The Spectrophotometric Determination of the pKa of Pyrrolidinedithiocarbamic and Piperidinedithiocarbamic Acids , Based on Diode Array Measurements

Neste trabalho apresenta-se um método espectrofotométrico, baseado em equipamentos com arranjo lin ear de diodos (“di ode ar ray”), para determinação de pKa dos ácidos ditiocarbâmicos derivados da pirrolidina (HPyr-I) e da piperidina (HPip-II). A elevada velocidade de leitura do equipamento, torna possível minimizar o problema da decomposição dos ditiocarbamatos, em meio ácido. Os valores de pKa obtidos foram 3.10(I) e 3.51(II), em I = 0.50 mol L (NaClO4) à 25.0 °C, sendo consistentes com os mecanismos de decomposição ácida. Apresenta-se também uma discussão baseada na forma dos espectros obtidos em meio ácido e básico e sua relação com a ordem dos valores de pKa, a qual é inversa em relação a valores determinados anteriormente.


In tro duc tion
Dithiocarbamates (DTC) have been widely ap plied in sev eral in dus tries (e.g.rub ber), ag ri cul ture, med i cine, organic syn the sis, and in an a lyt i cal chem is try (es pe cially inor ganic spe cies anal y sis) [1][2][3][4][5][6][7][8][9][10] .The low sta bil ity of the DTC in acidic me dia has been in di cated as the ma jor lim i ta tion to the use of these ver sa tile com pounds.
Zahradnick and Zuman 11 first stud ied the de com po sition re ac tion, and Chakrabarti et al. [12][13][14][15][16] pro posed a deg rada tion mech a nism which may be re lated to the acidic form, rep re sented by an un sta ble in ter me di ate and gen er at ing an amine and car bon disulfide, ac cord ing to the re ac tion: (1) These au thors pro posed that the in sta bil ity of the in terme di ate, whose struc ture is pre sented in the Fig. 1, is due to the pos i tive charge den sity in the car bon and ni tro gen atoms of the carbamate bond (N-C), which in duces the mol ecule de com po si tion in acidic me dia.
Ar ti cle Thus, the pK a value for the DTC is an im por tant fea ture since it is re lated to the com pound sta bil ity and var ies with the aminic substituent.
The ki netic, polarographic, or math e mat i cal meth ods pro posed for the pK a mea sure ments [12][13][14][15][16] are time con suming.Con ven tional potentiometric and spec tro pho to met ric meth ods are not ap pli ca ble be cause of the rapid de com posi tion that oc curs in acidic me dia.
The pres ent work de scribes the ap pli ca tion of a rel atively eas ier spec tro pho to met ric method based on Volgel's pro posal 17 , but em ploy ing a di ode-array spectrophotometer, whose read ing speed per mits data acqui si tion in such a short time that it by-passes the prob lem of com pound de com po si tion.The method is ap plied to the pK a value de ter mi na tion of pyrrolidinedithiocarbamic (HPyr) and piperidinedithiocarbamic (HPip) ac ids.
The struc tural for mula of the ac ids are:

Ma te rials and Methods
All of the re agents used were of an a lyt i cal grade.

DTC syn the sis and char ac ter iza tion
The Pyr and Pip sodic salts were ob tained by the re action be tween the car bon disulfide and the re spec tive amine (pyrrolidine or piperidine), in the pres ence of so dium hydrox ide 18 in an eth a nol/wa ter 1:1 (v/v) re ac tion me dia.The white salt ob tained was washed with cold eth a nol and then recrystallized by dis solv ing in wa ter, fol lowed by the ad dition of eth a nol.The so lu tion was frozen and the white needle crys tals which formed were char ac ter ized by vi bra tional spec trom e try (lig ands), atomic ab sorp tion (sodium), and thermogravimetric anal y sis.

Buffer so lu tions
The McIlvaine buffer so lu tions with ionic strength adjusted to 0.50 mol L -1 (NaClO 4 ) were pre pared ac cord ing to Elving et al. 19 .The pH of these buff ers was mea sured with a glass elec trode cal i brated with con di tional buff ers.These con di tional buff ers were so lu tions with an ionic strength of 0.50 mol L -1 (NaClO 4 ), and hy dro gen ion concen tra tions of 1.00 x 10 -2 and 1.00 x 10 -4 mol L -1 (HClO 4 ), cor re spond ing to pH 2.00 and 4.00, respectivel y 20 .

