A. TITLE: Precipitation Titration and its ApplicationB. PURPOSE: 1. To make and determine (standardization) AgNO3 solution 2. To determine Cl- content on tap water.C. BASIC THEORY: Precipitation TitrationThus far we have examined titrimetric methods based on acidbase, complexation, and redox reactions. A reaction in which the analyte and titrant form an insoluble precipitate also can serve as the basis for a titration. We call this type of titration a precipitation titration.One of the earliest precipitation titrationsdeveloped at the end of the eighteenth centurywas the analysis of K2CO3 and K2SO4 in potash. Calcium nitrate, Ca(NO3)2, was used as the titrant, forming a precipitate of CaCO3 and CaSO4. The titrations end point was signaled by noting when the addition of titrant ceased to generate additional precipitate. The importance of precipitation titrimetry as an analytical method reached its zenith in the nineteenth century when several methods were developed for determining Ag+ and halide ions.Basic reaction of precipitation titration is the happen of precipitate in the reaction between analyte substance, example:Ag+ + X- AgX(s)where x: halogenAg+ + CrO42- Ag2CrO4(s)(sorrel)Ag+ + SCN- AgSCN(s)Fe3+ + SCN- FeSCN2+(red)Indicator of K2CRO4 used in the titration between halide ion and silver ion, where excess of Ag+ ion will be react with CrO42- forms silver chromate and the color is brick red (Mohr way).At equivalent point:Equivalent Ag+ = equivalent Cl-Indicator of Fe3+ can be used in the titration between silver ion and SCN- ion where the excess of SCN- ion will be react with Fe3+ ion that give a red color. Or can be used in the titration between halide ion and excess of silver ion, and the excess of silver ion will be titrated with thiosianat ion (Volhard way).At equivalent point:Sum of equivalent Ag+ recidu = sum of equivalent SCN-OrSum of equivalent Ag+ total = sum of equivalent (Cl- + SCN-)Selecting and Evaluating the End pointAt the beginning of this section we noted that the first precipitation titration used the cessation of precipitation to signal the end point. At best, this is a cumbersome method for detecting a titrations end point. Before precipitation titrimetry became practical, better methods for identifying the end point were necessary.In any titration, it is necessary to have some method of detecting when just enough of the titrant has been added -- a procedure known as detecting the endpoint of the titration. The endpoint of this titration can be detected if the rapid change in either the concentration of silver ion or the concentration of chloride ion which occurs at the endpoint can be made apparent to an observer. Either instrumental methods or equilibrium methods can be used. The equilibrium methods are fairly straightforward. In this case we can use Ag2CrO4, because a solution of CrO42- is yellow while Ag2CrO4 is blood-red.Finding the End point With an IndicatorThere are three general types of indicators for precipitation titrations, each of which changes color at or near the titrations equivalence point. The first type of indicator is a species that forms a precipitate with the titrant. In the Mohr method for Cl using Ag+ as a titrant, for example, a small amount of K2CrO4 is added to the titrands solution. The titrations end point is the formation of a reddish-brown precipitate of Ag2CrO4.The Mohr method was first published in 1855 by Karl Friedrich Mohr.

Because CrO42 imparts a yellow color to the solution, which might obscure the end point, only a small amount of K2CrO4 is added. As a result, the end point is always later than the equivalence point. To compensate for this positive determinate error, an analyte-free reagent blank is analyzed to determine the volume of titrant needed to affect a change in the indicators color. Subtracting the end point for the reagent blank from the titrands end point gives the titrations end point. Because CrO42 is a weak base, the titrands solution is made slightly alkaline. If the pH is too acidic, chromate is present as HCrO4 instead of CrO42, and the Ag2CrO4 end point is delayed. The pH also must be less than 10 to avoid the precipitation of silver hydroxide.A second type of indicator uses a species that forms a colored complex with the titrant or the titrand. In the Volhard method for Ag+ using KSCN as the titrant, for example, a small amount of Fe3+ is added to the titrands solution. The titrations end point is the formation of the reddish-colored Fe(SCN)2+ complex. The titration must be carried out in an acidic solution to prevent the precipitation of Fe3+ as Fe(OH)3.The Volhard method was first published in 1874 by Jacob Volhard.

