Ch 27 Gravimetric Analysis - Cal State LA

1 11Ch 27 Gravimetric Analysis2 Analytical chemistryClassification by the techniques:1. Classical Analysis2. Instrumental AnalysisGravimetric, Titration(Volumetric) AnalysisElectrochemical Analysis ,Spectrochemical Analysis ,Chromatographic Separation and Analysis 23Ch 12 Gravimetric Analysisgravi metric (weighing - measure)Definition: A pprecipitation or volatilizationmethod based on the determination ofweight of a substance of known compositionthat is chemically related to the (1) The desired substance: completelyprecipitated.

2 "common ion" effect can be utilized:Ag+ + Cl- AgCl(s)excess of Cl- which is added(2) The weighed form: known composition.(3) The product: "pure", easily 1 Procedure 3512. 1 Procedure 7 Steps in Gravimetric Analysis1) Dry and weigh sample2) Dissolve sample3) Add pprecipitating reagent in excess4)Coagulate precipitate usually by heating5) Filtration-separate precipitate from mmother liquor6) Wash precipitate7) Dry and weigh to cconstant weight ( mg)6 Suction Filtration Filter flask Buchner funnel Filter paper Glass frit Filter adapter Heavy-walled rubbertubing Water aspirator 47 Suction Filtration Mother Experimentally simple and elegant Accurate Precise ( %) Macroscopic technique-requires at least10 mg ppt to collect and weigh properly Time-consuming (1/2 day?

3 Calculation Design of experiment Content Calculation Evaluation of the Calculation % of analyte, % A %A = weight of analyte x 100 weight of sample weight of ppt directly obtained ->%A 611 How Do We Get %A? % A = weight of ppt x Gravimetric factor ( ) x 100 weight of sample = a FW[analyte] b FW[precipitate] = # gms of analyte per 1 gm ppt12 The Gravimetric Factor = a FW[analyte] b FW[precipitate] (NO3) 713 Gravimetric Factor (NO3)2U3O83UO2(NO3)2/U3O8Cr2O3Ag2 CrO4Cr2O3/2Ag2 CrO414 Problem Consider a g sample containing75% potassium sulfate (FW )and 25% MSO4.

4 The sample isdissolved and the sulfate is precipatedas BaSO4 (FW ). If the BaSO4ppt weighs , what is the atomicweight of M2+ in MSO4? ANS: Mg2+ 815 Answer The hard part is setting up the correctequation Rearranging and * + * + + ;x= (2+Mg)16 Problem A mixture of mercurous chloride (FW )and mercurous bromide (FW ) g. The mixture is quantitatively reducedto mercury metal (At wt ) which g. Calculate the % mercurous chlorideand mercurous bromide in the originalmixture. ANS: g 917 Answer Again, important to set up correctequation: Rearranging and * * +2* 2 x() + 2 x()= Example: How many mL of 1% potassiumchloride would be needed to precipitate allof the silver in a g ore sample thatcontains parts per thousand silver?

5 Allow for a 50% excess of the chloridesolution. Representative GravimetricAnalyses20 The PrecipitationProcess1. Particle Size / Filterability produce particles large enough to be'caught ideally, produce crystals avoid colloidal suspension 1223 24 13252. Mechanism of PrecipitationCrystal Forming:nucleationparticle growth26 142728 152930 163132 1733 Techniques to promote crystal growth(1).Raising the temperature to increase Sand thereby decrease relativesupersaturation.(2).Adding precipitant slowly with vigorousmixing, to avoid a local, highlysupersaturated condition where the streamof precipitant first enters the analyte.

6 (3).Keeping the volume of solution large sothat the concentration of analyte andprecipitant are Other ways to obtain a good precipitate(1) Increasing the electrolyte concentration 1835 High Electrolyte Concentration to AidPrecipitation(1) Decreasing the volume of the counter-ion layer(2) Increasing the chance for coagulation363. Other ways to obtain a good precipitate(2) Using a volatile electrolyteAgCl, wash with HCl. Drying precipitate at 110 Cwill remove displace the less volatile, excess counter ion. 1937(3)38(4) pH control of precipitationCa2+ + C2O42- CaC2O4 (s)H2C2O4 2 H+ + C2O42- 2039(5) HomogeneousPrecipitation Fe3+ + 3 HCO2- Fe(HCO2)3 nH2O(NH2)CO + 3 H2O + heat HCOOH + OH- + CO2 + 2 NH4+40 Exercises1.

7 To obtain a precipitate which is useful for Gravimetric Analysis , theanalyst tries to obtain conditions to encourage crystal growth, asopposed to the formation of a colloid. Which of the followingstatements aids in the formation of a crystalline precipitate?(a) The solution is kept cool to slow down the movement of ions toallow crystals to form.(b) The solutions are made as dilute as practical to allow crystals toform slowly.(c) The solutions are mixed rapidly to allow the appropriate ions tomake contact without interference of other materials.

8 2141 Exercises2. Which of the following processes is true regarding the digestionof a solution containing a precipitate?(a) Digestion tends to increase particle size of the precipitate.(b) Digestion is used to slow down the coagulation of a colloidalprecipitate because of the increased energy added to the solution.(c) A long digestion is not recommended because the highertemperatures enable impurities to become trapped in the crystallattice of the precipitate42 Exercises3. In the Gravimetric Analysis of iron, hydroxide may be added to asolution containing Fe3+ to precipitate a gelatinous mess which is ignitedto form Fe2O3.

9 If grams of Fe2O3 were formed from the ignitionof the precipitated mess, how many grams of FeCO3 were contained inthe original sample analyzed?(a) g FeCO3(b) The answer cannot be computed because the sample did not containany Fe2+.(c) g FeCO3 2243 Exercises4 A sample of ore containing manganese weighed sample was heated in a solution of nitric acid, whichdissolved the manganese ion producing manganese(III) nitrate(Mn(NO3)3). The acidity of the solution was adjusted, andHydrogen sulfide gas was bubbled into the solution grams of manganese(III) sulfide (Mn2S3).

10 What is thepercentage of manganese in the original sample?(a) Mn(b) Mn(c) Mn44 Exercises5 Why do you reheat/recool the solid unknown until its weightstabilizes? increase the precision of your increase the accuracy of your ensure that there is no water in your convert any nickel metal to nickel oxide. 2345 Exercises6 In Gravimetric determination of Nickel, heating gently duringthe coagulation stage would aid what process?a. Precipitation -- more Nickel diglyme would come out Digestion -- particle size and purity increases due Evaporation -- the solution becomes more concentrated so moreprecipitation occurs and filtering is Calculation -- the waiting gives you time to work exampleproblems and ask A certain barium halide exists as the hydrated salt ,where X is the halogen.