Ammonia, ionisation

Liquid ammonia, which boils at 240 K, is an ionising solvent. Salts are less ionised in liquid ammonia than they are in water but, owing to the lower viscosity, the movement of ions through liquid ammonia is much more rapid for a given potential gradient. The ionisation of liquid ammonia... 

Liquid ammonia. This can be prepared by compressing ammonia gas. It has a boiling point of 240 K and is an excellent solvent for many inorganic and organic substances as well as for the alkali metals. Liquid ammonia is slightly ionised. ... 

Note. All values are with reference to the molarity scale. Data for bases are expressed as acidic ionisation constants e.g. for ammonia we quote pK at =9 245 for the ammonium ion... 

The addition of half a mole of ammonium chloride to 1 litre of a 0.1M solution of aqueous ammonia has decreased the degree of ionisation from 1.35 to 0.0036 per cent, and the hydroxide ion concentration from 0.00135 to 0.000 0036 mol L-1. 

Pipette 25 mL of the bismuth solution (approx. 0.01 M) into a 500 mL conical flask and dilute with de-ionised water to about 150 mL. If necessary, adjust the pH to about 1 by the cautious addition of dilute aqueous ammonia or of dilute nitric acid use a pH meter. Add 30 mg of the xylenol orange/potassium nitrate mixture (see Section 10.50) and then titrate with standard 0.01 M EDTA solution until the red colour starts to fade. From this point add the titrant slowly until the end point is reached and the indicator changes to yellow. 

Procedure. Prepare an ammonia-ammonium chloride buffer solution (pH 10), by adding 142 mL concentrated ammonia solution (sp. gr. 0.88-0.90) to 17.5 g ammonium chloride and diluting to 250 mL with de-ionised water. 

Pipette 25 mL of the copper solution (0.01 M) into a conical flask, add 100 mL de-ionised water, 5 mL concentrated ammonia solution and 5 drops of the indicator solution. Titrate with standard EDTA solution (0.01 M) until the colour changes from purple to dark green. 

Pipette 25 mL nickel solution (0.01 M) into a conical flask and dilute to 100mL with de-ionised water. Add the solid indicator mixture (50mg) and 10 mL of the 1M ammonium chloride solution, and then add concentrated ammonia solution dropwise until the pH is about 7 as shown by the yellow colour of the solution. Titrate with standard (0.01 M) EDTA solution until the end point is approached, then render the solution strongly alkaline by the addition of 10 mL of concentrated ammonia solution, and continue the titration until the colour changes from yellow to violet. The pH of the final solution must be 10 at lower pH values an orange-yellow colour develops and more ammonia solution must be added until the colour is clear yellow. Nickel complexes rather slowly with EDTA, and consequently the EDTA solution must be added dropwise near the end point. 

Pipette 25.0 mL of the phosphate solution (approx. 0.05M) into a 250 mL beaker and dilute to 50 mL with de-ionised water add 1 mL of concentrated hydrochloric and a few drops of methyl red indicator. Treat with an excess of 1M magnesium sulphate solution (ca 2mL), heat the solution to boiling, and add concentrated ammonia solution dropwise and with vigorous stirring until the indicator turns yellow, followed by a further 2 mL. Allow to stand for several... 

Strong acid with a weak base. The titration of a strong acid with a moderately weak base (K sslO-5) may be illustrated by the neutralisation of dilute sulphuric acid by dilute ammonia solution [curves 1 and 3, Fig. 13.2(a)]. The first branch of the graph reflects the disappearance of the hydrogen ions during the neutralisation, but after the end point has been reached the graph becomes almost horizontal, since the excess aqueous ammonia is not appreciably ionised in the presence of ammonium sulphate. 

Ammonium citrate. Dissolve 50 g tri-ammonium citrate in 50 mL of concentrated ammonia solution added with care. Cool, and make up to 100 mL with de-ionised water. 

Potassium cyanide. (CAUTION ) Dissolve 25 g of the salt in 35 mL of de-ionised water to which has been added 5 mL of concentrated ammonia solution. Make up to 50 mL with de-ionised water and filter if necessary. 

In order to concentrate the lead extract, remove the lead from the organic solvent by shaking this with three successive 10 mL portions of the dilute hydrochloric acid solution, collecting the aqueous extracts in a 250 mL beaker. To the combined extracts add 5 mL of 20 per cent ascorbic acid solution and adjust to pH 4 by the addition of concentrated ammonia solution. Place the beaker in a fume cupboard, add 3 mL of the 50 per cent potassium cyanide solution and immediately adjust the pH to 9-10 with concentrated ammonia solution. Transfer the solution to a 250 mL separatory funnel with the aid of a little de-ionised water, add 5 mL of the 2 per cent NaDDC reagent, allow to stand for one minute and then add 10 mL of methyl iso butyl ketone. Shake for one minute and then separate and collect the organic phase, filtering it through a fluted filter paper. This solution now contains the lead and is ready for the absorption measurement. 

In the deformulation of PE/additive systems by mass spectrometry, much less fragmentation was observed with DCI-MS/MS using ammonia as a reagent gas, than with FAB-MS [69]. FAB did not detect all the additives in the extracts. The softness and the lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Applications of hyphenated FAB-MS techniques are described elsewhere low-flow LC-MS (Section 7.3.3.2) and CE-MS (Section 7.3.6.1) for polar nonvolatile organics, and TLC-MS (Section 7.3.5.4). 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Table 2.7.2. In (+ )-ionisation mode, apart from the [M + H]+ ion, the sodiated and potassiated ions could be detected. Furthermore, the fragment ion [M + H—H20]+ arising from the loss of one water molecule out of the carbohydrate moiety was formed. No ammonium adduct ion was observed although ammonia was used for pH adjustment of the eluent. In (+)-ionisation mode two fragments were detected, both containing the carbohydrate part of the molecule, [Gluc-NH2-CH3]+ and [(G1uc-H20)-NH2-CH3]+.

A weak base is only partially ionised in aqueous solution. A weak base is made up of molecules. Only some of the molecules are dissociated (ionised) when dissolved in water. Ammonia is a weak base. 

C. Ammonia is a weak base and so is not completely ionised (dissociated). However, an aqueous solution of ammonia is alkaline and so contains more hydroxide ions than hydrogen ions. 

There are a number of other methods for ionising the sample in a mass spectrometer. The most important alternative ionisation method to electron impact is Chemical Ionisation (Cl). In Cl mass spectrometry, an intermediate substance (generally methane or ammonia) is introduced at a higher concentration than that of the substance being investigated. The carrier gas is ionised by electron impact and the substrate is then ionised by collisions with these ions. Cl is a milder ionisation method than El and leads to less fragmentation of the molecular ion. 

Ammonia is 50% ionised at pH 9.25. In this case it is the protonated form of the base that is ionised and as the pH falls the base becomes more ionised. 

For example, in a general screen for acidic drugs, which includes most of the NSAIDs (Fig. 13.5), three mobile phases may be used. Table 13.3 shows the Rf values obtained for three NSAIDs in three different mobile phases. It can be seen from the data in Table 13.3 that even for closely related structures slight differences in polarity and lipophilicity can be exploited to produce separation. For instance, ibuprofen is the least polar drug in system 1 but is the most polar drug in system 3, where the carboxyl groups in the structures will be ionised due to the ammonia in the mobile phase. It can also be seen that the polarity of a mobile phase containing a mixture of chloroform and acetone is similar to that of pure ethyl acetate. 

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