Secondary standard solutions

The IUPAC Commission on Analytical Nomenclature refers to (3) and (4) respectively as Primary Standard Solutions and Secondary Standard Solutions. 

A secondary standard is a substance which may be used for standardisations, and whose content of the active substance has been found by comparison against a primary standard. It follows that a secondary standard solution is a solution in which the concentration of dissolved solute has not been determined from the weight of the compound dissolved but by reaction (titration) of a volume of the solution against a measured volume of a primary standard solution. 

A series of calibration standard solutions are prepared from this secondary standard solution as seen in Table 2.32.1. 

The secondary standard solution of NaOH of Exercise 30 was used to titrate a solution of unknown concentration of HCl. A 30.00-mL sample of the HCl solution required 34.21 mL of the NaOH solution for complete neutrahza-tion. What is the molarity of the HCl solution ... 

This is the process of determining the volume of a substance (usually of a primary standard solution or a standardised secondary standard solution) required to just complete the reaction... 

Secondary standard solutions are those which cannot be prepared by direct weighing in a chemical balance. These are usually prepared by standardising against some primary standard solution. Examples are solution of NaOH, KOH, HCl, HgSO, KMnO. ... 

To prepare primary standard solutions, weigh out separately 100 mg of each alkaloid (nicotine, 4-ethenylpyridine, etc.) into individual 100-ml volumetric flanks and dilute to volume with solvent. To prepare secondary standard solutions, pipet 1ml of nicotine primary standard into 100-ml volumetric flask. Also, pipet 0.50ml of 4-ethenylpyridine into the same volumetric flask (along with appropriate volume of any additional alkaloid primary solutions) and dilute with solvent. 

Transfer the primary section of XAD-4 resin together with the associated glass wool plugs from 20 sample tubes to 20 2-ml vials. Spike five vials with 10-pl portions of alkaloid secondary standard solution, five vials with 20-pl portions, and five vials with 50-pl portions. The remaining five vials are blanks. Cap and store all vials in the same manner in which samples are handled. Uncap, add quinoline (internal standard) solution, desorb with solvent, and analyze all spiked samples and blanks as described above. Correct the results obtained on spiked samples by subtracting the blank average from the average for each spiked level. Calculate desorption efficiency (DE) for each alkaloid as follows ... 

In practice, the pH measurements of the so-called secondary standard solutions are often used for calibrating the measuring electrodes. The pH values of these are established in comparison with the primary standards. Their pH values must be consistent with the pH values of the primary standards. The measurements are carried out with the hydrogen electrode and in cells without transference in determination of the pH of the secondary standard solution, if it is possible. The calibrating standards are commercially available, and their pH values at different temperatures are certified by the manufacturer. In Table 7.2, the pH values of a few secondary standard solutions are listed. They were determined with the Harned cell for different temperatures. The table is taken from the report of lUPAC 2002 [35]. 

Table 7.2 pH values of secondary standard solution at different temperature... 

The titrant solution must be standardized before the titration. In other words, its titer must be precisely known. (In certain countries, a standard solution is purely and simply what we called a titrated solution above). The standardization of the titrant solution is achieved by its titration with a standard solution. This can be a primary or secondary (standard) solution. 

As a result of a variable liquid-junction potential, the measured pH may be expected to differ seriously from the determined from cells without a liquid junction in solutions of high acidity or high alkalinity. Merely to affirm the proper functioning of the glass electrode at the extreme ends of the pH scale, two secondary standards are included in Table 8.14. In addition, values for a 0.1 m solution of HCl are given to extend the pH scale up to 275°C [see R. S. Greeley, Anal. Chem. 32 1717 (I960)] ... 

In titrimetry certain chemicals are used frequently in defined concentrations as reference solutions. Such substances are referred to as primary standards or secondary standards. A primary standard is a compound of sufficient purity from which a standard solution can be prepared by direct weighing of a quantity of it, followed by dilution to give a defined volume of solution. The solution produced is then a primary standard solution. A primary standard should satisfy the following requirements. 

