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Analytical Method for OTC products

Potentiometric Titration

It is desirable to simplify analytical methods used for topical drug forms in human OTC labels. This is possible by using innovative variants of titrimetric methods whic h eliminate the drawbacks of known slowness and being tedious since the operation must wait for a steady reading, even when an automatic-titrator is used. However its high precision and the dtermination of an absolute quantity, has been widely preferred in many industries including the pharmaceutical1) 2)

Very fast titrations are possible using potentiometric measurements of the change induced by analytes on buffered redox couples during a chemical reaction. The potential change due to redox reaction of an analyte with the buffer component is directly proportional to the analyte concentration. In addition the potential of the redox electrode is very stable in the potential buffer solution.The buffered method involves the use of a potential buffer solution consisting of a redox couple, which stabilizes the measuring potential of the redox electrode. Rapid determination of redox compounds such as concentrated hydrogen peroxide, ethanol in alcoholic beverages, reducing sugars, insulin, glúcose, mangan11ese(II), phenol and minoxidil have been proposed3) 4)5) 6) 7) 8)9)10).

Furthermore, the analytical methodology and mechanism based on the change of potential generated between the analyte and the redox couple can be measured after a simple redox reaction as an equilibrium potential, reached as just said “after” (the reaction, and a transient potential during a complexed redox reaction. Proportionate redox potential buffer composition relative to concentration of a wide range of small changes in the composition of the buffer solution and whose reaction causes a potential change, namely, the peak height proportional to concentrations of the ayalyte yielding linear responses E0. The ΔE from control potential is expressed in mV so that sets of mV values may be tabulated against comcentrations of the analyte expressed by the Nernst equation, as shown in (1), .

E1 =E0 + (59÷n) log[Ox1]÷[Red1] (mV, 25ºC)

where E0 is the formal redox buffe potential, and n is the number of moles of electrons involved in the electrode reaction, and [Ox1] and [Red1] are the concentrations of Ox1 and Red1, respectively. The baseline potential, E1 (the potential when a sample is not injected), can be written in terms of the initial concentrations of Ox1 and Red1, based on:

E2 =E0 + (59÷n) log{[Ox1]+m[Ox2]÷[Red1]−m[Ox2]} (mV)

where [Ox2] is the initial concentration of Ox2. Under the assumption that no dispersion of the sample and reaction products occur while flowing through the manifold, the potential change (ΔE) (peak height) of the redox electrode can be expressed by the following equation, which is derived from (1) and (2)

ΔE = (59÷n) log{(1+m[Ox2]/[Ox1])÷(1−m[Ox2]/[Red1])} (mV)

(4) The relationship between ΔE and [Ox2]depends on both the initial concentration ratio of Ox1 to Red1 and the value of n, which can be estimated from (4). The variation in ΔE with [Ox2], under the condition that the value of n is 1:1 becomes linear within ca. 40mV, when the value of [Ox1]/[Red1] is about 1:2. When [Ox1]/[Red1] is higher than 2:1 or lower than 1:5, the sensitivity, slope of the calibration curve, increases, but the shapes of the curves become concave or convex, respectively. Thus, the potential change and baseline potential of the redox electrode are governed by the change in the composition of the redox couple in the potential buffer solution, according to (3). The linear relationship can be utilized as a validation curve for an Ox2 sample. However, the upper limit for the measurable concentration of the Ox2 with linearity is limited by the concentration of the potential buffer, as predicted from (3). This means that the concentration of the potential buffer must be chosen in accordance with the concentration of the analyte so that the measurable concentration level of the Ox2 is not altered by incorrectly choosing the proportional concentration of the potential buffer11).

So that the precise determination of an absolute quantity of a given substance to be analyzed cab be maintained and now combined with the convenient swiftness that potentiometric measurement of <chen>E0</chem> provides. So simplicity is combined with precision.

Intrinsic Redox Potentiometry

Based on foresaid advances it is plausible to conduct intrinsic redox potentiometric measurements in a finished product formulated with a redox pair as one of its ingredients. The formal redox potential in such a construct would vary by reacting the redox pair with the active ingredient however the measurement done in conditions of equilibrium long after the reaction came to an end. Ideally it would be preferable to determine a value for ΔE under longstanding stability and best reflecting the active moiety`s concentration

This novel approach has been for the first time adopted at our laboratory recently and has been found quite practical and precise as it as expected. The fact that the pair is also an acud/base buffer adds to the method simplicity. Secure pH control goes in hand with correct E0 measurements.The vast majority of o ur products are brought to pH 7. The formal pH for the pair is 6.7.

Proprietary Phosphate buffer/Redox couple

]Therefore the redox couple is prepared as a salt of acyl phosphate in an aqueous solution by the process of acylation of diluted phosphoric acid (H.sub.3PO.sub.4) or its salts (Msub2 2HPO.sub.4 or MH sub.4 where M is monovalent cation) with diluted salts of an acid of the general formula RCOOX, where R can be hydrogen or a lower alkyl group having 1 to 5 carbon atoms, and X can be –OCOR. –OR, NR.sub.2, and in each particular case X can be an organic or inorganic radicals enhancing in each particular case the redox couple in combinations with sorbic acid, H2O2, acetic acid, DL-lactic acid, L-lactic acid, n-alkans, cholesterol and glycerol.

This method uses solely water as solvent for extraction of acetyl phosphate to the original water used and keeping the reaction controlled by dilution and with temperature control, avoiding hydrolysis by high reaction temperature and obtaining high yields of stable AcP that can be stored at room temperature with indefinite shelf life. The acidification brings the pH to 6.5 and captures high content of solids bearing the solution a specific gravity of 1.238 to 1.3. The stability of AcP extracted by this method contrasts with the difficulty of storing the compound at room temperature as obtained by prior methods whereby at pH 7.2 it would not endure beyond 21 hrs. Acetate ion may have unfavorable solvation or electrostatic interactions with the phosphoryl oxygen atoms and an unfavorable alignment of the Sp2 orbit of the nucleophilic oxygen atom.

If NH4OH is used in a second step the pair is generated as a di-ammonium salt, this allows the dianion to be held stably together by bridging of positively charged ammonium moieties to negatively charged oxygen atoms in the anion lowering charge repulsion to overcome the electrostatic repulsion forces in the dianion and allowing intramolecularity of the carbonyl moiety to occur in the presence of Mg++ cations.

The use of ammonia to isolate the acetyl phosphate is greatly preferred for a number of other reasons. First, ammonia is extremely cheap, not only compared to silver or lithium salts, but as compared to other suitable bases. Second, ammonium acyl phosphate salts, particularly diammonium salts, are very soluble in water, and the ammonium ion is innocuous to enzymes. Water solubility of the salts is congruent with its very low toxicity

Intramolecularity is fertile ground for a zwitterion the moeity best explaining the voñtannetric features of the pair and its acid/base buffering characteristics.


Description of method

After having determined the formal E0 for the redox pair (+160mV) at 23ºC and it is compared to the one set as standard for the finished product. A Temp/pH/Redux Potential apparatus is used with a single platinum electrode.

After manufacturing the finished product is left overnight for its stabilization. Once stabilized measurement is done and results sent to QA.

For each individual product a scale per each 10 mV of ΔE from formal <chen>E</chem> each manufactured lot of a given product must match such E2 for that particular product at the standard pH and temperature.


An accelerated stability study done for our veterinarian product MetriHeal which contains our proprietary phosphate buffer showed that acid/n and redox buffering properties remained substantially intact after 190 days. Therefore the redox pair is an excellent redox potential preparation on which to base intrinsic quantitative measurements of active ingredients.


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