Solubility isoth e rm s fo r Ca HPO ,· 2H 20, dical c ium phosphate dihydrate, DCPD , in th e tern ary sys te m Ca(OH ), -H"PO, -H2 0 were dete rmin ed at 5, 15, 25, a nd 37.5°C in th e pH range 3.5-7; th e relativ e pos iti ons of th e isotherm s indi cate th at DCI'D has a negati ve th e rmal coefficie nt of so lubility. Th e so lubility produ ct, K", of DCI'D an d th e stab ility co nstants Kx and Ky for th e ion pairs [Ca HPO~] and [Ca H2PO:t], res pectively, we re obtain ed as fun cti ons of te mpe ra ture by the use of a ge neralized leas t squares procedure subj ec t to three conditi on fun cti ons -constan cy of the solubili ty product, elec tri c al ne utralit y in the solution, and congru e nt di ssoluti on of the so lid. Th e equations ob tain ed areIn In Kx= 51090/7'-341.14 + 0.58807'The existe nce of a max imum in Ks in the neighborhood of 25°C is plau s ibl e on the basis of availabl e th e rmodynamic da ta for DCI'D. T he rmodynam ic fun cti ons are reported for th e solution of DCPD a nd for th e assoc iation of th e ion pairs. Ke y word s : Calcium phosp hate ; d icalcium phosp hate dihydrate; ion pa irs; solu bili ty ; solubility isothe rm s; solubility produ ct.
Solu bility isoth erms of beta-trica lcium phos ph ate, j3-Ca3(PO.)" prepared by heating mixtures of CaC0 3 an d CaHP O. above 800°C, were dete rmined in th e tern ary system Ca(OH),·H 3PO.-H 20 a t 5, 15,25, and 37°C in the pH range 6.0-7.5 by equilibrati on with dilute H 3PO ., solutions. The results in dicate that j3-Ca3(P0 4), has a negative therm al coefficien t of solu bility. The solubility produ ct , K" was de termin ed as a fun cti on of te mperature by a ge neral ized least-s qu a res procedure; the res ultin g equ ation is log Ks =-45723. 26/T + 287.4536 -0. 546763 T;th e valu es of K, a nd its dispersion at 25 a nd 37°C are 1.20(0.056), and 0.283(0.011 ) X 10-29. Thermo· dyna mi c fun ctions for the dissolution of the salt at th e four experim enta l te mpe ratures are repo rted. Wh e n treated as an adju stable constant, th e Ca/p ratio in these j3-Ca3(PO.}. solutions was fou nd to have th e value 1.514(0.010), confirmin g th at the stoichi ometry of th e high tempe rature form of thi s salt is correctl y indi cated by the above formula.The co mputed isotherm s at 25°C for se veral calcium phosphates are co mpared; it is shown tha t j3-Ca3(P0 4}, is , next to Ca5(P0 4)aOH, the most stabl e at pH's above 6.36. The singular points of th e j3-Ca3(PO.)2 isotherm with those of CaHP O • . 2H20 and CaHP04 are at pH 's 5.88 a nd 6. 36, respecti vely.
