5CJ OJ QJ ^J aJ #h`. Standardize against pure zinc (Bunker Hill 99.9985%) if high purity magnesium is not available. The hardness of a water source has important economic and environmental implications. To calculate magnesium solution concentration use EBAS - stoichiometry calculator. endstream endobj 267 0 obj <>/Filter/FlateDecode/Index[82 161]/Length 27/Size 243/Type/XRef/W[1 1 1]>>stream The Titration After the magnesium ions have been precipitated out of the hard water by the addition of NaOH (aq) to form white Mg(OH) 2(s), the remaining Ca 2+ ions in solution are titrated with EDTA solution.. 0000021647 00000 n which is the end point. Beginning with the conditional formation constant, \[K_\textrm f'=\dfrac{[\mathrm{CdY^{2-}}]}{[\mathrm{Cd^{2+}}]C_\textrm{EDTA}}=\alpha_\mathrm{Y^{4-}} \times K_\textrm f = (0.37)(2.9\times10^{16})=1.1\times10^{16}\], we take the log of each side and rearrange, arriving at, \[\log K_\textrm f'=-\log[\mathrm{Cd^{2+}}]+\log\dfrac{[\mathrm{CdY^{2-}}]}{C_\textrm{EDTA}}\], \[\textrm{pCd}=\log K_\textrm f'+\log\dfrac{C_\textrm{EDTA}}{[\mathrm{CdY^{2-}}]}\]. 0000001090 00000 n After filtering and rinsing the precipitate, it is dissolved in 25.00 mL of 0.02011 M EDTA. At a pH of 9 an early end point is possible, leading to a negative determinate error. EDTA forms a chelation compound with magnesium at alkaline pH. Add 12 drops of indicator and titrate with a standard solution of EDTA until the red-to-blue end point is reached (Figure 9.32). When the titration is complete, raising the pH to 9 allows for the titration of Ca2+. Next, we draw our axes, placing pCd on the y-axis and the titrants volume on the x-axis. A complexometric titration method is proposed to determine magnesium oxide in flyash blended cement. 0000001814 00000 n Figure 9.33 Titration curves for 50 mL of 103 M Mg2+ with 103 M EDTA at pHs 9, 10, and 11 using calmagite as an indicator. The accuracy of an indicators end point depends on the strength of the metalindicator complex relative to that of the metalEDTA complex. Procedure to follow doesn't differ much from the one used for the EDTA standardization. Eriochrome Black-T(EBT) is the metal ion indicator used in the determination of hardness by complexometric titration with EDTA. &=6.25\times10^{-4}\textrm{ M} We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. If the metalindicator complex is too weak, however, the end point occurs before we reach the equivalence point. Sketch titration curves for the titration of 50.0 mL of 5.00103 M Cd2+ with 0.0100 M EDTA (a) at a pH of 10 and (b) at a pH of 7. The calcium and magnesium ions (represented as M2+ in Eq. Dilutes with 100 ml of water and titrate the liberated iodine with 0.1M sodium thiosulphate using 0.5ml of starch solution, added towards the end of the titration, as an indicator. Most indicators for complexation titrations are organic dyesknown as metallochromic indicatorsthat form stable complexes with metal ions. Recall that an acidbase titration curve for a diprotic weak acid has a single end point if its two Ka values are not sufficiently different. 21 0 obj <> endobj The method adopted for the Ca-mg analysis is the complexometric titration. If the sample does not contain any Mg2+ as a source of hardness, then the titrations end point is poorly defined, leading to inaccurate and imprecise results. 