Calcium EDTA titration

Murexide may be employed for the direct EDTA titration of calcium at pH =11 the colour change at the end-point is from red to blue-violet, but is far from ideal. The colour change in the direct titration of nickel at pH 10-11 is from yellow to blue-violet. 

Calcichrome. This indicator, cyclotris-7-( l-azo-8-hydroxynaphthalene-3,6-disulphonic acid), is very selective for calcium. It is in fact not very suitable as an indicator for EDTA titrations because the colour change is not particularly sharp, but if EDTA is replaced by CDTA (see Section 2.26), then the indicator gives good results for calcium in the presence of large amounts of barium and small amounts of strontium.13... 

If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected, a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or where a suitable indicator cannot be found, back titration procedures are used. A measured excess of EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution. Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will displace the zinc or magnesium. 

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

In the original work [12], the reagent flow-rate was varied. An example of such a titration is the determination of calcium by titration with EDTA using the Cd ISE and the Cd-EDTA complex as an electrometric indicator [105]... 

The murexide method measures Ca2+ only Mg2+, at the concentration in milk, does not affect the indicator appreciably. Calculation of Mg2 + concentration is possible when the total calcium and magnesium (obtained by EDTA titration) is known. This is based on the assumption that the same proportion of each cation is present in the ionic form, which is justifiable since the dissociation constants of their citrate and phosphate salts are virtually identical. 

Hardness can be measured by either (1) calculation from the concentration of calcium and magnesium ions in the ample, or (2) EDTA titration. 

These complexes are weak enough that, in the complexometric titration determination using EDTA, they break and are included in the total calcium hardness reported. Thus, the total calcium hardness is composed of the legitimate cation, Ca, plus the complex ions as shown in the previous equations. This total calcium hardness must be corrected by the concentrations of the complexes in order to determine the correct activities of the calcium ions. Let the total concentration of the calcium species as determined by the EDTA titration be [Car]. Thus, the concentration of the calcium ion [Ca a )] is... 

From (11-35), and as illustrated schematically by Figure 11-9A, at constant [CaY ] the electrode potential decreases as calcium ion concentration increases. During an EDTA titration, pCa increases sharply near the equivalence point (Figure 11-9.B). If we consider now the entire titration and take into account the sharply increasing value of the ratio [CaY ]/[Ca ] during the early parts of the titration, the potential... 

Calcium in lithium salts has been determined by EDTA titration after a preliminary separation of the calcium from lithium using a chelating resin. 

An EDTA titration on the tap w ater will indicate the presence ot calcium and magnesium ion.s in the tap water, the eri(x hroine black I indicator changing colour from red to blue as these ions are complexed out of soiutitm. In soft water, the criochrome black T will l)e blue, since calcium and magnesium ions are absent. 

Figure 1 7-6 EDTA titration curves for 50.0 mL of 0,00500 M Ca-+ (i cav = 1.75 X 10 ) and Mg2+ = 1.72 X 10 ) at pH 10.0, Note that because of the larger formation constant, the reaction of calcium ion with EDTA is more complete, and a larger change occurs in the equivalence-point region. The shaded areas show the transition range for the indicator Eriochrome Black T.

C9. Copp, D. H., Simple and precise mioromethod for EDTA titration of calcium. 

Figure 3.4 Data from 10 measurements of calcium in milk by EDTA titration. The outlier is circled. The solid line is the mean of the data (x) and the dashed line is at x + Gcriticai > where s is the standard deviation of the data and Gcrideal = 2.29, which is the two-tailed G value at a = 0.05.

A strong acid cation exchange resin is converted to the H form. This then is used to remove the cations from a liquid sample such as mineral water, pond water, or the juices from low acid foods and exchange them with H. An aliquot of the sample containing H" will be titrated with standard base to determine the total amount of ions present. A second aliquot will be titrated with EDTA to determine the total calcium and magnesium ions present, and a third aliquot will be used to determine magnesium, again with an EDTA titration. 

The formation constants of the EDTA complexes of calcium and magnesium are too close to differentiate between them in an EDTA titration, even by adjusting pH (see Figure 9.4). So they will titrate together, and the Eriochrome Black T end point can be used as above. This titration is used to determine total hardness of water, (Ca " plus Mg " —see Experiment 9). Eriochrome Black T cannot be... 

The conditional formation constant for the calcium-EDTA chelate was calculated for pH 10 in Example 9.4 to be 1.8 X 10 °. Calculate the conditional formation constant at pH 3. Compare this with that calculated for lead at pH 3 in Problem 8. Could lead be titrated with EDTA at pH 3 in the presence of calcium ... 

