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Analytical chemistry Precipitation.

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1 Analytical chemistry Precipitation

2 Introduction We can use reactions that precipitate as a principle of titration if the required conditions are available. The most important condition is the existence of away for knowing the equivalence point in addition that requires the system to reach a state of equilibrium fast after every addition of the titrate. which means that the soluble product for the formed precipitate as small or possible. Any way, most of the precipitation reactions are slow, although it is possible to overcome that in some situations by adding some organic solvent such as ethanol, because that help

3 Reduce the soluble of the precipitate
Reduce the soluble of the precipitate. Although, many ways of titration have been invented that in clued precipitation, the most important of these ways and the most used are the halides titration and the pseudo-halides such as cyanide and thio- cyanide with silver, and titrating sulfate by barium

4 Titration Curve Precipitation curve are similar to neutralization titration curves. Since they are both found by drawing the relation ship between PM=-log[M] and the size of standard solution where the precipitated ion is symbolized by M these curves are very important because the clarity the changes that happen at the equivalence point, and by that it is able to judge the success or failure of the titration. Depending on that, the proper evidence is chosen

5 Example1 A titration curve derivation 50ml from absolution o f 0.1M of sodium chloride with 0.1M solution of silver nitrate. If the conditional solubility product silver chloride at 25c equals Ksp= 1.82 The solution You must notice that it is possible to calculate the changes in peg or pal that happen at the different additions to the silver chloride solution, since they both give the curve titration, but they depends on the type of evidence used. If the evidence is sensitive to the concentration of the chloride ion, we use pal. But if it is sensitive to the concentration of silver ion we use pAg.


7 1- Calculating the pM before adding any amount of reagent solution (silver nitrate):
Before adding any amount of silver nitrate the concentrations of silver chloride ion in the titration flask is equal to: [Cl-] =1x10-1 [Ag+] =0.0M based on that: pCl=-log [Cl]=-log1 x 10-1 = 1 pAg=-log [Ag] =-log 0.0 =

8 2- calculating the pM before eq.point:
At any point before eq.point. For example, after adding 10ml of silver nitrate solution we notice that the concentration of chloride ion is calculated as following: [Cl-] = 50 x _ x 0.1 It is clear that the concentration of chloride ion in the solution will decrease by adding silver nitrate which is caused by the precipitation silver chloride on one hand and the dilution on the other. It is also noticed that the silver chlorides precipitate is just like any other precipitate in which a small amount of it dissolves to give equal amounts of [Ag] , [Cl]. That is why [Ag] has been included in the equation above in order to express the amount of chloride ion that which is the result of silver chloride precipitate partial dissolving and which is calculated by using the dissolving result equation. But since its value is very small compared with the chloride ion remain in the solution ,it is possible to neglect it to simplify calculations as Follows: [Cl-] =6.7 x mole/l pCl=-log 6.7 x =1.17 calculate [Ag+], the value of [Cl] is replaced in the dissolving result equation as follows: [Ag+] = Ksp = x10 - [Cl-] x10-9 pAg =-log 2.7x10-9 =8.57

9 3- Calculating the pM at eq.point:
At eq.point we find that 50ml of 0.1M silver nitrate solution has been added to 50ml of 0.1M sodium chloride solution, which means that silver chloride ion concentrations in the solution are equal [Cl]=[Ag]. meaning that the system is saturated solution of silver chloride by that it is possible to calculate both [Ag+] [Cl-] = [Ag]2 =1.82x10-10 [Ag+] [Cl-]=[Ag]2=1.82x10-10- [Ag+]= [Cl-] = x =1-35x10-5 mol/l pAg =pCl =-log 1.35x10-5 =4.87