3 22. EDTA (mol / L) 1 mol Magnesium. Unfortunately, because the indicator is a weak acid, the color of the uncomplexed indicator also changes with pH. Next, we add points representing pCd at 110% of Veq (a pCd of 15.04 at 27.5 mL) and at 200% of Veq (a pCd of 16.04 at 50.0 mL). endstream endobj 22 0 obj<> endobj 24 0 obj<> endobj 25 0 obj<>/Font<>/XObject<>/ProcSet[/PDF/Text/ImageC/ImageI]/ExtGState<>>> endobj 26 0 obj<> endobj 27 0 obj<> endobj 28 0 obj[/ICCBased 35 0 R] endobj 29 0 obj[/Indexed 28 0 R 255 36 0 R] endobj 30 0 obj[/Indexed 28 0 R 255 37 0 R] endobj 31 0 obj<> endobj 32 0 obj<> endobj 33 0 obj<> endobj 34 0 obj<>stream Table 9.13 and Figure 9.28 show additional results for this titration. $d 7$ 8$ H$ a$gd, d 7$ 8$ H$ gd% | ~ zhY h, 5CJ OJ QJ ^J aJ #h, h, 5CJ OJ QJ ^J aJ #h, h% 5CJ OJ QJ ^J aJ +h;- h, 5CJ OJ QJ ^J aJ mHsH.h;- h% 5CJ H*OJ QJ ^J aJ mHsH +h;- h% 5CJ OJ QJ ^J aJ mHsH.h;- h, 5CJ H*OJ QJ ^J aJ mHsH .h;- h% 5CJ H*OJ QJ ^J aJ mHsH q t xcM8 (h, h% CJ# OJ QJ ^J aJ# mHsH +h The operational definition of water hardness is the total concentration of cations in a sample capable of forming insoluble complexes with soap. At any pH a mass balance on EDTA requires that its total concentration equal the combined concentrations of each of its forms. After the equivalence point, EDTA is in excess and the concentration of Cd2+ is determined by the dissociation of the CdY2 complex. This dye-stuff tends to polymerize in strongly acidic solutions to a red brown product, and hence the indicator is generally used in EDTA titration with solutions having pH greater than 6.5. Solving equation 9.11 for [Y4] and substituting into equation 9.10 for the CdY2 formation constant, \[K_\textrm f =\dfrac{[\textrm{CdY}^{2-}]}{[\textrm{Cd}^{2+}]\alpha_{\textrm Y^{4-}}C_\textrm{EDTA}}\], \[K_f'=K_f\times \alpha_{\textrm Y^{4-}}=\dfrac{[\mathrm{CdY^{2-}}]}{[\mathrm{Cd^{2+}}]C_\textrm{EDTA}}\tag{9.12}\]. Your TA will give you further information on how you will obtain your data. Otherwise, the calcium will precipitate and either you'll have no endpoint or a weak endpoint. Volume required to neutralise EDTA. Figure 9.29 Illustrations showing the steps in sketching an approximate titration curve for the titration of 50.0 mL of 5.00 103 M Cd2+ with 0.0100 M EDTA in the presence of 0.0100 M NH3: (a) locating the equivalence point volume; (b) plotting two points before the equivalence point; (c) plotting two points after the equivalence point; (d) preliminary approximation of titration curve using straight-lines; (e) final approximation of titration curve using a smooth curve; (f) comparison of approximate titration curve (solid black line) and exact titration curve (dashed red line). Titrate with EDTA solution till the color changes to blue. For example, an NH4+/NH3 buffer includes NH3, which forms several stable Cd2+NH3 complexes. First, however, we discuss the selection and standardization of complexation titrants. leaving 4.58104 mol of EDTA to react with Cr. (a) Titration of 50.0 mL of 0.010 M Ca2+ at a pH of 3 and a pH of 9 using 0.010 M EDTA. &=\dfrac{(5.00\times10^{-3}\textrm{ M})(\textrm{50.0 mL})}{\textrm{50.0 mL + 30.0 mL}}=3.13\times10^{-3}\textrm{ M} Adding a small amount of Mg2+EDTA to the buffer ensures that the titrand includes at least some Mg2+. h, 5>*CJ H*OJ QJ ^J aJ mHsH.h 0000011407 00000 n Solutions of EDTA are prepared from its soluble disodium salt, Na2H2Y2H2O and standardized by titrating against a solution made from the primary standard CaCO3. If the metalindicator complex is too strong, the change in color occurs after the equivalence point. (Show main steps in your calculation). 