KIMIA ANALITIK DASAR 2

SILABUSMetode titrimetri; titrasi netralisasi; analisis volumetri untuk reaksi pembentukan endapan dan senyawa kompleks (argentometri); titrasi pembentukan senyawa kompleks; analisis volumetri secara oksidasi-reduksi (redoksimetri); Analisis gravimetri

BUKU REFERENSI David Harvey, Modern Analytical Chemistry, Mc GrawHill Skoog, D. A. and West, D.M., Fundamentals of Analytical Chemsitry, Holt-Saunders International edition. Vogels Textbook of Quantitative Chemical Analysis, Longman and Scientific, Essex Christian, G.D., Analytical Chemsitry, 5th edition, John Wiley and Sons, Inc., New York.

INTRODUCTION TO TITRIMETRY

In a titration, increments of titrant are added to the analyte until their reaction is complete.From the quantity of titrant required, the quantity of analyte that was present can be calculated.

Most common types of titrations : neutralization titrations oxidation-reduction titrations

complex formation titration precipitation reactions

Figure 1 Titration Setup

TITRATIONS IN PRACTICE1 Accurately add of specific volume of sample solution to a conical flask using a pipette

Known: volume of sample Unknown: concentration of analyte in sample

Pro Pipet

2 Slowly add standard solution from a burette to the sample solution

Known: concentration of the titrant

3 Add until just enough titrant is added to react with all the analyte The end point is signalled by some physical change or detected by an instrument Note the volume of titrant used

Known: volume of the titrant

If we have:HA + BOH BA + H2Oanalyte titrant

Then from the balanced equation we know: 1 mol HA reacts with 1 mol BOH

We also know:CBOH, VBOH and VHA and

c1v1 c 2 v 2 n1 n2

cHA vHA cBOHvBOH 1 1

c HA

c BOHv BOH v HA

STANDARD SOLUTIONSStandard solution:

Reagent of known concentrationPrimary standard:

highly purified compound that serves as a reference material in a titration.Determine concentration by dissolving an accurately weighed amount in a suitable solvent of known volume.

primary standard 1. High purity 100.0% 2. Stability toward air 3. Absence of hydrate water 4. Available at moderate cost 5. Soluble 6. Large F.W.

Secondary standard:compound that does not have a high purity Determine concentration by standardisation. Titrate standard using another standard. Standard solutions should: Be stable React rapidly with the analyte React completely with the analyte React selectively with the analyte

EQUIVALENCE POINTThe amount of added titrant is the exact amount necessary for stoichiometric reaction with the analyte in the sample.

VS

END POINTAn estimate of the equivalence point that is observed by some physical change associated with conditions of the equivalence point.

Aim to get the difference between the equivalence point and the end point as small as possible. Titration error: Et = Veq Vep

Estimated with a blank titration

Indicators used to observe the end point (at/near the equivalence point)

Thymol blue indicator

Instruments can also be used to detect end points. Respond to certain properties of the solution that change in a characteristic way. E.g.: voltmeters, ammeters, ohmmeters, colorimeters, temperature recorders, refractometers etc.

BACK TITRATIONAdd excess titrant and then determine the excess amount by back titration with a second titrant.

Used when: end point of back titration is clearer than end point of direct titration

an excess of the first titrant is required to complete reaction with the analyte

If we have: HA + BOH BA + H2Oanalyte titrant

Then from the balanced equation we know: 1 mol HA reacts with 1 mol BOH If I add excess titrant and then react the excess with a second titrant as follows: HX + BOH BX + H2Otitrant 2 excess

Then from the balanced equation we know: 1 mol HX reacts with 1 mol BOH

We also know: c1v1 c 2 v 2 CBOH, CHX and VHX and n1 n2

cBOHvBOH (excess) cHX vHX 1 1vBOH (excess) cHX vHX cBOH

We also know vBOH(total) vBOH(reacted) = vBOH(total) vBOH(excess)

From our initial titration:HA + BOH BA + H2Oanalyte titrant

we then know: CBOH, VBOH(reacted) and VHA and we want to find CHA!

cHAvHA cBOHvBOH (reacted) 1 1 cHAcBOHvBOH (reacted) vHA

In summary: HA + BOH BA + H2Oanalyte titrant reacted

c1v1 c 2 v 2 n1 n2

VHA CHA ?titrant 2

CBOH

HX + BOH BX + H2Otitrant excess

CHX VHX

CBOH

cHX vHX vBOH (excess) cBOH

vBOH(reacted) = vBOH(total) vBOH(excess)

cHA

cBOHvBOH (reacted) vHA

Example: 50.00 ml of HCl was titrated with 0.02 M Ba(OH)2. The end point was reached (using bromocresol green as indicator after 20 ml Ba(OH)2 was added.

What is the concentration of the HCl?

c1v1 c 2 v 2 n1 n2

Example:A 0.8 g sample of iron ore is dissolve in acid. The iron is reduced to Fe2+ and titrated with 0.02 M KMnO4. 40 ml of titrant was added to reach the end point. Calculate the % Fe in the sample. MnO4- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O

MnO4-

+

5Fe2+ ..

mass of analyte %Fe 100 mass of sample

Example:The CO in a 20 L sample of gas was converted to CO2 by passing the gas over iodine pentoxide heated to 150oC: I2O5(s) + 5CO(g) 5CO2(g) + I2(g) The iodine distilled at this temperature was collected in an absorber containing 8 mL of 0.01 M Na2S2O3: I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq) The excess Na2S2O3 was back titrated with 2 mL of 0.001 M I2 solution. Calculate the mg CO per liter of sample.

I2O5(s) + 5CO(g) 5CO2(g) + I2(g) I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq)

I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq)

nreacted =

REACTED

I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq)

I2O5(s) + 5CO(g) 5CO2(g) + I2(g) I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq)

I2O5(s) + 5CO(g) 5CO2(g) + I2(g)

Calculate the mg CO per liter of sample.