Gravimetric AnalysisGravimetric Analysisa. simple, very little equipment or training is requireda. simple, very little equipment or training is required

b. fast b. fast c. can be fairly specific Example: A solution contains HgExample: A solution contains Hg2+2+. . We can determine the amount of We can determine the amount of HgHg2+2+ by precipitating it with a by precipitating it with a solution of Clsolution of Cl--. Assume there are . Assume there are no other insoluble chloride salts no other insoluble chloride salts present. A 25.00mL solution present. A 25.00mL solution containing Hgcontaining Hg2+2+ was treated with was treated with excess NaCl and precipitated excess NaCl and precipitated 0.4511g HgCl0.4511g HgCl22. What was the . What was the molarity of the Hgmolarity of the Hg2+2+ originally? originally?

Limitations of Gravimetric AnalysisLimitations of Gravimetric Analysis

Any interferents Any interferents must be removed must be removed prior to prior to precipitationprecipitation

Method is not very Method is not very sensitivesensitive

Species Species analyzeanalyzedd

Precipitated Precipitated formform

Form weighedForm weighed Some interfering Some interfering speciesspecies

KK++ KB(CKB(C66HH55))44 NHNH44++, Ag, Ag++, Hg, Hg2+2+, Tl, Tl++, ,

RbRb++, Cs, Cs++

MgMg2+2+ Mg(NHMg(NH44)PO)PO44.6H.6H22OO MgMg22PP22OO77 Many metals except Many metals except NaNa++ and K and K++

CaCa2+2+ CaCCaC22OO44.H.H22OO CaCOCaCO33 or CaO or CaO Many metals except Many metals except MgMg2+2+, Na, Na++, or K, or K++

BaBa2+2+ BaSOBaSO44 BaSOBaSO44 NaNa++, K, K++, Li, Li++, Ca2+, , Ca2+, Al3+, CrAl3+, Cr3+3+, Fe, Fe3+3+, Sr, Sr2+2+, , PbPb2+2+, NO, NO33

--

TiTi4+4+ TiO(5,7-dibromo-8-TiO(5,7-dibromo-8-hydroxyquinoline)hydroxyquinoline)22

samesame FeFe3+3+, Zr, Zr4+4+, Cu, Cu2+2+, C, C22OO442-2-, ,

citrate, HFcitrate, HF

VOVO443-3- HgHg33VOVO44 VV22OO55 ClCl--, Br, Br--, I, I--, SO, SO44

2-2-, CrO, CrO442-2-, ,

AsOAsO443-3-, PO, PO44

3-3-

CrCr3+3+ PbCrOPbCrO44 AgAg++, NH, NH44++

MnMn2+2+ Mn(NHMn(NH44)PO)PO44.H.H22OO MnMn22PP22OO77 Many metalsMany metals

FeFe3+3+ Fe(HCOFe(HCO22))33 FeFe22OO33 Many metalsMany metals

NiNi2+2+ Ni(dimethylglyoximaNi(dimethylglyoximate)te)22

samesame PdPd2+2+, Pt, Pt2+2+, Bi, Bi3+3+, Au, Au3+3+

Example:Example:

I have a solution of 100.0mL of a I have a solution of 100.0mL of a solution containing Lisolution containing Li++, Pb, Pb2+2+, Na, Na++, and , and KK++. I want to determine the amount of . I want to determine the amount of PbPb2+2+ is in the solution gravimetrically. is in the solution gravimetrically. The suggested procedure calls for The suggested procedure calls for precipitation with SOprecipitation with SO44

22. Would I be . Would I be able to use COable to use CO33

2-2- instead? What instead? What potential disadvantages are associated potential disadvantages are associated with using Nawith using Na22COCO33 instead of Na instead of Na22SOSO44??

Part 2 of examplePart 2 of example

The lowest reliable measurement The lowest reliable measurement possible with the electronic balance possible with the electronic balance is 0.0010g. What is the detection is 0.0010g. What is the detection limit for Pblimit for Pb2+2+ using SO using SO44

2-2- as the as the precipitating agent? COprecipitating agent? CO33

2-2- as the as the precipitating agent?precipitating agent?

Characteristics of Good Gravimetric Characteristics of Good Gravimetric TechniquesTechniques

Ideally Precipitates in a gravimetric analysis Ideally Precipitates in a gravimetric analysis should be:should be:

Insoluble, Ksp should be way below the Insoluble, Ksp should be way below the amount expected in solution. Limiting factor amount expected in solution. Limiting factor for detection is usually the balance, not Ksp. for detection is usually the balance, not Ksp. (For larger samples, this may not be true.) (For larger samples, this may not be true.)

Easily filterable, make large crystals which Easily filterable, make large crystals which separate from solution easilyseparate from solution easily

Pure with known composition, that is, they Pure with known composition, that is, they do not make a whole bunch of insoluble do not make a whole bunch of insoluble complexes.complexes.

