Preparation of Standard Acid

Hydrochloric acid is the most frequently used titrant in analytical acid-base work. According to Kolthoff and Stenger, 0.1 Af solutions of hydrochloric acid can be boiled for 1 h without loss of acid if the evaporated water is replaced. Even 0.5 M hydrochloric acid can be boiled for 10 min without appreciable loss. Sulfuric acid has the disadvantage of a relatively weak second step of ionization (pAT 2.0). Moreover, a number of metallic and basic sulfates are sparingly soluble. Nitric acid is relatively unstable, though useful in special procedures such as the alkalimetric method for phosphorus. 

Although the determinate preparation of standard acids is possible, the use of a standardization method is usual practice. 

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This book is intended as a practical manual for chemists, biologists and others whose work requires the use of pH or metal-ion buffers. Much information on buffers is scattered throughout the literature and it has been our endeavour to select data and instructions hkely to be helpful in the choice of suitable buffer substances and for the preparation of appropriate solutions. For details of pH measurement and the preparation of standard acid and alkali solutions the reader is referred to a companion volume, A. Albert and E. P. Serjeant s The Determination of Ionization Constants (1971). 

Hydrochloric acid and sulphuric acid are widely employed in the preparation of standard solutions of acids. Both of these are commercially available as concentrated solutions concentrated hydrochloric acid is about 10.5- 12M, and concentrated sulphuric acid is about 18M. By suitable dilution, solutions of any desired approximate concentration may be readily prepared. Hydrochloric acid is generally preferred, since most chlorides are soluble in water. Sulphuric acid forms insoluble salts with calcium and barium hydroxides for titration of hot liquids or for determinations which require boiling for some time with excess of acid, standard sulphuric acid is, however, preferable. Nitric acid is rarely employed, because it almost invariably contains a little nitrous acid, which has a destructive action upon many indicators. 

For the preparation of standard cobalt solutions, use analytical grade cobalt(II) chloride or spectroscopically pure cobalt dissolved in hydrochloric acid subject solutions containing 0, 5, 10, 25, 50, 100, 150, and 200 jug of Co to the whole procedure. 

Preparation of standard titanium solution. Weigh out 3.68 g potassium titanyl oxalate K2Ti0(C204)2,2H20 into a Kjeldahl flask add 8g ammonium sulphate and 100 mL concentrated sulphuric acid. Gradually heat the mixture to boiling and boil for 10 minutes. Cool, pour the solution into 750 mL of water, and dilute to 1 L in a graduated flask 1 mL = 0.50 mg of Ti. 

The standard directions for the preparation of phenylacetic acid specify that the benzyl cyanide is to be treated with dilute sulfuric acid prepared by adding three volumes of sulfuric acid to two volumes of water. There action, however, goes so vigorously that it is always necessary to have a trap for collecting the benzyl cyanide which is blown out of the apparatus. The use of the more dilute acid, as described in the above directions, is more satisfactory. 

The standard method of preparation of phenylacetic acid is by the hydrolysis of benzyl cyanide with either alkali1 or acid.2 The acid hydrolysis runs by far the more smoothly and so was the only one studied. There are numerous other ways in which phenylacetic acid has been formed, but none of them is of practical importance for its preparation. These methods include the following the action of water on phenyl ketene 3 the... 

The condensation of 4-aminopyridine (145) with diethyl ethoxymethylenemalonate (EMME) gives 3-ethoxycarbonyl-l,6-naphthyridin-4-one (146) from which compound (2) was obtained by standard procedures (50JOC1224,60JCS1790,58LA(612)153,65JHC393). A recent application of this synthesis is for the preparation of nalidixic acid analogues (147) (82CPB2399). 

One feature of NIOSH method descriptions in general is an elaborate exercise to prepare a primary standard. This disincentive to setting up particular methods for occasional needs can be obviated if, as may have to be verified, bought-in ampules of standard acid are trustworthy. 

Preparation of Standards. Standards for ash analysis were prepared from commercially available pure salts in aqueous solution with appropriate acids addition where necessary to match acid concentrations in the samples as well as to hold materials in solution. Master standard solutions were prepared so that serial dilutions for the construction of working curves were possible. A constant amount of silicon and aluminum (equivalent to 20% Si-5% Al) interference solution was added to each set of standards along with lithium tetraborate to carefully match... 

As previously discussed in this chapter, the molecular size of protein or nucleic acid samples may be determined by electrophoresis. This requires the preparation of standard curves of log molecular weight versus fi (mobility) using standard proteins or nucleic acids. 

Preparation of Picric Acid (Standard Method). Twenty-five grams of phenol and 25 grams of concentrated sulfuric acid (d. 1.84) in a round-... 

The standard procedure is illustrated by the preparation of cinnamic acid and furylacrylic acid (Expt 6.138). The cinnamic acid obtained is the more stable ( )-isomer. It may be readily reduced to the saturated acid (3-phenylpropanoicacid) and two procedures are described. Catalytic hydrogenation is a convenient method, but the conjugated double bond may also be reduced with, for example, sodium amalgam in the presence of alkali. 

ISO (2002b). Milk Fat—Preparation of Fatty Acid Methyl Esters (Standard ISO 15884/IDF 182). International Organization for Standardization, Geneva. 

The calibration blank in trace element analysis is prepared by acidifying reagent water with the same concentrations of the acids used in the preparation of standards and samples. It serves as a calibration point in the initial calibration. As part of an analytical batch, the calibration blank is analyzed frequently to flush the analytical system between standards and samples in order to eradicate memory effects. Calibration blanks are also used in inorganic compound analysis, where they are prepared with the chemicals specified by the method. 

It is very important to run at least duplicate blanks with each batch of digestions, especially for trace element determinations. This allows correction to be made for contamination from apparatus and/or reagents. The loss of acid is small during the digestion if electrical heating is used, so that it is not essential to use digestion blanks for preparation of standards for flame spectrometric analysis. The matrix matching (see Chapter 3) is adequate if appropriate amounts of sulfuric acid and of the 4% perchloric acid in sulfuric acid solution are added to all standards. 

The methods described above have been used principally to quantify FFAs in cheese, but can be used for other milk products with some slight modifications. All the above methods use internal standards (typically FFAs which are not present in milk fat), and the recovery of all FFAs is based on the recovery of these internal standards. It is best to use both volatile and non-volatile FFAs as internal standards. Currently, the International Standard for the extraction of lipids and lipo-soluble compounds from milk and milk products is ISO 14156 (ISO, 2001) and involves solvent extraction. Determination of the fatty acid composition of milk fat involves the preparation of fatty acid methyl esters (FAME) by transesterification (ISO, 2002a), followed by quantification by GC (ISO, 2002b). 

Preparation of Standard Solutions Prepare a standard solution of each of the organic compounds to be quantitated in Hydrochloric Acid (known to be free of interfering impurities) at approximate concentrations of 5 mg/kg, or within 50% of the concentrations in the samples to be analyzed. 

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... 

Determination of iron The most important applications of dichromate involve either directly or indirectly the titration of Fe(II). An excess of standard Fe(II) can be added to determine oxidants, or an excess of Fe(III) to determine reductants. These determinations usually can be carried out equally well with Ce(IV). For routine applications, however, the low cost and ease of preparation of standard solutions and the great stability of dichromate offer some advantages. Permanganate is at a disadvantage, expecially if hydrochloric acid solutions are to be used. 

A method for the simple synthesis of phosphonothioates (310) or phospho-nothioic acids (311) has been reported. It uses standard reagents and should be applicable to the preparation of phosphonothioic acids bearing a range of functional groups (Scheme 81). ... 

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