10 Calculating the pM at eq.point:
At any addition of the silver nitrate solution after eq.point will remain dissolved in the titration solution. flask That is why silver ion total concentration in the solution equals the concentration of the added nitrates in addition to the silver ion concentration resulted from the dissolving of silver chloride precipitate. Although the last concentration may be considered very small compared in the zones far from eq.point. For that it is possible to neglect it to simplify calculations, it may also be calculated using conditional solubility product if necessary. For example, after adding 52.5ml of silver nitrate solution the total concentration for the silver ion in the solution as follows: [Ag +] =5 2,5x0.1-50x0.1 +[Cl-] 102.5 =2.4x10-3 mol/l [Cl-] = Ksp =1.82x10-10 =7.6x10-8 mol/l [Ag+] 2.4x10-3 pAg = -log 2.4x10-3 =2.62 pCl=-log 7.6x10-8 =7.12

11 Titration including silver (Argentimetry): there are three titration methods where silver ion is used. These methods are Mohr ,s method, Fijian's method and Volherd’s method. Mohr's method: The principle of this method depends on forming a second precipitate a particular color at eq.point in result of the increased reagent with the indicator. This method is used to titrate chloride and bromide ions using a standard solution of silver nitrate while using the chromate ion as an indicator. In the beginning of a titration any addition of silver nitrate on chloride or bromide solution which contains chromate will lead to precipitate. Silver chloride or silver bromide which is of a white color or not. That is because the molar disseverment of silver chromate is five times the molar disseverment of silver chloride ion and after precipitate the whole chloride or bromide ions from the solution at any eq.point, any increase in the silver nitrate solution will precipitate the red silver chromate which indicates the end point: Ag++Cl AgCl (White precipitate) Titration reaction 2Ag++CrO Ag2CrO (Red precipitate) Indicator reaction

12 Important Note on Mohr’s Method:
1-The indicator concentration is very important in this method, because the silver chromate must precipitate exactly at the eq.point. Where there is a saturated solution of silver chloride or silver bromide. The silver ion concentration in the saturated solution of silver chloride approximately equals 1x10-5 M. That can be calculated using the conditional solubility product equation. For that, silver chromate must precipitate exactly when the concentration of silver ions in the solution equals 1x10-5 M. Sins the conditional solubility product of silver chromate approximatelyequals1x10-12, it is possible to calculate the concentration of chromate needed to precipitate silver chromate under these circumstances: Ksp =[Ag+][CrO-2 ] 1x =[10-5 ]2 [CrO4] [CrO4] =1x10-2 M Meaning that the chromate concentration must be 1x10-2 M, because if it was more than that, silver chromate will precipitate before eq.point. But if it was less, we will need amore amount than 1x10-5 M of silver ion for the precipitation to occur which means, that the precipitate will not from until it -passes the eq.point. Although, in practical practices we find that usually the chromate concentration is around 2-5x10-3 M. that is because any higher than that, will give a dark yellow color to the solution that will cover the red color of the silver chromate precipitate. When using this concentration we will be using up more than we need of silver ion . For that, we have to make up for that by performing a titration at first, using an empty solution that dose not contain chloride ion but contains only indicator. After estimating the size of the silver solution needed to form the silver chromate precipitate , which can clearly be seen in the yellow solution, it is to be subtracted from the silver solutions size used to titrate the chloride ion solution

13 Which is of an un known concentration
Which is of an un known concentration. It is preferred to use a saturated solution of chloride over a blank solution, in order to also consider the effect of the white precipitate of silver chloride on the clearness of red color of silver chromate 2- . The solution acidity is very important in this method, because in an acidic medium chromate transform to dichromate: 2CrO4 +2H CrO7 +H2Odes, Since silver dichromate are more dissolvent than silver chromate, the indicators reaction in the acidic medium requires a high concentration of silver ion. If the medium was a strong basic, there is danger of silver precipitating as oxide or hydroxide: 2Ag+ +2OH AgOH Ag2O+H2O That is why this method must be performed in an neutral solution or nearly of neutral solution PH Adding calcium carbonate sodium bicarbonate or dichromate help adjust the pH solution around pH=8. When ammonium salt are present, the pH during the titration must not exceed 7.2, that is because ammonia is freed from these salts in the alkaline mediums which in return increases the dissolving of silver chromes end halides, that last eq.point.