0000028404 00000 n Finally, a third 50.00-mL aliquot was treated with 50.00 mL of 0.05831 M EDTA, and back titrated to the murexide end point with 6.21 mL of 0.06316 M Cu2+. CJ OJ QJ ^J aJ h`. Figure 9.29a shows the result of the first step in our sketch. A red to blue end point is possible if we maintain the titrands pH in the range 8.511. Next, we solve for the concentration of Cd2+ in equilibrium with CdY2. The stoichiometry between EDTA and each metal ion is 1:1. Solutions of Ag+ and Hg2+ are prepared using AgNO3 and Hg(NO3)2, both of which are secondary standards. h? (b) Titration of a 50.0 mL mixture of 0.010 M Ca2+ and 0.010 M Ni2+ at a pH of 3 and a pH of 9 using 0.010 M EDTA. The evaluation of hardness was described earlier in Representative Method 9.2. In this experiment you will standardize a solution of EDTA by titration against a standard Add 1 mL of ammonia buffer to bring the pH to 100.1. 21 19 Use the standard EDTA solution to titrate the hard water. 0000021829 00000 n Our goal is to sketch the titration curve quickly, using as few calculations as possible. In section 9B we learned that an acidbase titration curve shows how the titrands pH changes as we add titrant. 243 26 Calcium can be precipitated as carbonate or oxalate, although presence of oxalates may make end point detection difficult. Thus, by measuring only magnesium concentration in the Standardization of EDTA: 20 mL of the standard magnesium sulfate solution is pipetted out into a 250 mL Erlenmeyer flask and diluted to 100 mL . Conditions to the right of the dashed line, where Mg2+ precipitates as Mg(OH)2, are not analytically useful for a complexation titration. Just like during determination of magnesium all metals other than alkali metals can interfere and should be removed prior to titration. 0000021034 00000 n U! dh 7$ 8$ H$ ^gd 0000022320 00000 n A 0.50 g of sample was heated with hydrochloric acid for 10 min. The sample was acidified and titrated to the diphenylcarbazone end point, requiring 6.18 mL of the titrant. The actual number of coordination sites depends on the size of the metal ion, however, all metalEDTA complexes have a 1:1 stoichiometry. Add 4 drops of Eriochrome Black T to the solution. 0000001156 00000 n In this method buffer solution is used for attain suitable condition i.e pH level above 9 for the titration. MgSO4 Mg2++SO42- Experimental: given: Devarda alloy= 0.518g [EDTA] = 0.02 moldm^3 average titration 1ml of 0.1N potassium permanganate is equivalent to 0.2 mg of calcium Therefore, X3 ml of' Y' N potassium permanganate is equivalent to. The charged species in the eluent will displace those which were in the sample and these will flow to the detector. This leaves 5.42104 mol of EDTA to react with Fe; thus, the sample contains 5.42104 mol of Fe. The obtained average molarity of EDTA (0.010070.00010 M) is used in Table 2 to determine the hardness of water. zhVGV9 hH CJ OJ QJ ^J aJ h 5CJ OJ QJ ^J aJ #h hH 5CJ OJ QJ ^J aJ #hk h(5 5CJ OJ QJ ^J aJ h(5 CJ OJ QJ ^J aJ $h(5 h(5 5B* The reaction of Mg2+ with EDTA may be expressed as: Mg2+ + H2Y2- = MgY-2 + 2H+ The structure of EDTA and the magnesium-EDTA complex (without the hydrogen atoms) is shown below: The endpoint of the titration is determined by the . 