The third type of end point uses a species that changes color when it adsorbs to the precipitate. In the Fajans method for Cl using Ag+ as a titrant, for example, the anionic dye dichlorofluoroscein is added to the titrands solution. Before the end point, the precipitate of AgCl has a negative surface charge due to the adsorption of excess Cl. Because dichlorofluoroscein also carries a negative charge, it is repelled by the precipitate and remains in solution where it has a greenish-yellow color. After the end point, the surface of the precipitate carries a positive surface charge due to the adsorption of excess Ag+. Dichlorofluoroscein now adsorbs to the precipitates surface where its color is pink. This change in the indicators color signals the end point.The Fajans method was first published in the 1920s by Kasimir Fajans.

In a precipitation titration, the stoichiometric reaction is a reaction which produces in solution a slightly soluble salt that precipitates out. To determine the concentration of chloride ion in a particular solution, one could titrate this solution with a solution of a silver salt, say silver nitrate, whose concentration is known. The chemical reaction occurring is Ag+(aq) + Cl-(aq) --> AgCl(s) A white precipitate of AgCl is deposited on the bottom of the flask during the course of the titration. Since the chemical reaction is one Ag+ to one Cl-, we know that the amount of Ag+ used to the equivalence point equals the amount of Cl- originally present. Since n = cV, the number of moles of either Ag+ or Cl- can be calculated from the number of moles of the other, and the molar concentration or the volume ofadded solution can be calculated for either ion if the other is known. Application of Precipitation TitrationA precipitation reaction called finished, if the solubility of the precipitate is quite small. Behind the equivalent point, the concentration of ions that is titrated will have a big change. The problem that will be faced is choosing a good indicator.There are some ways to determining when reach the equivalent point in the precipitation titration:a. With determination of coloring precipitateb. With determination of coloring compound that is dissolvec. With adsorption indicatorIn the process of disinfection of water is often use chlor, because the prices is cheap and have a disinfectant power through several hours after chlor recidu. Through that process, chlor is reducted until becomes chloride (Cl-) that have no disinfectant, beside chlor is reacted with ammonia. Chlor active in a solution is available in a free situation (Cl2, OCl-, HOCl) and bonded situation (NH2Cl, NHCl2, NCl3)

D. TOOLS AND MATERIALS: Balance Piala glass Coloring bottle Weight bottle Volumetric flask Burette Volumetric pipette Erlenmeyer Picnometer AgNO3 Aquades NaCl K2CrO4 K2CrO4 5% Tap water Neraca analitis

E. PROCEDURE:a. Determination (Standardization) AgNO3 Solution 0,1 N with NaCl as primary standard solution.

0,5 grams NaCl Weighing in weights bottle Moved into volumetric flask 100 mL Dissolved with aquades and dilute until the limit sign Mix it well Burette

Washed with AgNO3 solution Pipette with volumetric pipette 10 mL of NaCl solution Entered into Erlenmeyer 250 mL Added 10 mL aquades Added 1 mL K2CrO4 indicator Titrated with AgNO3 and mix it Sorrel precipitateStop

Read and note the number on the buret at the beginning and the end of titration Determined and note the volume of AgNO3 that used in titration Calculated the concentration of AgNO3 solution Repeated 3 times Average of AgNO3Calculated the average of AgNO3 solution

Application of Precipitate a. Determination of the content of Cl- in the tap water

Tap Water

Measured the density of tap water with picnometer Noted the place of sample Pipette 10 mL SolutionDiluted in the volumetric flask 100 mL

Took 10 mL Added 5 drops of K2CrO4 5% Brick Red PrecipitateTitrated with AgNO3 Repeated 3 times Average of Cl- in the tap waterCalculated the content of Cl- in the tap water

F. EXPERIMENT RESULTNo.Procedure of ExperimentExperiment ResultHypothesisConclusion

Burette1.Determination AgNO3 Solution 0,1 N with NaCl as primary standard solution

1,5 grams NaCl

Weighing in weights bottle Moved into volum-etric flask 100 mL Dissolved with aquades and dilute until the limit sign Mix it well

Washed with AgNO3 solution Pipette with volumetric pipette 10 mL of NaCl solution Entered into Erlenmeyer 250 mL Added 10 mL aquades Added 1 mL K2CrO4 indicator Titrated with AgNO3 and mix it Stop

