Volumetric primary standards

Table 11.27 Primary Standards for Aqueous Acid-Base Titrations Table 11.28 Titrimetric (Volumetric) Factors...

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

The history of reference materials is closely linked with the development of analytical chemistry. In the 19th Century all chemicals were, in comparison with those of today, of poor purity. Thus, for volumetric analysis suitable purified materials as primary standards had to be specified. One of the first examples was the recommendation of As(III) oxide by Gay-Lussac (1824) for this purpose. Somewhat later, Sorensen (1887) proposed criteria for the selection of primary chemical standards. These were further elaborated by Wagner (1903) at the turn of the last century. It is worthwhile mentioning that their criteria were quite similar to those used today. 

The molarity of the EDTA solution, MEDTA in Equation (5.52), can be known directly through its preparation with the use of an analytical balance and a volumetric flask. That is, one can purchase pure disodium dihydrogen EDTA and use it as a primary standard. In that case, the solution is prepared and the concentration calculated according to the discussion in Chapter 4 (see Section 4.3, especially Example 4.2, and Section 4.4.1). 

Example An unknown solution of HC1 may be standardized volumetrically in two ways, namely (/) by the help of AnalaR -grade NajCOj i.e., purity is known- Primary Standard , and (//) by the help of another standard solution of NaOH— Secondary Standard . 

Ceric ammonium nitrate is used as a volumetric oxidizing reagent in many oxidation-reduction titrations. Cerium(IV) ion is a strong oxidant simdar to permanganate ion. It is the most widely-used primary standard among all Ce(IV) compounds. Other apphcations of this compound are in organic oxidation reactions and as a catalyst in polymerization of olefins. 

Sodium carbonate [497-19-8] M 106.0. Crystd from water as the decahydrate which was redissolved in water to give a near-saturated soln. By bubbling CO2, NaHCO3 was ppted. It was filtered, washed and ignited for 2h at 280° [MacLaren and Swinehart JACS 73 1822 7957]. Before being used as a volumetric standard, analytical grade material should be dried by heating at 260-270° for 0.5h and allowed to cool in a desiccator. For prepcU ation of primary standard sodium carbonate, see PAC 25 459 1969. 

Amidosulphonie acid reacts quantitatively with the alkalis, and with carbonates and borates, and its use as a primary standard in volumetric analysis has been suggested.3... 

The validity of an analytical result depends on knowing the amount of a primary standard. A solution with an approximately desired concentration can be standardized by titrating a primary standard. In a direct titration, titrant is added to analyte until the reaction is complete. In a back titration, a known excess of reagent is added to analyte, and the excess is titrated with a second standard reagent. Calculations of volumetric analysis relate the known moles of titrant to the unknown moles of analyte. 

Standardization of a solution using a primary standard pipet, buret, Erlenmeyer flasks, volumetric flask, wash bottle, analytical balance, drying oven, desiccator, support stand, pH meter... 

Standardization of EDTA Solution Transfer about 1 g of primary standard calcium carbonate (CaC03), accurately weighed, into a 1000-mL volumetric flask, dissolve in 800 mL of water containing 5 mL of hydrochloric acid, dilute to... 

Potassium Dichromate Solution (0.1 N, primary standard) Transfer 4.9032 g of K2Cr207 (National Institute of Standards and Technology No. 136) to a 1-L volumetric flask dissolve in and dilute to volume with water. 

Potassium Acid Phthalate, 0.1 N [20.42 g KIICfin4(COO)2 per 1000 mL] Dissolve 20.42 g of primary standard potassium biphthalate [ KHC6H4(COO)2], previously dried at 105° for 2 h, in glacial acetic acid in a 1000-mL volumetric flask, wanning on a steam bath if necessary to effect solution and protecting the solution from contamination by moisture. Cool to room temperature, dilute to volume with glacial acetic acid, and mix. 

Sodium Methoxide, 0.02 N, in Toluene (1.08 g CH3ONa per 1000 mL) Weigh 2.5 g of freshly cut sodium metal, and cut into small cubes. Place about 200 mL of anhydrous methanol in a 1000-mL volumetric flask, chill in an ice bath, and add the cubes one at a time to the methanol. When the last cube is dissolved, dilute to the mark with toluene, and mix. Standardize the solution as follows Weigh accurately about 20 mg of primary standard benzoic acid, transfer it into a 50-mL conical flask, and dissolve it in 25 mL of dimethylformamide. Add 2 drops of a solution of 100 mg of thymol blue in 10 mL of dimethylformamide, and titrate immediately with the sodium methoxide solution to a blue endpoint. Titrate a blank solution of dimethylformamide in the same manner, correct the volume of sodium methoxide solution consumed by the blank, and calculate the normality. Each 2.442 mg of benzoic acid is equivalent to 1 mL of 0.02 N Sodium Methoxide in Toluene. 

A primary standard is a highly purified compound that serves as a reference material in volumetric and mass titrimetric methods. The accuracy of a method is critically dependent on the properties of this compound. Important requirements for a primary standard are the following ... 

In this section, we describe two types of volumetric calculations. The first involves computing the molarity of solutions that have been standardized against either a primary-standard or another standard solution. The second involves calculating the amount of analyte in a sample from titration data. Both types are based on three algebraic relationships. Two of these are Equations 13-1 and 13-3, both of which are based on millimoles and milliliters. The third relationship is the stoichiometric ratio of the number of millimoles of the analyte to the number of millimoles of titrant. 

However, 0.1 (N) approx NaOH can also be standardised versus 0.1 (N) accurately prepared oxalic acid which is a primary standard using phenolpthalein indicator. Equivalent weight of Oxalic acid (HgCgO. 2H2O) is 63 [63 g in one litre gives 1(N) or 6.3 g in 1 litre gives O.l(N)]. Therefore 1.575 gm is to be weighed accurately in 250 ml volumetric flask and volume made upto the mark with distilled water to prepare 250 ml of O.l(N) oxalic acid solution. Then a known volume say 25 ml of this acid will be taken by pipette plus (1 drop phenolpthalein) and volume of NaOH required to neutralise from burette is noted in ml at the end point when pink colour just appears with one drop of NaOH. 

Standard Ca solution (0.0 IN) primary standard Dissolve 0.2502 g of pure calcium carbonate (analytical grade, dried at 110°C overnight) with minimum quantity of concentrated HCl drop wise (10 ml of 3N HCl may be used). Warm the solution to expel C02 and then dilute to 500 ml in volumetric flask. 

Reagent-grade chemicals are almost always used in analyses. Primary standards are used for preparing volumetric standard solutions. 

---