Equip ment
A di ode-array Hewlett-Packard HP 8451A spectrophotometer and quartz cells 1.00 cm in length were used in the spec tro pho to met ric mea sure ments.
The lig ands were char ac ter ized in a Nicolet 5SXC FTIR spectrophotometer equipped with a TGS de tec tor in the 4.000-400 cm -1 range.The spec tra were ob tained from KBr pel lets.
The num ber of hydration wa ter mol e cules was de termined in a DuPont TGA-951 thermogravimetric mod ule, cou pled with a DuPont 9900 ther mal an a lyzer sys tem.
The atomic ab sorp tion ex per i ments were per formed in an Intralab AA12/1475 spectrophotometer.

Pro ce dures
All of the spec tro pho to met ric mea sure ments were carried out at 25.0  0.1°C, us ing the buffer so lu tions pre viously thermostated at this tem per a ture as blanks.
The stock NaDTC so lu tions (50.0 mL), at a 5.0 x 10 -4 mol L -1 con cen tra tion, were used for the prep a ra tion of the work ing so lu tions, with con cen tra tions rang ing from 8.33 x 10 -6 to 6.67 x 10 -5 mol L -1 at pH 2.05 to 4.47.A 3.33 mol L -1 so lu tion of each DTC was used in the de ter mina tion of the max i mum wave length ab sorp tion of the acidic and ba sic forms, at pH 0.98 and 7.82, re spec tively.
In or der to min i mize the de com po si tion of the compounds, the so lu tions were pre pared di rectly in the quartz cells.Thus 0.20 mL of the stock so lu tion of NaDTC was trans ferred with an au to matic pi pette and in tro duced into 2.80 mL of the pre vi ously thermostated buffer.The mean time spent in the pro ce dure and the read ing was about 5 s, since the equip ment reads the spec tral range in 0.1 s.

Re sults and Dis cus sion
The NaDTC char ac ter iza tion In Ta ble 1 the prin ci pal bands found in the vi bra tional spec tros copy mea sure ments are pre sented, com pared with the lit er a ture val ues for the groups C=S, N-CS 2, and C-N for the com pounds NaPyr and NaPip.
The atomic ab sorp tion ex per i ments ev i denced the presence of 10.80% Na in the NaPyr.2H20 (cal cu lated = Fig ure 1.The acid de com po si tion in ter me di ate, ac cord ing to Joris et al. 13 . Ta ble 1.The prin ci pal IR bands ob served, com pared with the lit er ature val ues.11.20%) and 9.94% (m/m) in the NaPip.2H 2 0 (cal cu lated = 10.49%).The thermogravimetric re sults con firmed the pres ence of two hydration wa ter mol e cules for both compounds.This is con sis tent with the work of Kudela et al. 21nd Siddiqi et al. 22 .
Mo lar ab sorp tivi ty ( ε) de ter mi na tion The mo lar ab sorp tivi ty was ob tained from an gu lar coef fi cients of the absorbance vs. con cen tra tion (mol L -1 ) curves for the acidic and ba sic forms of each DTC, in the max i mum wave length ( λmax ) re gion.The spec tra of these com pounds are pre sented in Fig. 2a (Pyr) and 2b (Pip), at dif fer ent pH val ues for the acidic and ba sic forms.Ac cording to the lit er a ture 23 , the pres ence of a sin gle max i mum in acidic me dia is due to protonation, which re duces the res onance in the N-CS 2 group.
In Fig. 3 the Lam bert-Beer curves for Pyr (3a) and Pip (3b) are pre sented at dif fer ent pH at λ max for the acidic and ba sic forms of each com pound.Ta ble 2 pres ents the fi nal re sults.
The val ues showed good agree ment with those found by Shankaranarayana and Patel 23 , be ing on the or der of 10 4 .The re sults in di cate that acidic forms have smaller absorptivities than ba sic forms.