Sorrel precipitate

Read and note the number on the buret at the beginning and the end of titration Determined and note the volume of AgNO3 that used in titration Calculated the con-centration of AgNO3 solution Repeated 3 times Calculated the average of AgNO3 solution

average of AgNO3 solution

Before:- NaCl: white powder- K2CrO4: yellow- AgNO3: colorless- Aquades: colorless

After:- NaCl + aquades: colorless- NaCl + aquades + K2CrO4: yellow- NaCl + aquades + K2CrO4 + AgNO3: sorrel

V1 AgNO3=5.6mLV2 AgNO3=5.7mLV3 AgNO3=6.0mLAt the beginning of titration:AgNO3 + NaCl AgCl(s) + NaNO3(aq) white precipitate

Approach equivalent point:2AgCl + K2CrO4 Ag2CrO4 + 2 KCl sorrel precipitateConcentration of AgNO3 =0.147 N

2.Application of Precipitate Determination of the content of Cl- in the tap water

Tap Water

Measured the density of tap water with picnometer Noted the place of sample Pipette 10 mL Diluted in the volum-etric flask 250 mL

Solution

Took 10 mL Added 5 drops of K2CrO4 5% Titrated with AgNO3

Sorrel Precipitate Repeated 3 times Calculated the content of Cl- in the tap water

Cl- content in the tap water

Before:- NaCl: white crystal- Aquades: colorless- K2CrO4: Yellow- AgNO3: colorless

After:- NaCl + aquades: colorless- NaCl + aquades + K2CrO4 = yellow- NaCl + aquades + K2CrO4 + AgNO3 = sorrel precipitateV1 AgNO3=0.5mLV2 AgNO3=0.8mLV3 AgNO3=0.5mLPrecentage Cl- in tap water = 0.0051%

G. ANALYSIS AND DISCUSSIONDetermining AgNO3 solution with NaCl as primary standard solution.In our experiment of determining AgNO3 solution with NaCl solution. In argentometry titration, a solution of AgNO3 used to specify halogen salts and cyanide so, this one is include on Mohr Method. First step that we done is, we dissolving 0.0506 grams of NaCl in 100 mL volumetric flask and dilute until the limit sign. After that pipette 10 mL of the solution and entered into Erlenmeyer 250 mL. then added 10 mL aquades. Added 1 mL of K2CrO4 indicator to the Erlenmeyer the solution became bright yellow because there(CrO4) 2- ion. Selected K2CrO4 indicator because the condition tends to be neutral system.Potassium chromate can only be used in a neutral condition. If potassiumchromic react with the acid condition, the chromate ions become bichromate ionby the reaction: 2 CrO4 2- + 2 H+ Cr2O7 2- + H2OWhereas in the alkaline conditions, Ag+ ions will react with OH-from the base and form a precipitate Ag (OH) and subsequently oxidizedbe A2O with the reaction:2 Ag+ + 2OH- H2OThen titrated with AgNO3 to until there is change color from yellow become sorrel. The sorrel precipitation is shown there is precipitate of Ag2CrO4. But in the equivalent point where the number of moles of AgNO3 equal to the number of moles of NaCl, there is white precipitate of AgCl. The equivalence point of AgCl shown with this equation :AgNO3 + NaCl AgCl+ NaNO3But in the end point of the titration , the indicator reacts with Titrant a sorrel precipitate formed ( Ag2CrO4 ) by the reaction :2AgCl + K2CrO4Ag2CrO4 + 2KClThis the solubility of Ag2CrO4 (8.5 x10-5 mol / L) is greater than the solubility of AgCl ( 1x10-5 ) so that the first AgCl precipitate. Titration was repeated 3 times with same volume of NaCl and we got AgNO3 volume 5.6 mL ; 5.7 mL , and 6.0 mL with the average of AgNO3 is 0.147N

Determining Cl- Content On Tap WaterIn the experiment of Determining Cl- Content On Tap Water we used tap water that we brought from our dorm. first pipette 10 ml of tap water and diluted in 100 mL volumetric flask up to the limit sign. Then pipette 10 ml of tap water which has been diluted and moved into Erlenmeyer. Than added 5 drops of 5% K2CrO4 (yellow), the solution became yellow and titrated with AgNO3 intil there is sorrel precipitate the red brick (Ag2CrO4). Titration was repeated 3 times with the same volume of tap water, so we got the AgNO3 volume are 0.5 mL; 0.8 mL, and 0.5 mL and the precentage of Cl- in tap water is 0.0051%

