Analisis gravimetri

Prinsip Dasar AnalisisGravimetri

Salah satu metode analisis kuantitatif makro yangpaling akurat

Analit (bahan yang dianalisis) secara selektifdikonversi menjadi endapan yang stabil (tidaklarut)larut)

Pengukuran massa endapan Mengubungkan massa endapan dengan masa

analit Masih banyak digunakan dan sering lebih akurat

Contoh

Sampel garam dapur (NaCl)Larutan sampel ditambah larutan perak nitrat,

endapan terbentuk adalah perak kloridaendapan terbentuk adalah perak klorida(AgCl)

Cl- + Ag+AgCl(s)

Langkah-langkah umum

1. Sampel kering ditimbang dengan akurat2. Penyiapan larutan3. Pengendapan4. Penyempurnaan endapan (digestion)4. Penyempurnaan endapan (digestion)5. Peyaringan6. Pencucian7. Pengeringan atau pembakaran (ignition)8. Penimbangan9. Perhitungan kadar

Konversi analit menjadi endapan Pengendapan secara kuantitatif dengan perekasi

yang sesuai Penguapan dan pengumpulan endapan Tanpa kehilangan massa selama penanganan

dan prosesdan proses Bebas dari pengotor dan pelarut

Penentuan massa Langsung (BaSO4, AgCl, dll) Selisih (CaC2O4.H2O dikonversi menjadi

CaC2O4.)

Langkah penting

Teknik Pengendapan Penyiapan pereaksi untuk mengendapkan analit Mereaksikan analit menjadi senyawa atau unsur yang

tidak larut Endapan mempunyai komposisi yang telah diketahui

(atau bisa diubah menjadi senyawa atau unsur yang(atau bisa diubah menjadi senyawa atau unsur yangkomposisinya diketahui)

Perlakuan endapan Pengumpulan endapan secara kuantitatif (tidak ada

yang hilang) Isolasi produk murni

Menimbang massa Menghitung kadar analit awat (konsentrasi)

Endapan yang akan ditimbang Siap disaring dan dimurnikan Kelarutan sangat rendah Stabil (tidak mudah bereaksi, misalnya

oksidasi)oksidasi) Komposisi diketahui setelah pengeringan atau

pembakaran (ignition)

Agen pengendapan Selektif Ag+ + Halida (X-) AgX(s) Ag+ + CNS- AgCNS(s) Spesifik Dimethylglyoxime (DMG) Dimethylglyoxime (DMG) 2 DMG + Ni2+ Ni(DMG)2(s) + 2 H+

Endapan bisa disaring

Suspensi koloidal Partikel berdiameter 10-7 sd 10-4 cm Biasanya sulit mengendap Sukar untuk disaring

Suspensi kristalin Suspensi kristalin Diameter > (1/10) mm Secara spontan mengendap Mudah disaring

Gravimetric Analysis Kemudahan endapan untuk disaring:

Pembentukan endapan dikendalikan olehsupersaturasi relatif (RELATIVESUPERSATURATION disingkat R.S.)

R.S. = (Q-S)/S R.S. = (Q-S)/S S = Kelarutan pada saat kesetimbangan (dari

Ksp) Q = Konsentrasi sesaat

Semakin besar Q akan mengarah kesuspensi koloid

Faktor penting dalam analisis gravimetri

Nukleasi Ion/atom/molekul bergerombol (bergabung membentuk inti

endapan) Pertumbuhan kristal

Kondensasi ion/atom/molekul dengan inti endapan yang adamembentuk partikel lebih besar yang kemudian mengendap

Nukleasi

membentuk partikel lebih besar yang kemudian mengendap Endapan koloid

Partikel koloid masih tertinggal tidak mau mengendap (suspended)karena terjadi penyerapan ion sehingga memiliki muatan (+) atau(-)

Pertumbuhan kristal

Ilustrasi pertumbuhan kristal

Foto pertumbuhan kristalSuspensi koloid

Faktor penting dalam analisis gravimetri

Koagulasi, aglomerasi Endapan partikel menyatu (begabung) membentuk

partikel lebih besar, bisa difilter (dalam elektrolitinert)

Peptizasi Pelarutan kembali koloid dengan pencucian dan

pemisahan elektrolit inert

Koagulasi koloid

Gambar ini menunjukkanbahwa koloid yangmengalami koagulasipermukaannya yang luasterus kontak dengan larutandimana kolid ini terbentuk