The standardisation of thiosulphate solutions may be effected with potassium iodate, potassium dichromate, copper and iodine as primary standards, or with potassium permanganate or cerium)IV) sulphate as secondary standards. Owing to the volatility of iodine and the difficulty of preparation of perfectly pure iodine, this method is not a suitable one for beginners. If, however, a standard solution of iodine (see Sections 10.112 and 10.113) is available, this maybe used for the standardisation of thiosulphate solutions. 

Titrimetric analysis - continued general discussion of, 257 primary and secondary standards for, 261 storage and preservation of solutions, 108 technique of, 286... 

The prepared NaOH solution is a secondary standard because its concentration is determined by titration against a primary standard. 

Secondary Standard is another standard solution that is used for standardization of an unknown solution. 

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 . 

A sample for which the true response is already known or is established is called a standard. A standard can be a primary standard, which is a standard through which other substances or solutions are made to be standards. It can also be a secondary standard, a solution whose concentration is known accurately either because it was prepared using a primary standard or because it was compared to another standard. All standards must ultimately be traced to a standard reference material (SRM). Standard reference materials are available from the National Institute of Standards and Technology (NIST) and should not be used for any other purpose in the laboratory (Section 5.4). Standardization is an experiment in which a solution is compared to a standard in order for itself to be a standard. The solutions used to establish a standard curve are often called reference standards and these must also be traceable to an SRM. 

Part of this chain is formed by the analyst in his/her laboratory (the "end user"), while part of it may be formed between NIST and the vendors. For example, a laboratory analyst can purchase a primary standard acid (which a vendor can certify as traceable to an SRM) for solution standardization and then base a number of secondary standardizations, such as acids and bases, on that one primary standard. Similarly, an analyst can purchase an atomic absorption reference standard (which a vendor can again certify as being traceable to an SRM) and then make one or more dilutions of this reference standard before creating the final series for the standard curve. 

Standard reference material (SRM) for wavelength accuracy, stray light, resolution check, and photometric accuracy can be purchased from NIST. Certified reference materials (CRMs) which are traceable to NIST and recertification services can be purchased from instrument manufacturers and commercial vendors [12]. The cost of neutral-density filters and prefabricated standard solutions in sealed cuvettes can be substantial. When purchasing performance verification standards from a secondary supplier other than a national standard organizations such as NIST in the United States and National Physical Laboratory (NPL) in the United Kingdom, make sure that the traceability of the standards are available in the certificates. The traceability establishes the relationship of individual results to the national standard through an unbroken chain of comparisons. 

The most-used secondary standard is the calomel electrode, shown in Fig. 7.40. It consists basically of a pool of mercuiy on top of which is spread a thin layer of Hg2Cl2 (calomel), a substance only slightly soluble in water. A KC1 solution (either at the unit activity with respect to Cl- or saturated) is in contact with the calomel Hg system and a Pt wire connects the electrode to the rest of the circuit. 

Sorbent Elution. The loaded extraction columns were disconnected and eluted with solvents individually. The elution procedure was as follows First, 1 column bed volume of pesticide-grade pentane was added to the sorbent to displace any residual water. Then, 3 column bed volumes of ethyl ether were added. The eluting solvents were collected in a 2-mL volumetric flask. The eluates from the primary and secondary columns were combined in the volumetric flask. The internal standard solution was added, and the final extract volume was adjusted to 2 mL. Air pressure was again used to push the solvents through the sorbents to avoid volatility losses. 

The most common electrode of this type is the saturated calomel electrode (SCE) which consists of mercury in contact with a layer of insoluble Hg2Cl2 immersed in a saturated aqueous solution of KC1. The SCE is used as a secondary standard reference electrode. At 25°C it has an electrode potential of + 0.2415 V. 

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