14 3- It has been experimentally proved that the chloride or bromide precipitate t. dose not effect the precipitation of silver chromate before eq.poin. In other words, silver chromate precipitate separately from silver chloride. This fact is unpredictable because of the nature of the atesilver halides precipitate. In the iodide, the formed precipitate ate the end point is an adsorption complex of silver chromate and iodide which is of a faded color compared with silver chromate, and that prevents the clearness of the end point. Meaning that the silver chromate precipitate formed at the end point adsorbs on the surface of the silver iodide precipitate which causes the color of silver chromate to fade . What has been said about iodide can be said about thiosyanate. Because of that, they cannot be estimated using Mohr’s method . 4-Some anions that form precipitate with silver such as arsenate and phosphate have not that big of an effect, because silver salts of these anions precipitate after silver chromate. On the other hand; iodide, sulphitesand carbonates have a big effect on the titration and must be eliminated before performing the titration. Oxalate can be removed by precipitating it by calcium and sulphite can be oxidized to sulphate which have no effect. Sal bides and carbonates can be eliminated by acidifying the solution and by boiling it. Colored metal chlorides nickel and cobalt cannot be titrated since the end point is unclear in these situations.

15 2- Volherd’s method. This method depends on forming a complex color ful combination at eq. point where the silver solution is titrated by a saturated solution of potassium thiosyanoic in the presence of ferric ion as an indicator. At titration, thiosyanate ion reacts with silver ion form a white precipitate from silver thiosyanate: SCN- +Ag AgSCN ( The titration reaction) At eq.point, any increase of thiosyanate reacts with ferric ion to form complex red combination which characterizes the end point. SCN- + Fe Fe(SCN) ( The indicator reaction) This titration must be in an acidic medium to prevent hydrolysis of ferric ion (precipitating as hydroxide). This condition is considered as an advantage for this method because some ions like carbonate, oxalate and arsenate that form precipitate which silver in the neutral solution do not reference with halides in the acidic medium because its precipitate with silver, dissolve in this medium. Other characteristics of this method is that ferric ion are very sensitive to light concentration of thiocyanate which malces this method accurate , because the amount needed to create the red color after ea.p. is very small. There might be danger of silver ions adsorbing on the surface of the silver thioscyanate is precipitate causing the end point to appear in an early stage. To avoid that, you must shake after every addition of saturated solution.

16 3- Fijian's Method: This method is also know as the adsorption indicators method. In this me the silver solution is from borate to halides solution in the titration flask whish also contains indicator anion( whish is a weak organic acid). In the beginning of the titration and before eq.point, we find that silver halides precipitates in a solution that contains of halide ions x - , and since most of precipitate tend toadsorp there ions from the primary adsorption layer stronger then ano other ions in the solution. Since the primary adsorption ion (which is halides in this state) carries a negative charge, indicator anion will also move away from the precipitate surface because of the conflict. The precipitate charge is equalized by adsorption a positive ion such as Na+ or H+ in the solution which is called counter ion, and this position may be represented by the following equation: Ag+ +2X AgX:X- : Na (before eq.point) At eq. point any increase in the silver ion will cause an extreme change on the precipitate surface, while the precipitate now is in a solution containing an addition of silver ion, so in this state we find that silver ion represents the primary adsorption ion. Based on that the primary adsorption layer becomes charged with appositive charge. When this happens, the anion of the negative indicator In- will adsorption the surface as a conjugate adsorption ion in order to neutral the charge, as shown in the adsorption ion in order to neutral the charge, as shown in the following equation: Ag+ + AgX + In AgX: Ag+ :In- This adsorption is accompanied with a change in color, for an unknown reason. That is because many colored organic compound anions undergo this titration without a change in color. :

17 It might be (and this is the possibility most accurate) that the cause of change in the color of indicator at adsorption, forms a colored complex compound with silver ion. This complex is not fixed, because it does not form in the solution at these low concentrations .But the direction of the compounds equilibrium has been changed by precipitating the compound on precipitate surface as follows: Ag+ + In AgIn AgX: AgIn (in the solution) (precipitate) Note, that the adsorption process is an adverse process. Hence, it is possible to titrate the increase in the silver ion by a saturated solution of halide until the color vanishes.