Tips for good Gravimetric AnalysesTips for good Gravimetric Analyses

Things which can be Things which can be done for best done for best gravimetric results:gravimetric results:

Cool solution after Cool solution after precipitation has precipitation has started.started.

Add precipitating Add precipitating reagent slowly so reagent slowly so crystals are largercrystals are larger

Keep the volume of Keep the volume of solution large so solution large so concentration of analyte concentration of analyte and precipitant is low.and precipitant is low.

Laboratory proceduresLaboratory procedures

Transfer of solidsTransfer of solids Single chunk handle with Single chunk handle with

tweezerstweezers Powdered transfer Powdered transfer

washings with at least 3 washings with at least 3 times with solventtimes with solvent

Weighing bottles: tight Weighing bottles: tight fitting ground glass joints fitting ground glass joints prevents contamination or prevents contamination or loss of sample. When loss of sample. When handling ground glass handling ground glass joints use either paper or joints use either paper or tongstongs

Handling precipitates Handling precipitates Solution is precipitatedSolution is precipitated

Laboratory Technique Laboratory Technique ConsiderationsConsiderations

1. Filters1. Filters Paper is very hydroscopic, hard to get reliable tare weightPaper is very hydroscopic, hard to get reliable tare weight GFF made from glass fibers less hydroscopic but should still GFF made from glass fibers less hydroscopic but should still

be kept in desicator. Very fine pores, different pore sizes be kept in desicator. Very fine pores, different pore sizes available.available.

Sintered glass fibers cannot be heated to high Sintered glass fibers cannot be heated to high temperaturestemperatures

2. Transferring2. Transferring decant: Pour majority of supernatant through without decant: Pour majority of supernatant through without

disturbing precipitate.disturbing precipitate. Wash precipitate: decant wash solution.Wash precipitate: decant wash solution.

3. Control of moisture3. Control of moisture Moisture is ubiquitous in the laboratory, in order to Moisture is ubiquitous in the laboratory, in order to

control for this a dry atmosphere will have to be control for this a dry atmosphere will have to be maintained in a reproducibly dry atmosphere by maintained in a reproducibly dry atmosphere by storage in a desicator or evaporation or ignitionstorage in a desicator or evaporation or ignition

Laboratory Considerations Laboratory Considerations continuedcontinued

4. Drying: Occurs by putting an open weighing 4. Drying: Occurs by putting an open weighing bottle (may be covered with a watch glass) at a bottle (may be covered with a watch glass) at a temperature slightly above 100oC to get rid of temperature slightly above 100oC to get rid of water or waters of hydration.water or waters of hydration.

5. Weighing to constant weight means with 5. Weighing to constant weight means with ++0.1mg0.1mg Requires more than one weighing!!!Requires more than one weighing!!!6. Desicators 6. Desicators Objects cannot be weighed when hot or warm. Objects cannot be weighed when hot or warm.

But if you were to take a sample out and wait for But if you were to take a sample out and wait for it to cool it would pick up moisture like that so put it to cool it would pick up moisture like that so put it in a desicator. Don’t seal descicator, creates a it in a desicator. Don’t seal descicator, creates a vacuum Desicant blue dry pink wetvacuum Desicant blue dry pink wet

Common DesiccantsCommon DesiccantsMechanism of ActionMechanism of Action

HydrationHydration ANHYDRONE® ANHYDRONE® (Magnesium (Magnesium Perchlorate Perchlorate anhydrous), CaCl2, anhydrous), CaCl2, MgO, MgSO4, K2CO3, MgO, MgSO4, K2CO3, KOH, Drierite, KOH, Drierite, Na2SO4 (anhydrous), Na2SO4 (anhydrous), H2SO4, ZnCl2H2SO4, ZnCl2

Absorption and/ or Absorption and/ or AdsorptionAdsorption

BaO, CaSO4, BaO, CaSO4, Molecular Sieve, Molecular Sieve, H3PO4, NaOH PelletsH3PO4, NaOH Pellets

ChemisorptionChemisorption CaO, P2O5CaO, P2O5

Silica gel goes from Silica gel goes from blue to pink as it blue to pink as it absorbs moisture Can absorbs moisture Can be regenerated in be regenerated in ovenoven

Anhydrous sodium Anhydrous sodium sulfate gets clumpy as sulfate gets clumpy as it absorbs waterit absorbs water

More Information about desiccants including common interferents and regeneration temperature can be found at:http://www.jtbaker.com/techlib/documents/3045.html

Further laboratory considerationsFurther laboratory considerations

7. Evaporating: Evaporating dishes 7. Evaporating: Evaporating dishes allow rapid evaporation but also are allow rapid evaporation but also are easy to tip otherwise cause loss of easy to tip otherwise cause loss of analyte. Glass beads may be added analyte. Glass beads may be added to avoid bumping and loss of analyte.to avoid bumping and loss of analyte.

8) Ignition: Hotter temperature for 8) Ignition: Hotter temperature for elimination of water or other reactantelimination of water or other reactant

Muffle furnaces that go up to 1100Muffle furnaces that go up to 1100ooC C are used primarily for this Need to are used primarily for this Need to use porcelain crucible.use porcelain crucible.

