GRAVIMETRI PENGUAPAN

GRAVIMETRI PENGUAPAN

• Analit dipisahkan dari sampel sebagai gas (zat volatil) yang diketahui komposisinya

• Pengukuran massa– Produk volatil yang dijebak– Residu

• Ex. Termogravimetri analisis massa sampel diukur secara kontinyu dengan kenaikan T

Chemistry 215 Copyright D Sharma

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TGA Instrumentation1. A sample (0.1 to 15 mg) is

placed into a tared TGA sample pan, which is attached to a sensitive microbalance.

2. The sample holder (connected to the balance) is subsequently placed into a high temperature furnace.

3. Balance assembly measures the initial sample weight at room temperature and continuously monitors changes in sample weight (losses or gains) as heat is applied to the sample (up to 1500 °C).

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Thermal Gravimetric Analysis (TGA)

• Precisely monitoring weight loss of a sample in a given atmosphere as a function of temperature and/or time

• Temperature: ambient to 1000 °C• Records the first derivative of the

mass loss

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TGA cont. • Evaluate the thermal decomposition and stability of

materials– Polymers, resins, rubbers, explosives

• Information on bulk composition of compounds– Thermal oxidation, heat resistance– Residual water or solvents – Compositional analysis – Ash content in a sample– Quantity of inorganic filler in a polymer

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• The percent weight loss of a test sample is recorded while the sample is being heated at a uniform rate in an appropriate environment. 

• The loss in weight over specific temperature ranges provides an indication of the composition of the sample, including volatiles and inert filler, as well as indications of thermal stability.

• The gas environment is pre-selected for either a thermal decomposition (inert – He or N2 gas), an oxidative decomposition (air or O2), or a combination therein. 

TGA cont.

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H2O, C7H3O6, C6H4, CO2 are consecutively lost.

TGA cont.

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TGA tests may be run in a heating mode at some controlled heating rate, or isothermally. Typical weight loss profiles are analyzed for the amount or percent of weight loss at any given temperature, the amount or percent of noncombusted residue at some final temperature, and the temperatures of various sample degradation processes.

TGA cont.

Contoh Soal• Dalam termogram penguapan kaslium oksalat monohidrat (146,11

g/mol) dengan berat awal 24,60 mg dipanaskan dari suhu ruang – 1000oC dengan 5o/menit. Perubahan massa sbb– Suhu 100 – 250oC= 3,03 mg

– Suhu 400 – 500oC = 4,72 mg

– Suhu 700 – 850oC =7,41 mg

• Penyelesaian

OHmolgmolgx

xmg

mg

2/00,18/11,146%32,12

%32,12%10060,24

03,3

COmolgmolgx

xmg

mg

/04,28/11,146%19,19

%19,19%10060,24

72,4

2/01,44/11,146%12,30

%12,30%10060,24

41,7

COmolgmolgx

xmg

mg

Penentuan Silika (SiO2) dalam bijih/aliase

• Prinsip dasar– Si SiO2

– SiO2 + 4HF SiF4 + 2 H2O

• Massa SiO2 = massa awal – massa akhir

contoh• A sample slag from blast furnace is analyzed for SiO2 by decomposing

0,5003 g sample with HCl, leaving a residue with a mass of 0,1414 g. after treating with HF and H2SO4 and evaporating volatil SiF4, residue with a mass of 0,0813 g remains.

• Determine %w/w SiO2 dalam sample?