pKa cal cu la tion s
Sub sti tu tion of the ε val ues in the Eqs. 2 and 3 AA = εA,RSH(RSH) + εA,RS-(RS-) (2) A B = εB,RSH(RSH) + εB,RS-(RS-) which con sider that the absorbance is due only to the presence of the free and protonated spe cies at each wave length, made it pos si ble to cal cu late the con cen tra tion of acidic and ba sic forms in equi lib rium.In the above equa tions, AA is the absorbance at wave length A, ε A,RSH is the mo lar ab sorptivi ty for the protonated form at wave length A, εA,RS-is the mo lar ab sorp tivi ty for the free form at wave length A,  and (RSH) and (RS -) are the ac tiv i ties of the protonated and free forms, re spec tively.
Using the ra tio of these con cen tra tions it is pos si ble to cal cu late the pKa ac cord ing the Handerson-Hasselbach equa tion: A sim ple com puter pro gram in the Quick Ba sic language (©Microsoft Co.) was de vel oped to per form the calcu la tions, giv ing the re sults pre sented in Ta ble 3, in I = 0.50 mol L-1 (ad justed with NaClO4) and at 25.0 °C.This pro -gram uti lizes the con cen tra tions of acidic and ba sic forms given by Eqs. 5 and 6, de rived from Eqs. 2 and 3 sys tem: ( The lit er a ture val ues and the con di tions un der which they were ob tained are pre sented in the Ta ble 4.This ta ble shows that the val ues are not in agree ment for HPip, pos sibly due to com pound de com po si tion.Also, the or der pKPyr pKPip is in verted in re la tion to the pres ent re sults, which are con sis tent with the pro posal of the acid de compo si tion in ter me di ate, since ac cord ing to the pres ent pKa re sults the Pyr is protonated at a pH more acid than the Pip, hav ing a lower pKa value. It is widely known that Pyr is much more sta ble in acidic me dia than Pip 3,8,9,10 , and is fre quently ap plied in sev eral meth ods of anal y sis in acidic con di tions.The solvatation of acidic in ter me di ates 12 is not suf fi cient to explain this dif fer ence, since the lig ands have sim i lar structures.For in stance, τ (Pyr) = 85.6 min in pH = 3.60 and τ (Pip) = 3.78 min in pH = 4.85 25 .Other ki netic data are pre sented in Ta ble 5, which are in agree ment with the re sults from Vandebeek et al. 14 .Ac cord ing to these au thors, the ap par -Ta ble 3. pK a val ues de ter mined by the spec tro pho to met ric method, us ing Eqs. 5   ent rate con stant (k ap ) and the half-life time (τ ap ) var ies with the pH up to a cer tain value, at which they be came con stant (k lim and τ lim , re spec tively).
A sec ond point in fa vor of the pres ent re sults is the spectral shape of the com pounds at dif fer ent pHs, pre sented in Figs.4a and 4b for Pyr and Pip, re spec tively.From these curves it is pos si ble to note that for the Pyr case at pH 3.06, a sin gle peak ap pears at 247 nm (acidic form), and that two peaks are pres ent at pH 3.61 and 4.47 at 258 and 276 nm (ba sic form).Thus, a spec tral change cor re spond ing to an al ter ation in the pre dom i nance of the forms oc curred between pH 3.06 and 3.61.
In the Pip case, this spec tral change ap pears clearly only at pH 4.47.At pH 3.06 there is only one peak at 280 nm (acid form), while at 3.61 there are two peaks, with out def ini tion, sug gest ing that the spec tral change oc curs near, but be fore pH 3.61.
Based on these com ments it is pos si ble to con clude that the pres ent val ues are more co her ent with the acid de compo si tion the ory and the spec tral change than the ear lier proposed val ues.This has been at trib uted to the use of the di ode ar ray spectrophotometer, with high read ing speed, tech nol ogy which was not avail able when pre vi ous research was de vel oped.

Fig ure 4 .
Fig ure 4. Vari a tion in the spec tral shape at pH 3.06, 3.61, and 4.47 for the com pounds (4a Pyr and 4b Pip).
Ta ble 2. Mo lar absorptivities, ε, de ter mined for each DTC to both acidic and ba sic forms.
Sum mary of pK a val ues from the lit er a ture and the pres ent work.