Bawah koloid Al(OH) Bawah koloid Al(OH)3

Pengaruh suhu pada massa endapan

Faktor penting dalam analisis gravimetri Ko-prespitasi

Umumnya senyawa yang terlarut mengendap bersamadengan endapan yang diinginkan (adsorpsi permukaan,oklusi, inklusi, campuran kristal, terperangkap),

Misalnya ion nitrat terperangkap dalam endapan AgCl

Penghancuran (Digestion) Precipitation heated for hour(s) in contact with solution

form which it was formed

Lihat ilustrasi di bawah

Gb Ilustrasi Ostwald ripeningGary Christian,Analytical Chemistry,6th Ed. (Wiley)

Lihat proses terbentuknya adsorpsi ion di bawah

Gb Representasi partikel AgCl dan penyerapan ion

Gary Christian,Analytical Chemistry,6th Ed. (Wiley)

Bebarapa senyawa yang bisa digunakan untukmengendapkan analit

Gary Christian,Analytical Chemistry,6th Ed. (Wiley)

Analisis elektrogravimetri.

Analisis ini mendasarkan teknikpengendapan dengan menggunakanpotensial searah (DC). Pada potensialpotensial searah (DC). Pada potensialreduksi tertentu (sesuai dengan potensialreduksi standar), ion logam diendapkanpada katoda. Endapan yang ada pada katodaadalah berat kuantitatif unsur tersebut didalam sampel. Perhitungan kadar bisadilakukan dengan mudah.

Perhitungan dalam analisis gravimetrimemerlukan skill

Pengetahuan reaksi kimia Stoikiometri Komposisi kimia endapan Komposisi kimia endapan

Examples of calculations (for tutorial) Gravimetric factor(GF)-the weight of

analyte per unit weight of precipitate

a(g/mol)analytewtf

eprecipitatanalyte/gg

e)precipitatlanalyte/momol(ba

/mol)(geprecipitatwtf(g/mol)analytewtfGF

AgCl)/gCl(gGFAgClg

AgCl)/molClmol(21

)AgClAgCl/mol(gAgClwtf)Cl/molCl(gClwtfAgClgClg 22222

So, if Cl2 in a sample is converted to chloride andprecipitated as AgCl, the weight of Cl2 that gives 1 g ofAgCl is

AgCl)/gCl(g0.2473AgClgAgCl)/gCl(gGFAgClg

27

2

Example 1

calculate the grams of analyte per gram of precipitate for thefollowing conversion:

Analyte PrecipitateP Ag3PO4K2HPO4 Ag3PO4Bi2s3 3BaSO4

Solution

GFPOAgP/gg0.0739911

/mol)POAgg(418.58)P/mol(g30.97

)POAgP/molmol(11

(g/mol)POAgwtg(g/mol)Pat wt

POAgP/gg

43

43

4343

43

GFPOAg/gHPOKg0.4161211

/mol)POAgg(418.58)(g/molHPOK(g174.18

)POAg/molHPOKmol(11

(g/mol)POAgwtf(g/mol)HPOKwtfPOAg/gHPOKg

4342

43

42

434243

424342

GFBaSO/gSBig0.7342931

)BaSO(g233.40/mol)SBi(g15.514

)BaSO/molSBimol(31

(g/mol)BaSOwtf(g/mol)SBiwtfBaSO/gSBig

432

4

32

4324

32432

In order to calculate the percent composition byweight of analyte in the sample, the weight ofsubstance sought was divided with the weight ofsample and times with 100%.The weight of sample sought can be obtain fromthe weight of precipitate and the correspondingweight/mole relationship, as in equation below:

100%(g)sampleofweight

e)precipitatsought/g(gGF(g)eprecipitatofsought%

weight

Weight of substance sought (g) Weight of precipitate(g)f wt substance sought (g/mol)

f wt precipitate (g/mol)

( mol substance sought/mol precipitate)

Weight of precipitate (g) GF (g sought/g precipitate)

ab

Thus, the general formula would be:

Example 2An ore is analyzed for the manganese content by converting the manganeseto Mn3O4 and weighing it. If a 1.52g sample yeilds Mn3O4 weighing0.126g,what would be the percent Mn2O3 in the sample? The percent Mn?

Solution:

100%sampleg52.1

)OMng/OMn(gO2MnO3MnOMng0.126

OMn%4332

43

3243

32

5.97%100%

g52.128.83(54.94)/2g0.126

100%sampleg1.52

)OMngMn/g(OMn

3MnOMng0.126Mn%

8.58%100%g52.1

(228.8)3(157.9)/2g0.126sampleg52.1

4343

43

Example 3What weight of pyrite ore (impure FeS2 ) must be taken for analysis so thatthe BaSO 4 precipitate weight obtained will be equal to one-half that of thepercent S in the sample?