18 The Factors Effecting Fijians' Method.
There are several conditions that should be obtained in the precipitate ,indicator and the titration medium to make it possible to exercise Fijians' Method to titrate an ion precisely and accurately. 1-The clearness of color at eq.point depends on the quantity of the adsorbed indicator on the precipitate surface which depends on the surface area exposed to the precipitate. So by the increase in the area eof the exposed precipitate surface, there will be an increase in the amount of the adsorbed indicator and the color will become clearer and darker. For that cause, it is preferred that the precipitate in these kinds of titration are colloidal ( little with grains). That is why it is preferred that the titrations occurs under the conditions of forming the precipitate. As making the titration happen fast, avoiding heat that helps in the precipitate solidification, using some organic solvents that decrease dissolving and at the same time helps forming a colloidal Precipitate and some substances that prevent solidification of the colloidal precipitate such as gelatin or starch. 2-The anion of the indicator In- must not replace the titrated ion X- in the primary adsorption layer during the titration. But it adsorb as conjugate ion at eq.point. Every anion has the ability to replace the anion beneath it.

19 3- Due to the fact that most of the adsorption indicator are considered to be weak organic acids, the pH of the solution shouldn't decrease too much. Meaning , that the solution acidity shouldn't be strong or else the equilibrium will be: Hin H+ + In- It will turn to the left toward the formation of the undissociate indicator HIn (the common ion effect). Hence, the concentration of the anion In the solution decreases. Due to that, the color intensity at the end point. 4- Although the increase in the surface area exposed to the precipitate helps clarify the end point, but it also helps in the photodecom position of silver chloride while exposed to direct sunlight. So in this cause.

20 In condition, Volherd’s method is the most method used to titrate halides since the existence of many odd ions. It is also a method of high accuracy though it is considered uncomfortable compared with Mohr’s method. Hence, The latter method is preferred in titrating halides in the state when interferences don’t occur and in state where results of reasonable accuracy are accepted. The adsorption indicator method is very suitable in the states where titrations are performed in an acidic medium. Especially, in the case of titrating iodide and thiosyanate

21 Analytical Chemistry Gravimetric Analysis

22 Introduction Gravimetric method of analysis is one of the classical methods of quantitative analysis which depends on used analytical balance for determination the components . Gravimetric analysis is done for substances by one of the following methods. 1- Increasing in weight method or direct weighting. As applied when determined the elements which is very pure such as Gold or silver elements by the direct weight of these elements. Or by calculating the increasing in weight of some substances which absorb some gases for example (CO2 – H2O) after the absorption. This increasing equals gas weight which is required to determination. 2-Decrease of weight method. ِِAs applied in determination crystallization water or some gases such as CO2within the chemical form for certain compound which is elevate as a result of heating or raising the temperature of a certain a mount of this compound. The product decreasing in the compound weight after heating equals to the gas weight which is required to determine that show the compound amount. E.g.. CaCO CO3+CaO

23 3- Chemical precipitation method
3- Chemical precipitation method. It should be distinguish between the electro deposition and chemical precipitation. The chemical precipitation method It depends on the ion separation which is determined of the solution by a chemical reaction between the ion and precipitating to produce sparingly soluble compound which is known with precipitate. Then the ion a mount is calculated by knowing the precipitate weight and its chemical form. Electro deposition method It follows the direct weight method which depends on the calculation of increasing of electrode weight in elements determination after element precipitation electronically.

24 Electro deposition method

25 Gravimetric analysis steps.
Gravimetric analysis is done by chemical precipitation in several steps could be summarized as follow: 1- Sample Solubility or Sample Solution. 2- The Primary treatment for the sample solution. 3- Precipitation. 4- Digestion. 5-Filtration. 6- Washing. 7- Drying Ignition. 8- Weighing. 9- Calculation.