Th e io niza ti on co nstant, K" fo r th e reacti on HF:;:2 H + + F -was ca lc ul ate d on th e ba s is of pote nti o· metric me as ure me nt s in th e cell Ag; AgCl , CI -, F -II La F"II Na F, HC I, 1-1 , 0 1 KC I(Sa td .), I-I g,Cl 2 ; Hg at 25°C. A leas t s qu a res procedure wa s a ppli e d to th e ex pe rim e nt a l da ta yie ldin g a bes t es tim a te fo r K, of 5 .85 X 10 -4 with a s ta nd a rd e rror of 0.03 X 10 -" . Key word s : H ydro flu ori c ac id ; ioni za ti on c onst ant ; lanth a num flu orid e e lec t rode; le as t s quares pro· ce d ure; potenti ome tri c me as ure me nt s . . IntroductionThe ch e mical properties of dilute aqueo us soluti ons of hydrofluori c ac id are described [1 , 2, 3] J by th e equilibria (1) (2) in wh ic h pare ntheses indi c ate ac tiviti es of th e various c he mical species. Mos t of the evid e nce for th e pres· e nce of th e variou s s pecies involved in th e above equilibria has been obtained from s tudi es [3,4] at hi gh co nstant ion ic stre ngth s. Unfo rtun a tely, the values for the apparent equilibrium co ns tants derived from s uc h studies are of limited use.Th ermodyn a mi c valu es of K J and K2 have bee n reported on th e basis of pote ntiome tric [5 , 6] or conductance meas ure me nts [7] . However, there are uncertainties in th e valu es of the constants originating from the graphical approximations [5 , 6] used in the calculation s, or from the assumptions regarding the species res pon sible for the transport of c urrent [7].The prese nt investigation was prompted by the observation of s mall but consistent discre pan cies between the valu es of fluoride ion activities obtained with Ce nt e r, Bos lon, Ma ss. 02 100. t Fi gures in b rac ket s ind ica t e t he lit e ratu re re re re nces at th e e nd of thi s paper.a lanthanum Auorid e me mbran e electrode and those calculated on the basis of th e re ported ionization constants of hydroAuori c a cid. Pote ntiom etri c meas ureme nts were mad e in th e ternary syste m aF-HCl-H20 . The use of th e th ermodynamic value of KJ d erived from such meas ure ments elimin ated th e di screpanc y betwee n meas ured a nd calc ulated Au orid e ion activiti es. Th e leas t squares procedure used in th e present study obviates the un certa int ies inh ere nt to gra phi cal me thod s and permits th e es timation of e rrors on a so und stati sti cal basis. Experimental Methods and Procedures . 1. NaF-HCI-H20 SystemsAll the che micals used in the present study were reagent grade. Stock solutions of sodium flu orid e were prepared by weighing the salt whi c h was dri ed at 100°C for 24 h; the salt was dissolved with condu ctivity water in borosilicate volumetri c flas ks and immediately transferred to polye thyle ne bottles for storage. The systems used in th e potentiometric measurements were mad e by takin g 100 ml of sodium fluoride solutions of suitable conce ntrations and adding various aliquots of standard hydrochloric acid. In this way it was possible to obtain syste ms with a wide co ncentration ran ge in bo...
Influence of some hydrophilic polymers on dissolution characteristics of furosemide through solid dispersion: An unsatisfied attempt for immediate release formulationFurosemide (FRMD) is 5-(aminosulfonyl)-4-chloro-Background: The objective of the present investigation was to enhance dissolution characteristics of water insoluble drug, furosemide (FRMD), by solid dispersion in various hydrophilic carriers. Materials and Methods: The solid dispersions were prepared by solvent evaporation technique using urea and Hydroxy Propyl Methyl Cellulose (HPMC E50 LV). Physical mixtures of drug with above-mentioned polymers were also prepared. Phase solubility studies were performed for drug in the presence of excipients in different ratios. The formulations were evaluated for drug content, in vitro dissolution, Fourier transform infrared (FT-IR) and differential scanning calorimetry (DSC). Similarity factor (f2) was calculated for comparison between dissolution of pure drug and drug-polymer physical mixtures with solid dispersions. Results: Phase solubility studies indicated linear increase in the drug solubility with increase in carrier concentration. In vitro release studies revealed that dissolution characteristic of FRMD was improved by solid dispersion technique. All the preparations of FRMD exhibited significant improvement in its dissolution profiles. Solid dispersion of FRMD with HPMC E50 LV exhibited the highest rate and extent of dissolution. Optimized batches of solid dispersions of both the carriers were characterized by FT-IR and DSC analysis, which indicated absence of major interactions between FRMD and carriers. Conclusion: Solid dispersion technique is one of the finest techniques to improve dissolution of poorly soluble drugs. AbstractAccess this article online Quick Response Code: Website:www.pnrjournal.com
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