Replacement titrations

Substitution Titrations. Upon the introduction of a substantial or equivalent amount of the chelonate of a metal that is less stable than that of the metal being determined, a substitution occurs, and the metal ion displaced can be titrated by the chelon in the same solution. This is a direct titration with regard to its performance, but in terms of the mechanism it can be considered as a substitution titration (or replacement titration). 

Since a number of factors are involved in the optimisation, some time is required to develop the method. However, when set up, the method can replace titrations and replicate analyses can be conducted very quickly with minimal consumption of reagents. 

Determination of barium by displacement of zinc ions in a strongly anuno-niacal solution. In general, replacement titrations seem to offer Uttle advantage over back titrations. 

Successful attempts have been made to replace titration by direct colorimetric determination of ammonia. Berthelot s reaction has been carried out on the neutralized Kjeldahl digest (E6, L19), but interference by H2O2 and by Ca++ can occur (R13). The Berthelot reaction has been speeded up by the introduction of a catalyst, sodium nitroprusside (L19) it must be carried out at pH 10-12 (C5) under which conditions it is specific for NH4+ (W12). The reaction between NH4+ and indanetrione hydrate may also be used the digest is neutralized, buffered at pH 5.0, heated with the reagent at 110°C for 30 minutes, diluted with 50% ethanol, and the color measured at 570 nm (Jl). 

As sodium acetate is titrated, the acetate ion is replaced by the chloride ion, which, owing to its slightly higher ionic-equivalent conductance, causes a slight increase in conductivity up to the endpoint. Beyond the endpoint, excess hydrochloric acid causes large increases. Such titrations are useful where the ionization constant of the liberated weak acid or base, when divided by the salt s concentration, does not exceed 5 x 10" . If a difunctional acid is formed, then the two ionization constants should differ by about 1Q- if two endpoints are to be observed NajS is an example of this type. Figure 5.9 shows typical replacement titration curves. 

Replacement titrations are useful for titrating such salts as oxalates, phosphates, benzoates, etc. 

The basis for titrations with indirect indicators of metallic ions is different from one that supports replacement titrations (see below). 

Replacement titration When a particular metal ion does not form a sufficiently stable complex with E.D.T.A., magnesium EDTA complex is added and the ions set free are titrated with E.D.T.A. ... 

The stability of the alkaline-earth EDTA complexes decreases down the group as expected from the increased ionic radius. However, Ca ions are exceptional in forming the most stable EDTA complex. For this reason, calcium cannot be titrated using the Erio T indicator, since no colour change to steel blue is observed. However, when a suspension of magnesium complexone is added, the calcium replaces magnesium in the complexone releasing Mg ions which can be titrated. This replacement titration may be used to standardise EDTA solution. 

Chloride ion may be an impurity in anionic surfactants, depending on the process used for sulfonation and the purity of the alkali used to neutralize the product after sulfonation. Chloride ion is determined by titration or ion chromatography. Ion chromatography conditions must be worked out for each type of surfactant because of differences in polarity and water solubility. Titration is more universally applicable. Potentiometric titration has replaced titration to a color change end point in most laboratories. 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

Any Fe + lost in this fashion must be replaced by the additional reduction of Fe +, reducing the current efficiency and increasing the time needed to reach the titration s end point. The net result is that the reported concentration of Cr207 is too large. 

The titration curve of phosphoric acid in the presence of sodium hydroxide is shown in Figure 1. Three steps, corresponding to consecutive replacement of the three acidic hydrogens, and two inflection points, near pH = 4.5 and 9.0, are evident. Dissociation constants are = 7.1 x 10 = 6.3 x 10 ... 

Bases on the data of curve of the potentiometric titrations of [AuBr ] ions by thiourea (Thio), it consistently replaces bromide ions in [AuBr ] ion. They are formed mixed bromide-thiourea complexes of Au(III) AuBr Thio, AuBiyr/iio AuBrThio, AuBrThioJ. ... 

There are two types of fluoride lon-selective electrodes available [27] Onon model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Onon model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride lon-specific electrode can be used for direct measurement [2S, 29] or for potenPometric titration with Th" or nitrate solutions, with the electrode as an end point indicator... 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

Discussion. When a solution of an orthophosphate is treated with a large excess of ammonium molybdate solution in the presence of nitric acid at a temperature of 20-45 °C, a precipitate is obtained, which after washing is converted into ammonium molybdophosphate with the composition (NH4)3[P04,12Mo03]. This may be titrated with standard sodium hydroxide solution using phenolph-thalein as indicator, but the end point is rather poor due to the liberation of ammonia. If, however, the ammonium molybdate is replaced by a reagent containing sodium molybdate and quinoline, then quinoline molybdophosphate is precipitated which can be isolated and titrated with standard sodium hydroxide ... 

C. Replacement or substitution titration. Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium. The metal cation M + to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs ... 

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... 

Determination of calcium. Pipette two 25.0 mL portions of the mixed calcium and magnesium ion solution (not more than 0.01M with respect to either ion) into two separate 250 mL conical flasks and dilute each with about 25 mL of de-ionised water. To the first flask add 4 mL 8 M potassium hydroxide solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 30 mg each of potassium cyanide (Caution poison) and hydroxylammonium chloride and swirl the contents of the flask until the solids dissolve. Add about 50 mg of the HHSNNA indicator mixture and titrate with 0.01 M EDTA until the colour changes from red to blue. Run into the second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1 mL. Now add 4 mL of the potassium hydroxide solution, mix well and complete the titration as with the first sample record the exact volume of EDTA solution used. Perform a blank titration, replacing the sample with de-ionised water. 

Treat the arsenate solution (say, 20.0 mL of ca 0.025M) in a glass-stoppered conical flask with concentrated hydrochloric acid to give a solution in 4M hydrochloric acid. Displace the air by introducing two 0.4 g portions of pure sodium hydrogencarbonate into the flask. Add 1.0 g of pure potassium iodide, replace the stopper, mix the solution, and allow to stand for at least 5 minutes. Titrate the solution, whilst stirring vigorously, with standard 0.1M sodium thiosulphate. 

Alternatively, in this and all subsequent titrations with 0.025M potassium iodate, a 250 mL conical flask may be used and the carbon tetrachloride or chloroform indicator replaced by 0.5 mL amaranth or xylidine ponceau indicator, which is added after most of the iodine colour has disappeared from the reaction mixture (see Section 10.125). 

Thus in the reaction between A+ ions and D ions, the A + ions are replaced by C+ ions during the titration. As the titration proceeds the conductance increases or decreases, depending upon whether the conductivity of the C + ions is greater or less than that of the A + ions. 

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