ExampleExample A raw sewage sample was brought in for A raw sewage sample was brought in for

total suspended solids and volatile solids total suspended solids and volatile solids analysis. A portion of the sample was analysis. A portion of the sample was poured into a tared evaporating dish and poured into a tared evaporating dish and weighed. The sample was then heated at weighed. The sample was then heated at 105105ooC for 4 hours, weighed, heated at C for 4 hours, weighed, heated at 550550ooC overnight, and weighed again. The C overnight, and weighed again. The data are as follows:data are as follows:

Tare wt.Tare wt. 42.9073g42.9073g Wet sampleWet sample 104.4680g104.4680g 105 wt.105 wt. 45.4140g45.4140g 550 wt. 550 wt. 43.3236g43.3236g What is the TSS in g/L? What percent What is the TSS in g/L? What percent

of the solids are volatile?of the solids are volatile?

Why are we concerned about solubility Why are we concerned about solubility considerations when doing gravimetric considerations when doing gravimetric

analysis?analysis? Precipitate must be insoluble so Precipitate must be insoluble so

that greater than 99.99% of that greater than 99.99% of analyte present in solution analyte present in solution precipitates in order for gravimetric precipitates in order for gravimetric analysis to be considered analysis to be considered quantitativequantitative

Solubility losses (to solution) can Solubility losses (to solution) can be minimized by carefully be minimized by carefully controlling the composition of the controlling the composition of the solution in which the precipitate solution in which the precipitate forms. This requires understanding forms. This requires understanding the relevant equilibrium reactions the relevant equilibrium reactions affecting the precipitates solubility.affecting the precipitates solubility.

Solubility can be affected by pHSolubility can be affected by pH

Where is the Where is the solubility of CaCO3 solubility of CaCO3 expected to be the expected to be the least?least?

Be quantitativeBe quantitative

ExampleExample

For what pH range will PbCrO4 have For what pH range will PbCrO4 have its least solubility?its least solubility?

Application of Gravimetric AnalysisApplication of Gravimetric Analysis

A 0.3516g sample of a commercial phosphate A 0.3516g sample of a commercial phosphate detergent was ignited at red heat to destroy detergent was ignited at red heat to destroy the organic matter. The residue was then the organic matter. The residue was then taken up in hot HCl, which converts the P to taken up in hot HCl, which converts the P to HH33POPO44. The PO. The PO44 was precipitated out as was precipitated out as MgNHMgNH44POPO44

..6H6H22O by addition of MgO by addition of Mg2+2+ followed by followed by aqueous NHaqueous NH33. After being filtered and washed, . After being filtered and washed, the ppt. was converted to MgPthe ppt. was converted to MgP22OO77 (MW = (MW = 222.57g/mol) by ignition at 1000222.57g/mol) by ignition at 1000ooC. The C. The residue weighed 0.2161g. Calculate the % P in residue weighed 0.2161g. Calculate the % P in the sample.the sample.

Another exampleAnother exampleLong ago a workman at a dye factory fell into a vat containing Long ago a workman at a dye factory fell into a vat containing

hot concentrated Hhot concentrated H22SOSO44 and HNO and HNO33. He dissolved . He dissolved completely. Because no one witnessed the accident, it was completely. Because no one witnessed the accident, it was necessary to prove that he fell in so his wife could collect necessary to prove that he fell in so his wife could collect the insurance money. The man weighed 70kg. A human the insurance money. The man weighed 70kg. A human body contains 6.3ppt(housand) P. The acid vat was body contains 6.3ppt(housand) P. The acid vat was analyzed for P to see if it contained a dissolved human.analyzed for P to see if it contained a dissolved human.A. The vat had 8.00 x 10A. The vat had 8.00 x 1033L of liquid and 100.0mL was L of liquid and 100.0mL was

analyzed. If the man fell into the vat, what is the analyzed. If the man fell into the vat, what is the expected quantity of P in the vat?expected quantity of P in the vat?

B. The 100.0mL was treated with a molybdate reagent that B. The 100.0mL was treated with a molybdate reagent that caused (NHcaused (NH44))22[P(Mo[P(Mo1212OO4040)])]..12H12H22O to precipitate. This O to precipitate. This substance was dried at 110substance was dried at 110ooC to remove water of C to remove water of hydration and heated to 400hydration and heated to 400ooC until it reached a C until it reached a constant composition corresponding to the formula constant composition corresponding to the formula PP22OO55

..24MoO24MoO33 which weighed 0.3718g. The same acid which weighed 0.3718g. The same acid without a person dissolved in it and treated in an without a person dissolved in it and treated in an identical manner had a weight of 0.00331g. How much P identical manner had a weight of 0.00331g. How much P was present in the 100.0mL sample? Is this consistent was present in the 100.0mL sample? Is this consistent with a dissolved man?with a dissolved man?