Several analytical methods will differentiate the "free" (hydrated) metal ions from dissolved complexed metal ions. These methods include specific ion electrodes, polarographic, and other amperometric and voltammetric methods and various types of spectroscopy (see Section 7-10). Specific ion electrodes only respond to the free metal ion for which they are "specific." To determine the relative amounts of complexed and uncomplexed metal ion in a solution, we can use a "wet chemical" method to measure the total concentration of "free + complexed" ions, and then an ion-specific electrode to determine the free metal ion concentration (activity). Care must be taken to eliminate interferences that may affect these measurements. We deduce the concentration of the "complexed ions" by the difference between these two measurements. For example, in the EDTA titration method for hardness, free and complexed calcium and magnesium ion s are measured. 

Another advantage of potentiometric titrations is that substances to which the electrode does not respond can be determined, if the electrode responds to the titrant or to some low level of an indicator substance that has been added to the solution. For example, low levels of Al can be determined by titration with standard fluoride solution, using a fluoride electrode [22]. EDTA and other chelates can be determined by titration with standard calcium or copper solution. Manganese(II), vanadium(II), or cobalt(II) can be determined via EDTA titration if a small amount of CuEDTA indicator is added to the solution and a copper electrode is used. The electrode responds directly to the Cu activity which, however, is dependent on the activities of the EDTA and the other metal ion in solution. 

Ion-selective electrodes have a role for monitoring milk both for its calcium content and as a possible vehicle for health fluoridation programmes. The ion-selective electrode method appears to give higher ionic calcium levels (< 2.71 mmol dm" ) than the method of EDTA titration of ion-exchange eluates ( 2.52 mmol dm ) possibly because of the smaller pH changes in the former case. However, in passing from raw milk to sterilised milk ( 2.27 mmol dm ) and pasteurised milk ( 2.04 mmol dm" ) there is a fall in the ionic calcium level [275]. 

Lead oxide Lead oxide is an important constituent of glasses. Other constituents, especially zinc oxide, barium oxide, and calcium oxide, influence the determination of lead oxide in various glasses. Separation of lead oxide using electrolysis to form lead dioxide on the anode and subsequent EDTA titration is a popular method, applied to 50-100 mg of PbO after nitric acid-hydrofluoric acid treatment of the glass in a platinum dish. 

Alkaline-earth metal oxides Calcium and magnesium oxides are constituents of nearly all glasses and may be determined using EDTA titration. Neither determination is free of interference. 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

We studied the calcium—EDTA chelation using Calcein [4] as the fluorescent indicator. This indicator forms a fluorescent complex with free calcium ions. In order to determine calcium in a sample, the excess EDTA remaining after stoichiometric chelation of calcium in the sample is "back-titrated" with a known concentration of calcium solution. The equivalence point is reached when all of the excess EDTA is chelated and is detected by fluorescence of the Calcein-calcium complex formed by a slight excess of free calcium ions. The calcium content of the sample (in micrograms) is equal to the volume (in milliliters) of titrant added when no sample was introduced, minus the volume (in milliliters) of titrant added when the sample was introduced, times the calcium content of the titrant (in micrograms per milliliter). 

Twenty milliliter of EDTA solution (1 ml equivalent to 72 pLg calcium) were titrated using Calcein as the indicator of fluorescence end point. The titrant contained 400/xg calcium/ml and the entries in the table represent milliliters of this titrant added for automatic stop. Dade Reagents analysis was 100 4/xg/ml for Lab-trol, and 81.0 3.2/xg/mlfor Patho-trol. Procedure and instrumental parameters are given in the text. 

A 0.2000-g sample containing calcium is titrated against a 0.04672 Af EDTA solution. If 23.94 mL of the solution is required to reach an end point using a complexometric indicator, determine the percentage of calcium in the sample. 

Complexometric titration where the initially formed red calcium-eriochrome black T color is replaced vrith the blue calcium-EDTA color at the end point Spectrophotometric titration of the complex without preliminary extraction Spec A = 506 nm (pH = 11.3)... 

Calcium may be determined by atomic absorption spectrophotometry. This technique has been used for the estimation of calcium in biological fluids and agricultural materials. It is a speedy method, superior to the more tedious chemical determination by oxalate precipitation and more specific than the EDTA titration method. The accuracy is of the order of 2 per cent and sensitivity limits have been reported at 0 08 to 1 p.p.m. of calcium in solution. The interference problems are very similar to those experienced with the emission method although not quite so formidable. 

Injection of Calcium Gluconate, B.P. Contains about 10 per cent of calcium gluconate but up to 5 per cent of it may be replaced by a suitable calcium salt as stabiliser. The calcium content can be determined by the direct EDTA titration method given above. 

The following method may be used for determination of the calcium. The total calcium and magnesium is determined in an aliquot by EDTA titration, the calcium precipitated as oxalate from a further aliquot and the residual magnesium titrated to give the calcium by difference. 

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