External indicator

To ensure the presence of a slight excels of nitrous acid, potassium iodide-starch paper is sometimes used as an external indicator, a drop of the solution being removed from time to time during the addition of the sodium nitrite, and then dropped on to the paper. When an excess of nitrous acid is present, iodine is liberated, and gives the familiar... 

The experimental conditions necessary for the preparation of a solution of a diazonium salt, diazotisation of a primary amine, are as follows. The amine is dissolved in a suitable volume of water containing 2 5-3 equivalents of hydrochloric acid (or of sulphuric acid) by the application of heat if necessary, and the solution is cooled in ice when the amine hydrochloride (or sulphate) usually crystallises. The temperature is maintained at 0-5°, an aqueous solution of sodium nitrite is added portion-wise until, after allowing 3-4 minutes for reaction, the solution gives an immediate positive test for excess of nitrous acid with an external indicator—moist potassium iodide - starch paper f ... 

Indicators can be utilized to distinguish packages that have been processed from those that have not been processed. These are external indicators that do not have the capabiUty to detect critical shortcomings in cycle parameters because they are not located inside the packages. A weU-known example of this type is autoclave tape, which is also used to hold together packages wrapped in muslin or other kinds of wrap-type packaging materials (qv). 

Bromothymol blue or commercial universal indicator pH paper (graduated in 0.2-pH units) may be used as external indicators. 

The green colour due to the Cr3+ ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include AT-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution) the latter must be used in presence of phosphoric) V) acid. 

Ethylenedioxybis ethyliminodi( acetic acid)] see EGTA Evolution methods 444 External indicators 375 Extinction see Absorbance Extinction coefficient 649 molar, 649 specific, 649... 

The range of a meter can be increased by the use of floats of different densities, a given float covering a flowrate range of about 10 1. For high pressure work the glass tube is replaced by a metal tube. When a metal tube is used or when the liquid is very dark or dirty an external indicator is required. 

In other words, the small excess of HN02 present at the end-point can be detected visually by employing either starch-iodide paper or paste as an external indicator. Thus, the liberated iodine reacts with starch to form a blue green colour which is a very sensitive reaction. Besides, the end-point may also be accomplished electrometrically by adopting the dead-stop end-point technique, using a pair of platinum electrodes immersed in the titration liquid. 

Diethyl phenyl ethyl malonate. 1 mole of benzyl cyanide is added dropwise to a solution of 1 mole of ethyl carbonate in 2 liters of anhydrous ethanol containing 5 g of clean sodium metal. This mixture is refluxed (preferably on a steam bath) for 5 hours. It is then cooled and to it is added a cooled mixture of 40 g of sulfuric acid in 100 ml of anhydrous ethanol. This alcoholic solution is refluxed for 5 hours, cooled, neutralized with sodium ethylate (use external indicator). The mixture is evaporated to half bulk, filtered from the sodium sulphate and to it is added 1 mole of clean metallic sodium. Reflux while adding 1 mole of ethyl bromide dropwise. Heat for another 2 hours after the addition is completed. Remove the alcohol by distillation and dissolve the remaining residue in water. Extract the substance from the water with benzene and after drying, the benzene is recovered and the ester should be purified by distilling in vacuo. 

If U is obtained exactly, the effective Hamiltonian H has no matrix elements between the active and external spaces—that is, the two spaces are completely decoupled—and all matrix elements containing both active and external indices... 

In the absence of the halogens the selenium may be oxidised to selenious acid by heating with nitric acid in a Carius tube at 240° to 300° C. The resulting liquid is nearly neutralised, excess of zinc oxide added, and the mixture titrated with silver nitrate using chromate as external indicator 3... 

The titration is then repeated, all but 2 c.cs. of the volume of glucose solution used in the first determination being run in at once, and the remainder in drops until the blue colour just vanishes. The end point is more easily observed when the dish is slightly tilted. Several determinations are made until concordant results are obtained. If the end point is indistinct, a dilute acetic acid solution of potassium ferrocyanide spotted on a white plate may be used as external indicator. A brown coloration is observed so long as copper is present in solution. 