26 1- Sample Solubility or Sample Solution.
By doing several tests for sample solubility and choosing a suitable solvent after the sample checking by naked eye and mechanical separation by using drainer (when the sample contains crystals in different size). After that the sample is dried for two hours at ( co) and absorbing any changes in weight might happen as a result of losing the crystallization water or some volatnete contents. Finally a suitable a mount of the sample weighted and soluted in the chosen solvent. 2- The primary treatment for the sample solution. I t includes the preparation suitable conditions for the precipitate process such as pH solution volume and temperature which decrease the chances of solubility of precipitate. In addition, a preliminary separation may be necessary of eliminate interfering material, that may interfere in the main gravimetric analysis process.

27 3- precipitation. It is the most important step of the Gravimetric analysis. The principle purpose of the precipitate process is the contains separation which will be determined from the others contains by the formation precipitated form. The partial dissociation for some precipitates in which chemical form changes for pant of precipitate (precipitated form) during the drying process leading to precipitate trans formation to mixture of two compounds of indefinite ratios, which in turn leads to a mistake in calculations that are found by knowing molecular weight for one compound only. That happens when estimating Iron or Aluminum in the form of hydroxide which is the precipitated form, Al(OH)3 , Fe(OH)3 in which part of it transfers into oxide during drying process. 2Al(OH) Al2O3 +3H2O 2Fe(OH) Fe2O3+3H2O The precipitates are weighted in form of mixture of oxide or hydroxide. To avoid these resulting mistakes, the chemist dose an. Ignition process for these precipitates to ensure the conversion of all parts to one form only that has a fixed chemical form. Known as weighted form. Representing the weighed form are oxides Aluminum, iron and Calcium in the previous examples. Despite the previously mentioned dissociation phenomena, most of the precipitates used in gravimetric analysis are chemically compounds such as barium sulphate, and therefore, the weight form for such precipitates the same as the precipitated form.

28 Needs for Gravimetric analysis
1- Precipitated form. 2- Weighted form. Conditions for the Precipitated form. 1-The precipitated form must be sparingly soluble compound. And the solubility product of compound should not exceed 1x10-8 to avoid solubility process mistake. 2- The precipitated form must be in form of granules and crystals to simplify the process of washing and filtrating precipitate. The galytinic compound such as Iron hydroxide or Aluminum hydroxide have tiny granules which increase the impurity for precipitate and slowing the filtration process because the granules close the process of filter paper, 3- The precipitated form must be transferred to weighted form easily and completely. Conditions for the weighted form. 1- The weighted form should be one compound with knowing and fixed chemical formula and that because calculation to be done compound . E.g.: When determined Iron as considered hydroxide Fe(OH)3 not representing chemical formula for the precipitate as we suspected. The precipitate have a big amount of water molecular that differ according to precipitate conditions. That why to have to Wright the chemical formula for precipitate as (Fe2O3. x H2O ) which not suitable to be weighted form because the calculation of the molecular weight can not be done for …..

29 For theses form because of presences of in unknown number of water molecules, that why to make ignition to exclude the un known number of water molecules and to have weight form with known chemical formula which is Fe2O3. Fe2O3.xH2O Fe2O3 +xH2O 2- The weighted form should be fixed chemically to avoid the oxidation and reduction in the lap or during ignition, and weighted form not (hypo scopic) not absorb water, also should not absorb CO2 which may lead for increase the weight and mistake in result. E.g.. choice of CaO is not a good choice because this precipitate absorb water vapor and CO2 form the air. So we have to change CaO to CaSO4 by treatment will H2SO4 in the coach crucible and get ride from the extra of acid by evaporation. 3- The amount of the element or ion (meaning to be estimated) in the weighted form as little as possible. For example, when estimating the element of chrome by Gravimetric methods of analysis, it is possible to weigh the chrome in the form of chrome oxide (Cr2O3) or in the form of Barium chromes( BaCrO4). Although, it is preferred to use the later compound as a balanced picture because the content or amount of chrome in this compound is less than the amount of chrome in chrome oxide. There fore, the mistakes that might result in the loss of part of the precipitate while weighing, washing, drying, filtering or ignition is much less when using Barium chrome. Continuo ………………………