H. CONCLUSION Based on experiment that we have done, we can conclude that :1. AgNO3 concentration is 0.147N 2. The precentage of Cl- in tap water is 0.0051%I. ANSWER AND QUESTIONMaking and Determining (standardization) of AgNO3 solution1. Make a titration curve between AgNO3 volume dan pCl for titration 50 ml of 0.1 M NaCl solution with AgNO3 0,1 M.Answer :Initial titration pCl = 1After adding 25 ml AgNO30,1 MNaCl+AgNO3 AgCl+ NaNO3m5 mmol2,5 mmol r2,52,5 2,5s2,5- 2,5[Cl] = 2,5 / 75 = 0,033pCl = -log 0.033 = 1.48

After adding 49,9 ml AgNO30,1 MNaCl+AgNO3 AgCl+NaNO3m5 mmol4.99 mmol r4.994.99 4.99s0.01- 4.99[Cl] = 0.1 / 99.9 = 0.0001pCl = -log 0.0001 = 4

After adding 50 ml AgNO30,1 MNaCl+AgNO3 AgCl+sNaNO3m5 mmol5 mmol r55 5s-- 5[Cl-] [Ag+] = Ksp[Cl-] = [Ag+][Cl-]2 = 4. 10 -13[Cl-] = 6,32 . 10-7pCl = - log 6,32 . 10-7pCl = 6,199 ( equivalent point )

After adding 50,1 ml AgNO30,1 MNaCl+AgNO3 AgCl+NaNO3m5 mmol5,01 mmol r55 5s-0.01 5[Ag+] = 0. 01 / 100.1 = 9,99 . 10 -5pCl = -log 9,99 . 10 -5 = 4pAg + pCl = pKsp pCl = 8.4

After adding 60 ml AgNO30,1 MNaCl+AgNO3 AgCl+NaNO3m5 mmol6 mmol r55 5s-1 5[Ag+] = 1 / 10 = 9,09 . 10-3pCl = -log 9,09 . 10-3 = 2.04pAg + pCl = pKsp pCl = 10.36

Titration Curves

2. What is the concentration of NaCl salt in a solution, if 25 ml of the solution was reacted with 25ml AgNO3 0,2 M. and an excess of AgNO3 react precisely with a 28ml KSCN 0.1 M .Answer :mmol AgNO3 = mmol NaCl + mmol KSCN25 ml . 0,2 M = mmol NaCl + 28 ml . 0,1 M 5 mmol = mmol NaCl + 2,8 mmol mmol NaCl = 2,2 mmol [NaCl] = 2,2 / 25 = 0.088 M

Application of Titration Precipitation1. How to choose indicator on argentometric titration ?Answer:Indicators for Argentometric Titrations Chemical end point Consist of a color change occasionally, the appearance or disappearance of turbidity in the solution being titrated. (1) the color change should occur over a limited range in pfunction of the reagent or the analyte and (2) the color change should take place within the steep portion of the titration curvefor the analyte. Potentiometric end point Potential between a silver electrode and a reference electrode whose potential is constant is measured Independent of the added reagent Amperometric end point Current generated between a pair of silver microelectrodes in the solution of the analyte is measured Current plotted as a function of reagent volume Chromate Ion; The Mohr MethodSodium chromate can serve as an indicator for the argentometric determination of chloride, bromide, and cyanide ions by reacting with silver ion to form a brick-red silver chromate (Ag2CrO4) precipitate in the equivalence-point region. Adsorption Indicators: The Fajans Method An organic compound that tends to be adsorbed into the surface of the solid in a precipitation titration. Ideally, the adsorption (or desorption) occurs near the equivalence point and results not only in a color change but also in a transfer of color from the solution to the solid (or the reverse). Fluorescein is a typical adsorption indicator that is useful for the titration of chloride ion with silver nitrate. In aqueous solution, fluorescein partially dissociates into hydronium ions and negatively charged fluoresceinate ions are yellow-green. The fluoresceinate ions forms an intensely red silver salt.However, Its concentration is never large enough to precipitate as silver fluoresceinate. Iron(III) Ion; The Volhard Method In the Volhard method, silver ions are titrated with a standard solution of thiocyanate ion:

Iron(III) serves as the indicator:

The titration must be carried out in acidic solution to prevent precipitation of iron (III) as the hydrated oxide. The most important application of the Volhard method is the indirect determination of halide ions.