The corresponding sodium salts, which are cheaper, serve equally well. Excess of potassium bromide and hydrochloric acid is added to a solution of the substance to be estimated and the bromate, in the form of a volumetric solution (usually N/5), is run in. The end point (shown by the presence of free bromine) is detectable, quite sharply, by spotting on starch-iodide paper as external indicator. Alternatively the end point may be determined by adding a known amount of bromate in excess, then potassium iodide, and titrating the liberated iodine with thiosulphate. 

Oxidation of toluene-o-sulphonamide to saccharin. In a 600-ml beaker, mounted on an electric hot plate and provided with a mechanical stirrer, place 12 g (0.07 mol) of toluene-o-sulphonamide, 200 ml of water and 3g of pure sodium hydroxide. Stir the mixture and warm to 34-40 °C until nearly all has passed into solution (about 30 minutes). Introduce 19g (0.32 mol) of finely powdered potassium permanganate in small portions at intervals of 10-15 minutes into the well-stirred liquid. At first the permanganate is rapidly reduced, but towards the end of the reaction complete reduction of the permanganate is not attained. The addition occupies 4 hours. Continue the stirring for a further 2-3 hours, and then allow the mixture to stand overnight. Filter off the precipitated manganese dioxide at the pump and decolourise the filtrate by the addition of a little sodium metabisulphite solution. Exactly neutralise the solution with dilute hydrochloric acid (use methyl orange or methyl red as external indicator). Filter off any o-sulphonamidobenzoic acid (and/or toluene-o-sulphonamide) which separates at this point. Treat the filtrate with concentrated hydrochloric acid until the precipitation of the saccharin is complete. Cool, filter at the pump and wash with a little cold water. Recrystallise from hot water. The yield of pure saccharin, m.p. 228 °C, is 7.5 g (58%). 

When different laboratories obtain different values this can point to a real difference in value of the measu-rands, or it can be caused by variations in the measurement parameters of the method during the measurement process (for homogeneous samples). But differences, observed by different parties, do not, in themselves, provide proof of correctness . This requires an independent, external indicator or criterion, or correction, which enables one to convert the demonstrated interlaboratory reproducibility (a form of precision ) into accuracy . Failing to show proof of this makes it impossible to present a statement of accuracy . 

Procedure Unless otherwise directed, dissolve the specified amount of the test substance in 30 to 40 mL of water neutralize to litmus external indicator with nitric acid, if necessary and add 1 mL in excess. Add 1 mL of silver nitrate TS to the clear solution or filtrate, dilute to 50 mL with water, mix, and allow to stand for 5 min protected from direct sunlight. Compare the turbidity, if any, with that produced similarly in a control solution containing the required volume of Standard Chloride Solution and the quantities of the reagents used for the sample. 

Volumetric estimation4 in neutral solution can be effected by titration with standard sodium chloride, potassium chromate being employed as indicator and in nitric-acid solution with thiocyanate, using ferric alum as indicator, or with sodium chloride without any external indicator. 

An example of tests for chirality is provided by studies of mandelic acid crystals (Figure 14.23). These crystals are polar and noncentrosymmetric, space group T 2i, but no hemihedral faees develop and therefore there are no external indications that allow one to distinguish the two ends of its polar axis b. Other techniques have to be used. In order to differentiate between the two ends of the hexagonally-shaped crystals (which were shown, by X-ray diffraction studies to have the c axis along the unique axis of the crystal). The directions of the a and b axes with respect to crystal habit were also established by X-ray diffraction studies. 

We have implicitly used the permutational symmetry of the T matrix elements in writing Eqs. (4), (81), and (82). In particular, the restrictions on the external indices are a direct consequence of there being only one unique T matrix element for a given set of indices. 

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