30 This can be proven by the following mathematical process which proves that the amount of chrome in (1 mg ) of barium chromes is less than the amount of chrome in (1mg ) of chrome oxide. The weighted form(BaCrO4) The weighted form(Cr2O3). BaCrO Cr Cr2O Cr 253.3 mg mgCr mg Cr 1mgBaCrO xmg Cr mg Cr2O xmgCr 52x x1 X= = 0.2 mgCr x= =0.7mgCr 253, There fore, the amount of missing chrome in the case of losing1mg of Barium chromes, is much less than the some amount of loss of chrome oxide. The previous conditions of the precipitated and weighted form must be considered when choosing the precipitant, so it can be said that choosing the precipitant must be done in order for it to give sparingly soluble precipitate which considerable sized crystals or grains so it can be easily filtered and converted to the balanced form if it exists. In the case where only one form is present, the chemical formula for this precipitate has to be known, fixed and does not absorb steam or carbon dioxide from the air. The amount of element meaning to be estimated has to be as less as possible. In addition to the previous conditions, the precipitant must be at considerable level of volatile so that the used addition of this substance can be easily gotten rid of. For e.g.: In the case of the cupper precipitation in the form of cupper hydroxide Cu(OH)2,it is preferred to use sodium or potassium hydroxide instead of ammonium hydroxide although the latter is more volatile, that is because of the formation of a complex compound with cupper in the presence of additional ammonia, which causes the cupper hydroxide precipitate to dissolve when using ammonium hydroxide as a precipitant. When precipitating Fe in the form of hydroxide it is preferred to use NH OH instead of NaOH, KOH. When precipitating Ba in the form of BaSO it is preferred to use H SO instead of NaSO,KSO. When precipitating Ag in the form of AgCl it is preferred to use HCl instead of NaCl, KCl.

31 4- Digestion: The principle purpose of these process is to simplify filtration of precipitate by increasing the amount of precipitate crystals or grains, where the precipitate is left in the solution for period of time and the grains of precipitate will be clotted if the type of precipitates was or if the precipitate has very small grains, or if the precipitate is crystallized, recrystalization process can be done, when the small crystals will be soluted (that passes through filter paper) and it precipitate again in the form of large crystals or above the crystals in the solution. 5- Filtration: It is the process of separating the solid form precipitate that is formed by chemical precipitation process from the liquid form (solution). Where the Ion or element was present in before the precipitation process the filtration can be done by using filter paper, coach crucible or Buchner funnel when ever the precipitate of large grains it is filtered easier and vice versa.

32 6- Washing: The process of washing the precipitate is done by using specific liquid, in order to remove any impurities or any solution remains that have been formed in that might be attached to the precipitate that grains, when choosing the washing liquid we must absolve the following: 1- It must be electrolyte and easily violating and it must not contain non volatile salt such as NaCl, so it can be easily removed when drying or ignition. 2- It is not form non volatile product and insoluble with precipitate. 3 - It must not dissolve the precipitate. . When washing colloidal precipitate, the electrolyte substance must be adding that in order to equalize the charges surrounding the precipitate. And to clot the precipitate, because of that, water is not use to wash the precipitate sins it cause the crystals precipitate to dissolve. 7- Drying or Ignition: The principle purpose of drying or ignition is to remove any remains of the liquid which is used in the washing process water molecules and vomited impurities and the ignition is also used to convert the precipitate form to chemically fixed weighted form.

33 mwt for the precipitate
8- weighing: It is the process of using analytical balance to determine the precipitate and it comes ate the end of the practical steps for gravimetric analysis and that is after making sure the precipitate represents only one chemical compound and it must have a known and fixed chemical formula. 9- calculation: The amount of the content is meaning to be estimated is calculated by the knowledge of the precipitate weight and gravimetric conversion factor or chemical factor which can be calculated as follow: Gravimetric conversion factor = wt.atomic or mwt for the content is meaning to be estimated mwt for the precipitate wt. atomic for( Cl) Gravimetric conversion factor = mwt (AgCl) Wt(Cl) =Gravimetric conversion factor x wt for precipitate wt for precipitate(AgCl) x Gravimetric conversion factor (Cl) Cl / = x100 wt of sample (NaCl)


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