Volatile acid salts

For mass spectrometry-compatible mobile phases, volatile acid salts are acceptable (i.e., TFA, formic acid, acetic acid). The ammonium counterion works best. 

The scale of preparative HPLC is normally larger than that of conventional HPLC. Therefore, a practical starting point is to develop an analytical separation that optimizes the isolation conditions. Optimization of the analytical method implies seeking conditions that combine maximum resolution of the peak of interest and minimum elution time, under the restriction of a limited pressure drop. The optimized conditions determine the column, mobile phase, flow rate, and sample loading capacity for the particular column. The conditions may be either normal phase or reverse phase. The mobile phase should be chosen carefully to avoid salt complexation with the compound to be isolated. Volatile acid salts such as trifluoroacetic acid, formic acid, and acetic acid are acceptable mobile phase additives, and the ammonium counterion is preferred for pH adjustment to any of these acids. 

An analytical cleanup of the isolated sample is critical before NMR analysis. A clean sample improves the quality of NMR data. As was mentioned earlier, volatile acid salts such as trifluoroacetic acid, formic acid, and acetic acid are often used as mobile phase additives, which may cause salt formation if the pEl of the mobile phase is adjusted (i.e., using ammonium hydroxide). In addition, mobile phase solvents may also contain low-level impurities that become enriched during the concentration process. It is essential to remove any salts and/or impurities from the isolated product. A simple way to purify the isolated product is to reinject it onto the preparative column using a mobile... 

Concentrated sulphuric acid displaces more volatile acids from their salts, for example hydrogen chloride from chlorides (see above) and nitric acid from nitrates. The dilute acid is a good conductor of electricity. It behaves as a strong dibasic acid ... 

Ester of an alcohol Alcohol (volatile) Na salt of a carboxylic acid (non-volatile). 

The following are examples of the above procedure. A mixture of diethylamine and re-butyl alcohol may be separated by adding sufficient dilute sulphuric acid to neutralise the base steam distillation will remove the alcohol. The amine can be recovered by adding sodium hydroxide to the residue and repeating the distillation. A mixture of diethyl ketone and acetic acid may be treated with sufficient dilute sodium hydroxide solution to transform the acid into sodium acetate and distilling the aqueous mixture. The ketone will pass over in the steam and the non-volatile, stable salt will remain in the flask. Acidification with dilute sulphuric acid hberates acetic acid, which can be isolated by steam distillation or by extraction. 

Now distil the filtrate A) and collect the distillate as long as it is acid to litmus. Should any solid separate out in the distilhng flask during the distUlation, add more water to dissolve it. Set aside the residue B) in the flask. Identify the volatile acid in the distihate. A simple method is to just neutralise it with sodium hydroxide solution, evaporate to dryness and convert the residual sodium salt into the S-benzyl-iao-thiuTonium salt (Section 111,85,5). 

The residue (5) in the distilhng flask may stUl contain a water-soluble, non-volatile acid. Cool the acid solution, neutralise it with dilute sodium hydroxide solution to Congo red, and evaporate to dryness on a water bath under reduced pressure (water pump). Heat a httle of the residual salt (G) upon the tip of a nickel spatula in a Bunsen flame and observe whether any charring takes place. If charring occurs, thus... 

Step 3. The non-steam-volatile compounds. The alkaline solution (82) remaining in the distiUing flask from Step 2 may contain water-soluble, non-volatile acidic, basic or neutral compounds. Add dilute sulphuric acid until the solution is just acid to Congo red, evaporate to dryness, and extract the residual solid with boiling absolute ethyl alcohol extraction is complete when the undissolved salt exhibits no sign of charring when heated on a metal spatula in the Bunsen flame. Evaporate the alcoholic solution to dryness and identify the residue. 

Acetic acid (b.p. 118 °C) is not boiled off from open dyebaths as readily as ammonia but is rapidly flashed off in steam or dry heat processes, thus developing the maximum degree of alkalinity under these conditions. The sodium salts of less volatile acids, such as sodium citrate, can be used to develop a lower degree of alkalinity. 

This mechanism could be demonstrated via nitrogen elemental analysis of polymers and copolymers treated with amine acid salts and thermally cured (Table II and Experimental). In a control experiment, ammonium acetate was added In excess to a vinyl acetate/ethylene emulsion copolymer without amlnoplast crosslinker to confirm that essentially all of the ammonia volatilized from the unfunctlonallzed polymer during cure (much poorer volatilization was observed If NH4CI was used In place of NH4OAC). 

R. Bunsen, H. Erdmann, and others 15 have made rubidium hydrosulphate, RbHS04. It is obtained by heating the rubidium salt of a volatile acid with sulphuric acid to about 250°-257° the oily liquid, on cooling, forms a crystalline mass of this salt. R. Bunsen made caesium hydrosulphate, CsHS04, by treating caesium carbonate with sulphuric acid under similar conditions. Both salts crystallize from water in rhombic, crystals. 

---