0 2 4 seWEeee #hLS h% CJ H*OJ QJ ^J aJ hLS CJ OJ QJ ^J aJ hp CJ OJ QJ ^J aJ h`. U! startxref Let the burette reading of EDTA be V 3 ml. A new spectrophotometric complexometric titration method coupled with chemometrics for the determination of mixtures of metal ions has been developed. The red arrows indicate the end points for each analyte. This provides some control over an indicators titration error because we can adjust the strength of a metalindicator complex by adjusted the pH at which we carry out the titration. Complexation titrimetry continues to be listed as a standard method for the determination of hardness, Ca2+, CN, and Cl in waters and wastewaters. The third titration uses, \[\mathrm{\dfrac{0.05831\;mol\;EDTA}{L}\times0.05000\;L\;EDTA=2.916\times10^{-3}\;mol\;EDTA}\], of which 1.524103 mol are used to titrate Ni and 5.42104 mol are used to titrate Fe. 6ADIDnu1cGM?froF%a,;on_Qw!"eEA#z@$\Xx0f 80BUGc77 b`Y]TkEZt0Yu}5A\vm5Fvh5A/VbgvZd Before the equivalence point, Cd2+ is present in excess and pCd is determined by the concentration of unreacted Cd2+. The determination of the Calcium and Magnesium next together in water is done by titration with the sodium salt of ethylenediaminetetraethanoic acid (EDTA) at pH 8 9, the de- tection is carried out with a Ca electrode. The consumption should be about 5 - 15 ml. 2) You've got some . First, we calculate the concentration of CdY2. Sample solutions for the calculation of the molarity of EDTA and titer CaCO3 are shown in Appendix. We also will learn how to quickly sketch a good approximation of any complexation titration curve using a limited number of simple calculations. Buffer . Each ml of 0.1M sodium thiosulphate is equivalent to 0.02703 g of FeCI3,6H2O. Add 10 mL of ammonia buffer, 50 mL of distilled water and 1 mL of Eriochrome Black T indicator xref It is widely used in the pharmaceutical industry to determine the metal concentration in drugs. This is often a problem when analyzing clinical samples, such as blood, or environmental samples, such as natural waters. The next task in calculating the titration curve is to determine the volume of EDTA needed to reach the equivalence point. (not!all!of . Determination of Hardness: Hardness is expressed as mg/L CaCO 3. 1.The colour change at the end point (blue to purple) in the Titration I is due to [Mark X in the correct box.] Determination of Permanent hardness Take 100 ml of sample hard water in 250 ml beaker. The range of pMg and volume of EDTA over which the indicator changes color is shown for each titration curve. Before adding EDTA, the mass balance on Cd2+, CCd, is, and the fraction of uncomplexed Cd2+, Cd2+, is, \[\alpha_{\textrm{Cd}^{2+}}=\dfrac{[\mathrm{Cd^{2+}}]}{C_\textrm{Cd}}\tag{9.13}\]. The most widely used of these new ligandsethylenediaminetetraacetic acid, or EDTAforms strong 1:1 complexes with many metal ions. In addition magnesium forms a complex with the dye Eriochrome Black T. Menu. \end{align}\], \[\begin{align} 0.2 x X3 xY / 1 x 0.1 = Z mg of calcium. (b) Diagram showing the relationship between the concentration of Mg2+ (as pMg) and the indicators color. &=\dfrac{\textrm{(0.0100 M)(30.0 mL)} - (5.00\times10^{-3}\textrm{ M})(\textrm{50.0 mL})}{\textrm{50.0 mL + 30.0 mL}}\\ The burettte is filled with an EDTA solution of known concentration. We begin by calculating the titrations equivalence point volume, which, as we determined earlier, is 25.0 mL. 4 23. ! The concentration of Ca2+ ions is usually expressed as ppm CaCO 3 in the water sample. EDTA Titration You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution. Elution of the compounds of interest is then done using a weekly acidic solution. 0000031526 00000 n Why is the sample buffered to a pH of 10? 