Standards Standard solutions

Preparation of Standards. Standard solutions were proposed by diluting certified standards (1000 ppm, Fisher Scientific Co.) with nitric acid (3% Ultrex). These solutions were prepared when required and were not stored. 

Anhydrous sodium carbonate is an example of a primary standard. Standard solutions of primary standards can be used to standardize other solutions, as you... 

Standard solutions and gas mixtures are also available for many single- and multisubstance techniques for example, drug assay standards, standard solutions of metals in single- or multielement form, and carbon monoxide and nitrogen dioxide in air. 

Molecular Weight of an Acid by Titration with Standard Alkali Solution. 

The excess of unchanged acetic anhydride is then hydrolysed by the addition of water, and the total free acetic acid estimated by titration with standard NaOH solution. Simultaneously a control experiment is performed identical with the above except that the alcohol is omitted. The difference in the volumes of NaOH solution required in the two experiments is equivalent to the difference in the amount of acetic add formed, i.e., to the acetic acid used in the actual acetylation. If the molecular weight of the alcohol is known, the number of hydroxyl groups can then be calculated. 

C.HsNH, + CHjCO-O-COCHa = C.R NHCOCHs -h CHjCOOH then hydrolysed with water and the total free acetic acid estimated by titration with standard NaOH solution, the result being compared with that obtained in a control or blank experiment. 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

Method. An amino-acid such as glycine, NHjCH,COOH, cannot be estimated by direct titration with standard alkali solution, owing to the opposing effects of the basic and the acidic groups. If, however, the amino-acid is first... 

Let us first consider the standard Galerkin solution of Equation (2.80) obtained using the previously described steps. 

Hydrazine hydrate may be titrated with standard acid using methyl orange as indicator or, alternatively, against standard iodine solution with starch as indicator. In the latter case about 0-1 g., accurately weighed, of the hydrazine hydrate solution is diluted with about 100 ml. of water, 2-3 drops of starch indicator added, and immediately before titration 6 g. of sodium bicarbonate are introduced. Rapid titration with iodine gives a satisfactory end point. 

For further details as to the standardisation of the alkali and the storage of standard alkali solutions, see Vogel, A Text Book oj Quantitative Inorganic Analyti t Theory and Practice, Second Edition. 1951, 233 et eeq. (Longmans, Green and Oo. Ltd.). 

Table 8.33 Standard Solutions for Calibrating Conductivity Vessels 8.160...

Basic substances for standardizing acidic solutions continued)... 

Iodine, Q.IN (0 to 1 —). Dissolve 12.690 g of resublimed iodine in 25 mL of a solution containing 15 g of KI which is free from iodate. After all the solid has dissolved, dilute to 1 L. If desired, check against a standard arsenite or standard thiosulfate solution. 

Make solution alkaline with NaHC03 and titrate excess I2 with standard arsenite solution. HPH2O2/4 = 16.499... 

S H2S + F(excess) = S + 2 1 + 2 Back-titrate excess F with standard thiosulfate solution. 

H2S + 4 Br2 + 4 H2O = SO + 8 Br + 10 H" Use excess KBr and standard KBr03 solution. Fet stand until clear, add excess KI, and titrate with standard thiosulfate solution. 

Table 11.30 lists standard solutions for precipitation titrations and Table 11.31 lists specific reagents as indicators, adsorption indicators, and protective colloids for precipitation titrations. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Standard EDTA Solutions. Disodium dihydrogen ethylenediaminetetraacetate dihydrate is available commercially of analytical reagent purity. After drying at 80°C for at least 24 hr, its composition agrees exactly with the dihydrate formula (molecular weight 372.25). It may be weighed directly. If an additional check on the concentration is required, it may be standardized by titration with nearly neutralized zinc chloride or zinc sulfate solution. 

Standard Magnesium Solution. Dissolve 24.647 g of magnesium sulfate heptahy-drate in water and dilute to 1 E for O.lAf solution. 

Standard Zinc Solution. Dissolve exactly 13.629 g of zinc chloride, ACS reagent grade, or 28.754 g of zinc sulfate heptahydrate, and dilute to 1 E for O.IOOOM solution. 

The list given below includes the substances that are most used and most useful for the standardization of solutions for precipitation titrations. Primary standard solutions are denoted by the letter (P) in Column 1. 

---