Solution:

%100sampleg

gA%

A

6.869gsampleg

%100sampleg

40.233064.32

21

S%1

%100sampleg

)BaSOS/gg(BaSO

S)BaSOA(g21

S%A

%100sampleg

A%

44

4

Soal-soal latihan

What is the %KCl in a solid if 5.1367 g of solid gives rise to 0.8246 g AgCl? The calcium in a 200.0 mL sample of a natural water was determined by

precipitating the cation as CaC2O4. The precipitate was filtered, washed, andignited in a crucible with an empty mass of 26.6002 g. The mass of thecrucible plus CaO (fwt 56.077 g/mol) was 26.7134 g. Calculate theconcentration of Ca (fwt 40.078 g/mol) in the water in units of grams per100 mL.

An iron ore was analyzed by dissolving a 1.1324 g sample in concentrated An iron ore was analyzed by dissolving a 1.1324 g sample in concentratedHCl. The resulting solution was diluted with water, and the iron(III) wasprecipitated as the hydrous oxide Fe2O3.xH2O by addition of NH3. Afterfiltration and washing, the residue was ignited at high temperature to give0.5394 g pure Fe2O3 (fwt 159.69 g/mol). Calculate (a) the % Fe (fwt 55.847g/mol) and (b) % Fe3O4 (fwt 231.54 g/mol) in the sample.

A 0.2356 g sample containing only NaCl (fwt 58.44 g/mol) and BaCl2 (fwt208.23 g/mol) yielded 0.4637 g of dried AgCl (fwt 143.32 g/mol). Calculatethe percent of each halogen compound in the sample.

Soal-soal latihan

A mixture containing only Al2O3 (FM 101.96) and Fe2O3 (FM 159.69)weighs 2.019 g. When heated under a stream of H2, Al2O3 isunchanged, but Fe2O3 is converted into metallic Fe plus H2O (g). Ifthe residue weighs 1.774 g, what is the weight percent of Fe2O3 in theoriginal mixture?original mixture?

An ore is analyzed for the manganese content by converting themanganese to Mn3O4 and weighing it. If 1.52-g sample yields Mn3O4weighing 0.126-g, what would be the percent Mn2O3 in the sample?The percent Mn? Molar mass: Mn2O3=157.9; Mn3O4= 228.8;Mn=54.94

Orthophosphate (PO33-) is determined by weighing as ammoniumphosphomolybdate, (NH4)PO4.12MoO3. Calculate the percentage Pand P2O5 if 1.1682-g precipitate ere obtained from a 0.2711-g sample.Molar mass: (NH4)PO4.12MoO3 =1876.5; P2O5= 141.95; P=30.97

Soal-soal latihan The Aluminum in a 0.910g sample of impure ammonium aluminum sulfate was

precipitated with aqueous ammonia as the hydrous Al2O3.xH2O. The precipitate wasfiltered and ignited at 1000oC to give anhydrous Al2O3, which weighed 0.2001g.Express the result of this analysis in terms of: a. % Al2O3 b. % Al in the sample.

An iron ore was analyzed by dissolving a 1.1324-g sample in concentrated HCl. Theresulting solution was diluted with water and iron (III) was precipitated as the hydrousoxide Fe2O3.xH2O by the addition of NH3. After filtration and washing the precipitatewas ignited to give 0.5394-g of pure Fe2O3 (159.69 g/mol). Calculate the % Fe (55.847was ignited to give 0.5394-g of pure Fe2O3 (159.69 g/mol). Calculate the % Fe (55.847g/mol), % Fe3O4 (231.54 g/mol) in the sample.

The calcium in a 200.0 mL sample of a natural water was determined by precipitatingthe cation as CaC2O4. The precipitate was filtered, washed, and ignited in a cruciblewith an empty mass of 26.6002 g. The mass of the crucible plus CaO (fwt 56.077g/mol) was 26.7134 g. Calculate the concentration of Ca (fwt 40.078 g/mol) in thewater in units of grams per 100 mL.

An iron ore was analyzed by dissolving a 1.1324 g sample in concentrated HCl. Theresulting solution was diluted with water, and the iron(III) was precipitated as thehydrous oxide Fe2O3.xH2O by addition of NH3. After filtration and washing, the residuewas ignited at high temperature to give 0.5394 g pure Fe2O3 (fwt 159.69 g/mol).Calculate (a) the % Fe (fwt 55.847 g/mol) and (b) % Fe3O4 (fwt 231.54 g/mol) in thesample.