0000023793 00000 n Complexometric titration is used for the estimation of the amount of total hardness in water. The ladder diagram defines pMg values where MgIn and HIn are predominate species. Step 1: Calculate the conditional formation constant for the metalEDTA complex. Titration Method for Seawater, Milk and Solid Samples 1. EDTA, which is shown in Figure 9.26a in its fully deprotonated form, is a Lewis acid with six binding sitesfour negatively charged carboxylate groups and two tertiary amino groupsthat can donate six pairs of electrons to a metal ion. HWM6W- ~jgvuR(J0$FC*$8c HJ9b\I_~wfLJlduPl Given the Mg2+: EDTA ratio of 1 : 1, calculate the concentration of your EDTA solution. (Assume the moles of EDTA are equal to the moles of MgCO3) Chemistry Reactions in Solution Titration Calculations. Description . (i) Calculation method For this method, concentration of cations should be known and then all concentrations are expressed in terms of CaCO 3 using Eq. Thus one simply needs to determine the area under the curve of the unknown and use the calibration curve to find the unknown concentration. Calculate the number of grams of pure calcium carbonate required to prepare a 100.0 mL standard calcium solution that would require ~35 mL of 0.01 M EDTA for titration of a 10.00 mL aliquot: g CaCO 3 = M EDTA x 0.035L x 1 mol CaCO 3/1 mol EDTA x MM CaCO 3 x 100.0mL/10.00mL 3. 4 Sample Calculations (Cont.) End point of magnesium titration is easily detected with Eriochrome BlackT. To perform titration we will need titrant - 0.01M EDTA solution and ammonia pH10.0 buffer. The most likely problem is spotting the end point, which is not always sharp. Complexometric Determination of Magnesium using EDTA EDTA Procedure Ethylenediaminetetraacetic Acid Procedure Preparing a Standard EDTA Solution Reactions 1.Weighing by difference 0.9g of EDTA 2.Quantitatively transfer it to a 250 mL volumetric flask 3.Add a 2-3mL of amonia buffer (pH 10) 3. ^208u4-&2`jU" JF`"Py~}L5@X2.cXb43{b,cbk X$ Thus, when the titration reaches 110% of the equivalence point volume, pCd is logKf 1. The amount of EDTA reacting with Cu is, \[\mathrm{\dfrac{0.06316\;mol\;Cu^{2+}}{L}\times0.00621\;L\;Cu^{2+}\times\dfrac{1\;mol\;EDTA}{mol\;Cu^{2+}}=3.92\times10^{-4}\;mol\;EDTA}\]. The free magnesium reacts with calmagite at a pH of 10 to give a red-violet complex. Architektw 1405-270 MarkiPoland, free trial version of the stoichiometry calculator. To determine the concentration of each metal separately, we need to do an additional measurement that is selective for one of the two metals. where Kf is a pH-dependent conditional formation constant. Titration is a method to determine the unknown concentration of a specific substance (analyte) dissolved in a sample of known concentration. B = mg CaCO3 equivalent to 1 ml EDTA Titrant. The second titration uses, \[\mathrm{\dfrac{0.05831\;mol\;EDTA}{L}\times0.03543\;L\;EDTA=2.066\times10^{-3}\;mol\;EDTA}\]. It is vital for the development of bones and teeth. Figure 9.35 Spectrophotometric titration curve for the complexation titration of a mixture of two analytes. Prepare a standard solution of magnesium sulfate and titrate it against the given EDTA solution using Eriochrome Black T as the indicator. From the chromatogram it is possible to get the area under the curve which is directly related to the concentration of the analyte. Liebigs titration of CN with Ag+ was successful because they form a single, stable complex of Ag(CN)2, giving a single, easily identified end point. PAGE \* MERGEFORMAT 1 U U U U U U U U U. Calcium and Magnesium ion concentration determination with EDTA titration 56,512 views Dec 12, 2016 451 Dislike Share Save Missy G. 150 subscribers CHEM 249 Extra credit by Heydi Dutan and. Dissolve the salt completely using distilled or de-ionized water. EDTA. Titanium dioxide is used in many cosmetic products. 