Reagent impurities

In the analysis of metals in petroleum and petroleum products one of the most common sample preparation procedures is the dilution of the sample with an organic solvent such as xylene, methyl isobutyl ketone (MIBK) or white spirit. It is of great importance that the solvent system chosen is as free as possible from metallic contamination. Elements such as sodium and zinc are commonly found in many organic solvents. Similarly, other reagents such as mineral acids must be investigated for metal content before use. Where ultra-trace level determinations are to be attempted the reagents used may need to be purified. For solvents, the use of redistillation or extraction with mineral acid may improve the blank levels. 

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The halogen-containing alkylsilyl derivatives have never enjoyed extensive popularity for trace analysis. There are several probable reasons for this modest sensitivity difficulties in c x)veniently separating derivatives from excess reagent Interferences in the separation of low molecular weight analytes by reagent impurities and reaction byproducts (particularly the disiloxanes formed by reaction of the reagents with water) and poor hydrolytic stat Ky of some derivatives. In the case of the... 

Errors in the analytical laboratory are basically of two types determinate errors and indeterminate errors. Determinate errors, also called systematic errors, are errors that were known to have occurred, or at least were determined later to have occurred, in the course of the lab work. They may arise from avoidable sources, such as contamination, wrongly calibrated instruments, reagent impurities, instrumental malfunctions, poor sampling techniques, errors in calculations, etc. Results from laboratory work in which avoidable determinate errors are known to have occurred must be rejected or, if the error was a calculation error, recalculated. 

The homogeneity problem is specific to solid samples, as liquid and gaseous samples are considered homogeneous by nature. Thus, for solid samples, an automatic sampling process is recommended to obtain reliable analytical information. To select the most adequate system to obtain a homogeneous sample it is necessary to take into account the sample complexity and the stability of the sample within a certain period of time.37 Further, it is necessary to establish first the nature of analysis that will be used in sample control, especially when the limits of detection are low. One must also be wary of the many contamination risks from reagent impurities, laboratory vessels, laboratory climate, and the operator. 

Process models are also important components of reactor control schemes. Kiparissides et al. [17] and Penlidis et al. [16] have used reactor models for control simulation studies. Particle number and size characteristics are the most difficult latex properties to control. Particle nucleation can be very rapid and a strong function of the concentration of free emulsifier, electrolytes and various possible reagent impurities. Hence the control of particle number and the related particle surface areas can be a difficult problem. Even with on-line light scattering, chromatographic [18], surface tension and/or conversion measurements [19], control of nucleation in a CSTR system can be difficult. The use of a pre-made seed or an upstream tubular reactor can be utilized to avoid nucleation in the CSTR and thereby imjHOve particle number control as well as increase the number of particles formed [20-22]. Figures 8.6 and 8.7 illustrate open-loop CTSR systems for the emulsion polymerization of methyl methacrylate with and... 

A radiochemical separation has three important advantages compared with a common chemical separation (1) Inactive carriers can be added for the elements to be separated (B and D). This avoids the difficulties of a chemical separation at the trace level. (2) Reagent impurities (or blanks) do not influence the detection limit capabilities of the analytical method. (3) Separations may not be quantitative and even not reproducible (see below). 

The prime concerns in determining copper concentration are contamination and loss. The potential sources of contamination are the impurities in reagents, impurities on laboratory apparatus that the sample touches, and the dust in the laboratory. Consequently, particular attention should be given to the collection and treatment of the sample prior to analysis. The sample bottle should be thoroughly washed with detergent and tap water next it should be successively rinsed with 10% hydrochloric or nitric acid and then three times with distilled or demineralized water [26]. 

It was left to the ingenuity of R. Bruce Merrifield, an organic chemist at the Rockefeller University, to develop an innovative solution to this problem that totally revolutionized the field of peptide synthesis. His plan was to assemble the peptide chain in a stepwise manner by adding new amino acids at the A terminus while the Oterminal end was attached to a solid polymeric support of chloromethylated polystyrene, which is now referred to as the Merrifield. In this fashion, all of the excess reagents, impurities, and by-products could be easily removed by washing the resin after each operation, and the pure polypeptide could be cleaved from the solid support as the last step in the synthesis. Merrifield was awarded the Nobel Prize in Chemistry in 1984 in recognition of his contributions to the invention and development of the solid-phase method for the synthesis of peptides. 

Tertiary alcohols react with the reagent immediately, with the separation of alkyl chlorides, the formation of which is indicated first by incipient turbidity and after a while by the separation of a layer. Secondary alcohols react in this manner after 2 — 5 min after 10 min a distinct layer is formed. Primary alcohols do not react imder these conditions, and if they are lower than C, they are soluble in the reagent. Impurities can also be a cause of the formation of turbidity, but they do not produce a layer. Allyl alcohol behaves as a secondary alcohol, because its OH group is activated by the double bond of the neighboring carbon atom. 