Solution

g 0,1231

g 0,0183 - g 0,1414 SiO 2

massa

%61,24%1005003,0

1231,0% 2 x

g

gSiO

Analisis AnorganikMetode Langsung• Penentuan kadar abu (ash content) dalam polimer• Metode:

Sampel ditimbangcrusdibakarpengabuan

• Penentuan padatan terlarut (TDS)• Metode:

Sampel air crus residu 1 residu 2

%100sampelberat

abuberat abu kadar % x

mLxmLgTDS 100sampel volume

residuberat )100/(

%1001residu

2residu - 1residu content) (organic lzat volati% x

Metode tak langsung• Penentuan kadar air dalam produk

Metode:

Sampel (x gram) residu (y gram)

• Penetuan silika dalam air, bijih, slag, aliase• Penentuan karbon dalam baja

Prinsip dasar: C CO2

%100x

y -x moisture% x

Analisis Organik• Penentuan unsur C dan H dalam senyawa

organik combustion analysis

contoh• 101,3 mg sampel senyawa organik diketahui mengandung Cl

dianalisis dengan pembakaran. Gas-gas hasil pembakaran ditampung dalam penyerap. Tabung penyerap CO2 bertambah massanya 167,6 mg, tabung penyerap H2O bertambah massanya 13,7 mg. sampel kedua sebanyak 121,8 mg ditambah HNO3 menghasilkan Cl2 yang kemudian bereaksi dengan AgNO3 membentuk AgCl sebanyak 262,7 mg AgCl. Tentukan persentase unsur-unsur dan rumus empiris!

• Penyelesaian• Sampel 1=101, 3 mg

mgmgx 71,456,167mg/mmol 44

mg/mmol 12 C massa

%12,45%1003,101

mg 45,71 C % x

mg

mmolmmolmg

mol 809,3/12

mg 45,71 C

mgmgx 52,17,13mg/mmol 18

1mg/mmol x 2 H massa

mmolmmolmg

mol 52,1/1

mg 1,52 H

%50,1%1003,101

mg 1,52 H % x

mg

Cont.• Sampel 2 = 121,8 mg

• Penentuan rumus empiris perbandingan molMol C : mol H: mol Cl3,809 : 1,52 : 1,8335 : 2 : 2

Jadi rumus empiris senyawa organik tersebut= C5H2Cl2

%21,53%100mg 8,121

mg/mmol32,143mol35,453mg/m

x mg 262 Cl% x

mgmgx 98,647,262mg/mmol 143,32

mg/mmol 35,453 Cl massa

mmol833,1mg/mmol453,35

mg 64,98 Cl mol

Gravimetri Partikulat

• Pemisahan analit dari matriknya

• Metode– Filtrasi: partikel padat dipisahkan dari matriks

cair, gas, padat– Ekstraksi: cair-cair, cair-padat

Classical extraction method is: SOXHLET EXTRACTION

(named after developer). Apparatus

Sample in porous thimble. Exhaustive reflux for up to 1 - 2 days. Solution of analyte(s) in volatile solvent (e.g. CH2Cl2, CHCl3 etc.) Evaporate to dryness or suitable concentration, for separation/analysis.

Contoh soal

• 200 mL sampel air disaring dengan penyaring serat kaca, setelah dikeringkan pada 105oC dan berat konstan, maka berat penyaring bertambah sebesar 48,2 mg. tentukan total suspended solid dalam ppm (mg/L)

• Penyelesaian

ppmLmgL

mgppmTSS 241/241

200,0

2,48)(

Tugas Kelompok

Penguapan CO2 dalam Penentuan Natrium Bikarbonat

• NaHCO3 + H2SO4 CO2 + H2O + NaHSO4

Penentuan NH3 dalam Garam Amonium

• Larutan garam amonium + NaOH berlebih dan dipanaskan. Gas NH3 dialirkan ke dalam larutan asam standar berlebih, kelebihan asam dititrasi dengan larutan standar basa

• NH4+ + OH- NH3 + H2O

• NH3 + H+ NH4+

• H+ sisa + OH- H2O

Penentuan kadar nitrogen (N) dalam protein

• Metode Kjedahl

• Sampel didestruksi dalam labu kjedahl dengan H2SO4 pekat. Larutan hasil destruksi ditambah larutan basa berlebih dan dipanaskan. Gas NH3 yang dihasilkan dialirkan ke dalam larutan standar asam kemudian dititrasi