
you can also dissolved metal in HCl solution and adjust the volume (to 1 L for example) C = n / V in this case C = m * Mw / V
with this method your solution is acid.
I avoid to use chloride of metals in order to prepare standard stock solutions, I rather prefer the compounds in elementar form or the oxides. The salt form is usually complicated because many are hygroscopic. Few salts are no hygroscopic or do not have water of crystallyzation. If the salt is not hygroscopic , the water of crystallization of this salt is stable until 140º C and the salt is soluble in the solvent, I do not see any problem to use it.
when you have n.H2O (unknown amount of H2O), how do you prepare standard solutions? I mean do you consider the M.W. of anhydrous salt? We dont know the molecular weight of the actual compound.
You can estimate the concentration of the metal ion of the salt containing unknown molecules of water by: 1- atomic absorption, 2- complexometric titration using a known ammount or concentration of the complexing agent. or 3- precipitating agents are also well recomended by adding the specific ppting agent of known concentration and after filtration wash the ppt and dry it fot weighing . you can titrate the filtrate with a titrant of known concentration to know how much exactly the volume of the remaining pption agent
There are some analytical methods for quantitative determination of lattice water you can find them in experimental books of analytical chemistry
To avoid using salts of chloride form to prepare standard solution because you do not known the exact number of hydrate, and most of chloride salts are easily hygroscopic. Moreover, hydrolysis may occur when yoy try to heat to obtain anhydrous form using these salts. You can use pure metal or its oxides form to prepare stock solution. If you use these chloride salts as raw materials to prepare stock solution, you have to determine the concentration by means of instrumental analysis such as AAS, ICP-AOS and others.
Actually, an expeienced chemist should always learn from an undergraduate chemistry student.
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