0000001334 00000 n OJ QJ ^J ph p !h(5 h(5 B*OJ QJ ^J ph ' j h(5 h(5 B*OJ QJ ^J ph h(5 B*OJ QJ ^J ph $h(5 h(5 5B*OJ QJ ^J ph hk hH CJ OJ QJ ^J aJ hj CJ OJ QJ ^J aJ T! Suppose we need to analyze a mixture of Ni2+ and Ca2+. Although many quantitative applications of complexation titrimetry have been replaced by other analytical methods, a few important applications continue to be relevant. The sample is acidified to a pH of 2.33.8 and diphenylcarbazone, which forms a colored complex with excess Hg2+, serves as the indicator. Add 1 or 2 drops of the indicator solution. Because Ca2+ forms a stronger complex with EDTA, it displaces Mg2+, which then forms the red-colored Mg2+calmagite complex. 0000020364 00000 n Complexation Titration is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts. and pCd is 9.77 at the equivalence point. This is the same example that we used in developing the calculations for a complexation titration curve. Another common method is the determination by . For a titration using EDTA, the stoichiometry is always 1:1. The same unknown which was titrated will be analyzed by IC. When the reaction is complete all the magnesium ions would have been complexed with EDTA and the free indicator would impart a blue color to the solution. EDTA solution. Portions of the magnesium ion solution of volume10 mL were titrated using a 0.01000 M solution of EDTA by the method of this experiment. All Answers (10) 1) Be sure the pH is less than 10, preferably about 9.5-9.7. 268 0 obj <>stream T! Determination of Total Hardness by Titration with Standardized EDTA Determine the total hardness (Ca2+ and Mg2+) by using a volumetric pipet to pipet 25 mL of the unknown solution into a 250 mL Erlenmeyer flask. concentration and the tap water had a relatively normal level of magnesium in comparison. \end{align}\]. This may be difficult if the solution is already colored. ! The sample, therefore, contains 4.58104 mol of Cr. This is how you can perform an estimation of magnesium using edta. Add a pinch of Eriochrome BlackT ground with sodium chloride (100mg of indicator plus 20g of analytical grade NaCl). Having determined the moles of EDTA reacting with Ni, we can use the second titration to determine the amount of Fe in the sample. EDTA is a versatile titrant that can be used to analyze virtually all metal ions. 0000000676 00000 n At a pH of 3 EDTA reacts only with Ni2+. 3. Next, we draw a straight line through each pair of points, extending the line through the vertical line representing the equivalence points volume (Figure 9.29d). In this case the interference is the possible precipitation of CaCO3 at a pH of 10. A time limitation suggests that there is a kinetically controlled interference, possibly arising from a competing chemical reaction. The blue line shows the complete titration curve. Therefore the total hardness of water can be determination by edta titration method. xref OJ QJ UmH nH u h CJ OJ QJ ^J aJ h, h% CJ OJ QJ ^J aJ hs CJ OJ QJ ^J aJ R T V Z v x | qcU? The mean corrected titration volume of the EDTA solution was 16.25 mL (0.01625 L). Report the samples hardness as mg CaCO3/L. 7mKy3c d(jwF`Mt?0wKY{jGO.AW,eU"^0E: ~"G vPKD"(N1PzbtN]716.