2. Explain how adsorption indicator work. Whats the function of desktrin? Why pH have to be control ? Answer :How adsorption indicator works: before the equivalence point of the colloidal particles are negatively charged AgCl caused of adsorption Cl- ions. Cl-ions are adsorbed to form the primary solution, which resulted colloidal particles become negatively charged. After the equivalence point, the excess Ag + ions replace Cl-ions from the primary layer and the particles become positively charged and anions are interested in the solution to form a secondary layer. Dextrin Functions: protective colloid to keep the sediment is widespread. pH must be controlled : to guarantee an indicator ion concentration of weak acid or weak base are available enough.J. REFERENCE

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0/09_Titrimetric _Methods/9E_Precipitation_Titrations Accessed on Sunday, December 21st 2013 at 15.42http://dwb.unl.edu/Teacher/NSF/C14/C14Links/www.chem.ualberta.ca/courses/plambeck/ p101/p01194.htm Accessed on Sunday, December 21st 2013 at 15.57http://freshlysqueezedwater.org.uk/waterarticle_watercontent.php Accessed on Sunday, December 21st 2013 at 16.30Jeffery, GH, Bassett J, Mendhan J,Denny RC, 1989.VOGELs Textbook of Quantitative Chemical Analysis. Ed 5. London : Logman GroupTim.2013.Panduan Praktikum Kimia Analitik I Dasar-Dasar Kimia Analitik. Surabaya:Jurusan Kimia FMIPA UNESA

K. ATTACHMENTPICTUREEXPLANATION

Detrmining Standarization of AgNO3Solution 0.1N With NaCl As Primary Standard Solution

NaCl after adding by K2CrO4

NaCl after Titrated by AgNO3

Determining Cl- Content on Tap Water

Tap water after adding by K2CrO4

Tap water after adding by AgNO3

L. CALCULATION1. Known: V AgNO3= V1= 5,6 mL; V2= 5,7 mL; V3= 6,0 mLMr of NaCl= 100 mL = 0,1 LMass of NaCl= 0,5 gramsAsked: N AgNO3?Answer:NNaCl= M x n= 0,085 M x 1 = 0,085 N1) At V AgNO3= 5,6 mLMolek AgNO3= Molek NaClN1 . V1= N2 . V2N1 . 5,6 mL= 0,085 N . 10 mLN1= 0,151 N

2) At V AgNO3= 5,7 mLMolek AgNO3= Molek NaClN1 . V1= N2 . V2N1 . 5,7 mL= 0,085 N . 10 mLN1= 0,149 N

3) At V AgNO3= 6,0 mLMolek AgNO3= Molek NaClN1 . V1= N2 . V2N1 . 6,0 mL= 0,085 N . 10 mLN1= 0,141 NN Average= = 0,147 N

2. Known: V AgNO3 = V1= 0,5 mL; V2= 0,8 mL; V3= 0,5 mLAsked: Percentage of Cl- on tap water?Answer:tap water= = = 0,9 grams/mLm= . V= 0,9 . 10= 9 grams

1) At Volume AgNO3= 0,5 mLV Cl- . N Cl-= V AgNO3 . N AgNO310 . N Cl-= 0,5 . 0,147N Cl-= 0,00735 N

2) At Volume AgNO3= 0,8 mLV Cl- . N Cl-= V AgNO3 . N AgNO310 . N Cl-= 0,8 . 0,147N Cl-= 0,01176 N

3) At Volume AgNO3= 0,5 mLV Cl- . N Cl-= V AgNO3 . N AgNO310 . N Cl-= 0,5 . 0,147N Cl-= 0,00735 N

Vaverage= = 0,00882 NM Cl-= 0,00882 N / 1 eq= 0,00882 MMol Cl-= M Cl- . V= 0,00882 M . 0,1 L= 0,0008 gramsMass of Cl-= mol Cl- . WE NaCl= 0,0008 grams . 58 grams/mol= 0,0464 grams% Cl-= = 0,0051%

Precipitation Titration 20