Acetic acid, anhydrous, dissociation

Anhydrous ammonia is normally analy2ed for moisture, oil, and residue. The ammonia is first evaporated from the sample and the residue tested (86). In most instances, the amount of oil and sediment ia the samples are insignificant and the entire residue may be assumed to be water. For more accurate moisture determinations, the ammonia can be dissociated into nitrogen and hydrogen and the dewpoint of the dissociated gas obtained. This procedure works well where the concentration of water is in the ppm range. Where the amount of water is in the range of a few hundredths of a percent, acetic acid and methanol can be added to the residue and a Karl Fischer titration performed to an electrometricaHy detected end point (89—92). 

Example 8. What effect has the addition of 0.1 mol of anhydrous sodium acetate to 1 L of 0.1 M acetic acid upon the degree of dissociation of the acid ... 

Shatenshtein [102] drew attention to the fact that nitric acid in anhydrous acetic acid was much less dissociated than when in water, and that this could be explained by the protolytic properties of the solution components and by the low dielectric constant of acetic acid. 

The tendency of a solvent to accept or donate protons determines the strength of a solute acid or base dissolved in it. For example, perchloric and hydrochloric acids are strong acids in water. If anhydrous acetic acid, a weaker proton acceptor than water, is substituted as the solvent, neither of these acids undergoes complete dissociation instead, equilibria such as the following are established ... 

Ionisation and Dissociation Constants for Acid-Base Equilibria in Anhydrous Acetic Acid at... 

Useful exploratory studies of acid-base behaviour in solvents of low dielectric constant have been made by conductance " and potentio-metric " titrations. Association constants are usually obtained from spectrophotometric measurements. The strengths of various bases can be compared by means of their association with an indicator acid like 2,4-dinitrophenol. If both acid and base are colourless, a competition for the base can be established between the acid and an indicator acid like bromophthalein magenta In solvents like benzene, other reactions than simple 1 1 association between B and RX may occur. Self-association of the acid or base is one such auxiliary reaction. A classic example is the dimerisation of carboxylic acids in benzene. If allowance is not made for this, constant values of the quotient [BRX]j[B][RX] will not be obtained. (Variations in the quotient cannot be attributed to interionic forces or other nonideal behaviour BRX is scarcely dissociated into ions at all and in spectrophotometric work very low concentrations of B and RX can be used.) Evidence for association ratios other than 1 1 can be obtained from indicator studies. The method developed by Kolthoff and Bruckenstein for studying reactions in anhydrous acetic acid fails for reactions in benzene and similar solvents because more than one acid molecule reacts with the indicator to give complexes of the form/w J r"(HX)yi. In such studies it is generally a good approximation... 

There is less abundant information about HBr and HI, but if we apply Robinson s treatment to the vapour pressure data we find piC(HBr)= -8, pK(Hl)= -9, compared with pR (HCl) = -6, suggesting that the strengths of the three acids are approximately in the ratio 1 10 10. Further, investigations in non-aqueous solvents often show that HBr is a considerably stronger acid than HCl. Thus conductivity measurements in anhydrous acetic acid indicate a ratio of about 20 between the dissociation constants of these two acids, while in acetonitrile we have piC(HBr) = 5.5 and pK(HCl) = 8.9. It is also frequently found that HBr is a much more effective acid catalyst than HCl under conditions where both acids are undissociated. It is of interest that studies... 

Ion-pair dissociation of solvated bases and base perchlorates in anhydrous acetic 265 acid... 

---