^`[ The quantitative relationship between the titrand and the titrant is determined by the stoichiometry of the titration reaction. &=\dfrac{(5.00\times10^{-3}\textrm{ M})(\textrm{50.0 mL})}{\textrm{50.0 mL + 25.0 mL}}=3.33\times10^{-3}\textrm{ M} The availability of a ligand that gives a single, easily identified end point made complexation titrimetry a practical analytical method. In addition to its properties as a ligand, EDTA is also a weak acid. At the equivalence point all the Cd2+ initially in the titrand is now present as CdY2. Problem 9.42 from the end of chapter problems asks you to verify the values in Table 9.10 by deriving an equation for Y4-. Click here to review your answer to this exercise. 4. The procedure de-scribed affords a means of rapid analysis. Adjust the samples pH by adding 12 mL of a pH 10 buffer containing a small amount of Mg2+EDTA. 2.1 The magnesium EDTA exchanges magnesium on an equivalent basis for any calcium and/or other cations to form a more stable EDTA chelate than magnesium. Solving gives [Cd2+] = 4.71016 M and a pCd of 15.33. \[C_\textrm{EDTA}=[\mathrm{H_6Y^{2+}}]+[\mathrm{H_5Y^+}]+[\mathrm{H_4Y}]+[\mathrm{H_3Y^-}]+[\mathrm{H_2Y^{2-}}]+[\mathrm{HY^{3-}}]+[\mathrm{Y^{4-}}]\]. Table 2 Determination of Total Hardness of Water Trials Volume of Sample (mL) Nt. In an EDTA titration of natural water samples, the two metals are determined together. In the lab 1 ppm CaCO 3 is expressed as 1 mg CaCO 3 per 1 Liter of sample or ppm is mg CaCO . When the titration is complete, we adjust the titrands pH to 9 and titrate the Ca2+ with EDTA. A indirect complexation titration with EDTA can be used to determine the concentration of sulfate, SO42, in a sample. %Srr~81@ n0/Mm`:5 A)r=AKVvY Ri9~Uvhug BAp$eK,v$R!36e8"@` For example, as shown in Figure 9.35, we can determine the concentration of a two metal ions if there is a difference between the absorbance of the two metal-ligand complexes. The reaction between Mg2+ ions and EDTA can be represented like this. See Chapter 11 for more details about ion selective electrodes. Titration . Figure 9.30 is essentially a two-variable ladder diagram. A more recent method is the titration of magnesium solution with ethylene-diamine tetra-acetate(Carr and Frank, 1956). The end point is determined using p-dimethylaminobenzalrhodamine as an indicator, with the solution turning from a yellow to a salmon color in the presence of excess Ag+. For the titration of Mg2+, one must buffer the solution to a pH of 10 so that complex formation will be quantitative. Calcium is determined at pH 12 where magnesium is quantitatively precipitated as the hydroxide and will not react with EDTA. Table 9.10 provides values of Y4 for selected pH levels. From Table 9.10 and Table 9.11 we find that Y4 is 0.35 at a pH of 10, and that Cd2+ is 0.0881 when the concentration of NH3 is 0.0100 M. Using these values, the conditional formation constant is, \[K_\textrm f''=K_\textrm f \times \alpha_\mathrm{Y^{4-}}\times\alpha_\mathrm{Cd^{2+}}=(2.9\times10^{16})(0.37)(0.0881)=9.5\times10^{14}\], Because Kf is so large, we can treat the titration reaction, \[\textrm{Cd}^{2+}(aq)+\textrm Y^{4-}(aq)\rightarrow \textrm{CdY}^{2-}(aq)\]. mH nH uh7 j h7 Uh j h U h)v h0Z CJ OJ QJ ^J aJ h, CJ OJ QJ ^J aJ hB CJ OJ QJ ^J aJ hZ7 CJ OJ QJ ^J aJ Uh0Z CJ OJ QJ ^J aJ h)v CJ OJ QJ ^J aJ hp CJ OJ QJ ^J aJ f charge attraction. \[\begin{align} Note that after the equivalence point, the titrands solution is a metalligand complexation buffer, with pCd determined by CEDTA and [CdY2]. <<7daf3a9c17b9c14e9b00eea5d2c7d2c8>]>>