Does not reduce ammoniacal silver nitrate or Fehling s solution. If, however, the sucrose solution is warmed for some time with the reagent in question, slight hydrolysis to glucose and fructose does take place and reduction then occurs occasionally samples of sucrose will rapidly give a silver mirror, presumably owing to impurities. 

This type of extraction depends upon the use of a reagent which reacts chemically with the compound to be extracted, and is generally employed either to remove small amounts of impurities in an organic compound or to separate the components of a mixture. Examples of such reagents include dilute (5 per cent.) aqueous sodium or potassium hydroxide solution, 5 or 10 per cent, sodium carbonate solution, saturated sodium bicarbonate solution (ca. 5 per cent.), dilute hydrochloric or sulphuric acid, and concentrated sulphuric acid. 

Nitrobenzene. Nitrobenzene, of analytical reagent quality, is satisfactory for most purposes. The technical product may contain dinitrobenzene and other impurities, whilst the recovered solvent may be contaminated with aniline. Most of the impurities may be removed by steam distillation after the addition of dilute sulphuric acid the nitrobenzene in the distillate is separated, dried with calcium chloride and distilled. The pure substance has b.p. 210°/760 mm. and m.p. 5 -7°. 

To 5 ml. of water add 1-2 drops of the amine if the amine does not dissolve, add a drop or two of concentrated hydrochloric acid. Add 0-5-1 ml. of this amine solution to 2-3 ml. of the reagent an almost immediate precipitate indicates the presence of a primary amine. A slight turbidity indicates the presence of a primary amine as an impurity. (Primary aromatic amines generally require 2-3 minutes for the test. Urea and other amides, as well as amino acids, do not react.)... 

Examples of typical packaging labels from reagent grade chemicals. Label (a) provides the actual lot assay for the reagent as determined by the manufacturer. Note that potassium has been flagged with an asterisk ( ) because its assay exceeds the maximum limit established by the American Chemical Society (ACS). Label (b) does not provide assayed values, but indicates that the reagent meets the specifications of the ACS for the listed impurities. An assay for the reagent also is provided. 

Some substances under El conditions fragment so readily that either no molecular ions survive or so few survive that it is difficult to be sure that the ones observed do not represent some impurity. Therefore, there is either no molecular mass information or it is uncertain. Under Cl conditions, very little fragmentation occurs and, depending on the reagent gas, ions [M + X]+ (X = H, NH4, NO, etc.) or [M - H] or [M - H]" or [M -1- X] (X = F, Cl, OH, O, etc.) are the abundant quasi-molecular ions, which do give molecular mass information. 

The parameter r continues to measure the ratio of the number of A and B groups the factor 2 enters since the monofunctional reagent has the same effect on the degree of polymerization as a difunctional molecule with two B groups and, hence, is doubly effective compared to the latter. With this modification taken into account, Eq. (5.40) enables us to quantitatively evaluate the effect of stoichiometric imbalance or monofunctional reagents, whether these are intentionally introduced to regulate or whether they arise from impurities or side reactions. 

Although the book on reagent chemicals contains many tests for the determination of trace impurities in reagents, it is not intended to be a text on the techniques of trace analysis but rather to provide tests that are reproducible in various laboratories, and which are accurate, economic, and feasible (see... 

The 30% reagent-grade hydrogen peroxide is purer than the industrial grades, is covered by ACS reagent specification, and is used as a laboratory reagent and in some specialty uses (see Fine chemicals). Several grades are also marketed for electronics use and thus have exceptionally low impurity levels. Some of these latter contain very Httie or no stabilizers (see Electronic materials). 

Specifications and Standards, Shipping. Commercial iodine has a minimum purity of 99.8%. The Committee of Analytical reagents of the American Chemical Society (67) and the U.S. Pharmacopoeia XXII (68) specify an iodine content not less than 99.8%, a maximum nonvolatile residue of 0.01%, and chlorine—bromine (expressed as chlorine) of 0.005% (ACS) and 0.028% (USP), respectively. In the past these requirements were attained basicaHy only by sublimation, but with processing changes these specifications can be met by direct production of iodine. Previously the impurities of the Chilean product were chiefly water, sulfuric acid, and insoluble materials. Improvements in the production process, and especiaHy in the refining step, aHow the direct obtainment of ACS-type iodine. Also, because of its origin and production process, the Chilean iodine has a chlorine—bromine impurity level of no more than 0.002%. 

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