Titration forms back titrations

Three general methods for performing EDTA titrations are (1) direct titration, (2) back titration, and (3) displacement titration. Method (1) is simple, rapid, but requires one standard reagent. Method (2) is advantageous for those metals that react so slowly with EDTA as to make direct titration inconvenient. In addition, this procedure is useful for cations for which satisfactory indicators are not available. Finally, it is useful for analyzing samples that contain anions that form sparingly soluble precipitates with the analyte under analytical conditions. Method (3) is particularly useful in situations where no satisfactory indicators are available for direct titration. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

One of the most important applications of redox titrimetry is in evaluating the chlorination of public water supplies. In Method 9.3 an approach for determining the total chlorine residual was described in which the oxidizing power of chlorine is used to oxidize R to 13 . The amount of 13 formed is determined by a back titration with 8203 . 

The %w/w K in a 0.6712-g sample was determined by a Volhard titration. After adding 50.00 mb of 0.05619 M AgNOa and allowing the precipitate to form, the remaining silver was back titrated with 0.05322 M KSCN, requiring 35.14 mb to reach the end point. Report the %w/w K in the sample. 

Animal fats and vegetable oils are triacylglycerols, or triesters, formed from the reaction of glycerol (1,2, 3-propanetriol) with three long-chain fatty acids. One of the methods used to characterize a fat or an oil is a determination of its saponification number. When treated with boiling aqueous KOH, an ester is saponified into the parent alcohol and fatty acids (as carboxylate ions). The saponification number is the number of milligrams of KOH required to saponify 1.000 g of the fat or oil. In a typical analysis, a 2.085-g sample of butter is added to 25.00 ml of 0.5131 M KOH. After saponification is complete, the excess KOH is back titrated with 10.26 ml of0.5000 M HCl. What is the saponification number for this sample of butter ... 

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

B. Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

In the back-titration small amounts of copper and zinc and trace amounts of manganese are quantitatively displaced from the EDTA and are complexed by the triethanolamine small quantities of cobalt are converted into a triethanolamine complex during the titration. Relatively high concentrations of copper can be masked in the alkaline medium by the addition of thioglycollic acid until colourless. Manganese, if present in quantities of more than 1 mg, may be oxidised by air and forms a manganese(III)-triethanolamine complex, which is intensely green in colour this does not occur if a little hydroxylammonium chloride solution is added. 

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. 

In this work, halogens were added across the double bonds of unsaturated fatty acids (commercial available sunflower oil) to form addition compounds and the degree of uptake was measured. A measured excess of iodine monobrontide (IBr) was allowed to react with the oil in the dark. At the end of a timed period, an excess of K1 was added to convert the remaining IBr to The 1 formed was then titrated with standard thiosulfate. This was a back-titration method. A back-titration blank was also required, where the sample was omitted. The uptake of iodine was calculated... 

Phosphorus is a common component of additives and appears most commonly as a zinc dialkyl dithiophosphate or triaryl phosphate ester, but other forms also occur. Two wet chemical methods are available, one of which (ASTM D1091) describes an oxidation procedure that converts phosphorus to aqueous orthophosphate anion. This is then determined by mass as magnesium pyrophosphate or photochemically as molybdivanadophosphoric acid. In an alternative test (ASTM D4047), samples are oxidized to phosphate with zinc oxide, dissolved in acid, precipitated as quinoline phosphomolybdate, treated with excess standard alkali, and back-titrated with standard acid. Both of these methods are used primarily for referee samples. Phosphorus is most commonly determined using x-ray fluorescence (ASTM D4927) or ICP (ASTM D4951). 

Shake flasks were inoculated with mixed liquor suspended solids from activated sludge units in a Houston area domestic waste sewage treatment plant. Initial surfactant concentrations were 20 mg/ . CO2 formed from biodegradation was trapped in aqueous Ba(OH)2. The amount of CO2 formed was determined by back-titrating residual Ba(OH)2 with HCl at the end of each test period. Glucose was included as a positive biodegradation standard. 

Insoluble metal salts are estimated by back titration the sample is heated with excess EDTA to form the soluble EDTA complex of the metal and then the excess EDTA is titrated with salt solutions containing Mg + or Zn- of known concentration. 

This reaction may be used to determine fulminate quantitatively by back titration of the sodium hydroxide formed. It can also be used to destroy fulminate residues and waste material. The impurities in mercury fulminate (oxalate and nitrate) are insoluble in thiosulphate. 

Procedure for Ammonium Nitrate. Transfer a 25-ml aliquot from the bottle of step 6 to 100 ml of the neutralized form aldehyde-water soln in a 400-ml beaker, add 4 drops of 1% phenolphthalein (or thymolphthalein) indicator and titrate to a pink color with. 0.5N NaOH. Add 2ml of NaOH in excess, cover the beaker, and let stand for 5 mins. Back-titrate with 0.1N HCl to pH 8.5 (glass-calomel electrode system)... 

If iron is present, a ferric thiocyanate complex will form and the soln will change from light blue to deep red. Back-titrate the ferric thiocyanate complex with 0.2N TiCla soln from the same buret untii the light blue color reappears. Determine the mis of Ti Cl3 soln... 

The PMMA/anhydride copolymer produced in sample 1 was titrated as previously described, and showed 3.57 0.07 mmol/g of titratable acidic functionality. Under these titration conditions all anhydride is hydrolyzed to carboxylate and back titrated to carboxylic acid. Both NMR and IR analysis show about 27 weight % acidic functionality which is almost exclusively in the anhydride form. The NMR and IR measurements predict 3.53 and 3.48 mmol/g acidity, in good agreement with the measured titration value of 3.57 0.07 mmol/g. 

Chloride is analyzed by some form of reaction with silver to form insoluble silver chloride. Direct titration of milk with silver nitrate yields erroneously high and variable results, and pre-ashing cannot be used because chloride is lost by volatilization. Satisfactory procedures involve adding an excess of standardized AgN03 directly to milk and back titrating with potassium thiocyanate (KSCN), using a soluble ferric salt as the indicator (Sanders 1939). 

In the back-titration method, a measured amount of an excess standard EDTA solution is added to the sample. The analyte ion combines with EDTA. After the reaction is complete, the excess EDTA is back-titrated against a standard solution of magnesium or zinc ion. Eriochrome Black T or Calmagite is commonly used as an indicator. After all the remaining EDTA chelates with Mg2+ or Zn2+, ary extra drop of the titrant solution imparts color to the indicator signifying the end point. The cations that form stable complexes or react slowly with EDTA can also be measured by the back-titration method. 

RN is a base (e.g., pyridine), R R are alkyl groups, and I2 is generated separately in a different compartment. Hence, in the presence of water, RNHS03R is oxidized by the iodine. The amount of water is determined by a coulombic titration of the 1 formed, back to iodine ... 

The most important determination is normally the concentration of carbon-magnesium-bonded species in solution. For routine estimation of this concentration for freshly prepared solutions of organomagnesium compounds, an aliquot of the test solution may be added to an excess of standard acid, and then back-titrated with sodium hydroxide. However, this simple determination of total base will give a high estimate of organomagnesium content if products of hydrolysis or oxidation are present. Analytical methods based on the determination of the hydrocarbon formed on hydrolysis of the organomagnesium compound... 

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