Iodometric Quantitative Analysis Method of Ascorbic Acid in Tablets

COSTINELA VALERICA GEORGESCU1, CRISTIAN CATALIN GAVAT2*, DOINA CARINA VOINESCU1 1 Dunarea de Jos of Galati University, Faculty of Medicine and Pharmacy, Department of Pharmacology and Clinical Department, 47 Domneasca Str.,800008, Galati, Romania 2 Grigore T. Popa University of Medicine and Pharmacy, Faculty of Medical Bioengineering, Department of Biomedical Sciences, 16 Universitatii Str., 700115, Iasi, Romania

Ascorbic acid is a potent reducing and antioxidant agent that functions in fighting bacterial infections, in detoxifying reactions, and in the formation of collagen in fibrous tissue, teeth, bones, connective tissue, skin, and capillaries [1,2].
Vitamin C is a water-soluble compound with powerful antioxidant action that fulfills a significant immune protective role of the body against infections and various diseases. It increases general immunity of the human body and prevents different cancers appearance. Ascorbate supplementation in cancer patients, either enhances the antitumor effects of chemotherapy or reduces its toxicity [1][2][3][4].
This vitamin possesses powerful antioxidant action by making a redox oxidation-reduction system in the cells, which contributes to the non-enzymatic transport of hydrogen to the tissues. Antioxidant action is represented by the neutralization and destruction of free peroxide radicals and nitrosamines in the body. Also prevents free peroxide radicals synthesis and promotes enzymatic activity. Vitamin C functions, as a proton carrier in oxidoreductase-catalyzed reactions in human body (catalase, peroxidase, cytochrome-oxidase, various dehydrogenases) are important aspects of all biochemical processes based on electron redox transfers. Ascorbic acid has antitoxic action by effective body protection against harmful effects of organochlorinated, organo-mercury compounds, lead, cadmium, arsenic compounds [3][4][5].
Vitamin C is also referred to as hexuronic acid. Structurally, it is L-gulonic acid or L-glucuronic acid gammalactone. It is absorbed by active transport or passive diffusion and enters to the cells through a specific transport system that requires a glucose transporter. Insulin causes vitamin C entry into the cell. Patients with unbalanced hyperglycemia and diabetes have low levels of vitamin C in the cell, which causes increased oxidative stress with high risk of atherosclerosis This vitamin is brought to the body exclusively by food intake, especially by daily consumption of fresh fruits and vegetables [1][2][3][4][5].
Ascorbic acid is directly involved in corticosteroids production and certain neurotransmitters synthesis (substances that allow nervous influx transmission). It is known ascorbic acid active involvement in glucose metabolism, collagen synthesis (by converting proline to hydroxyproline, the core component of collagen), folic acid and some amino acids metabolism and in immunological reactions that facilitate iron absorption in digestive tract.
Vitamin C is directly involved in glycosaminoglycans biosynthesis (polysaccharides present at the cell surface and in the extracellular matrix), Hyaluronic acid, present in the connective, epithelial and nervous tissue, fulfills an important biological moisturizing role of the skin, in maintaining the suppleness, elasticity and tonicity of the epidermis [1][2][3]5].
Presence of vitamin C in the body prevents fat deposition in the liver and ensures normal liver cell function. Ascorbic acid has antiallergic action and significantly decreases the incidence of clots in the blood vessels. It keeps glutathione in reduced form, actively protects vitamins A, B, E against oxidation and converts (reduces) Fe 3+ to Fe 2+ ions. Also activates folic acid (vitamin B9) in the body. Vitamin C is directly involved in protein and carbohydrate metabolism, increases resistance to infections by leukocyte activation, *email : ccgavat70@yahoo.com; Phone: 0743-782544 interferon production and maintains mucosal integrity [1][2][3][4][5][6].
Ascorbic acid deficiency causes a disease called scurvy (scurvy disease), that manifests itself through frequent gingival bleeding, severe changes in connective tissue. Some of the characteristic signs consists of atypical structures involving collagen and glycosaminoglycans formation, blurry spots on the skin, frequently on thighs and legs, soft gums and bleeding of virtually all mucous membranes. There are also known disturbances of the osteogenesis process, with the appearance of spongy forms [3][4][5][6][7].
Recommended intake of vitamin C to the human body is 60 mg /Kg body weight/day in adults. This value is the safety limit that prevents scurvy symptoms occurrence. Minimum required ascorbic acid intake to prevent scurvy is about 10 mg /Kg body weight/day [4][5][6][7].
The main goal of this study was to exactly quantify pure ascorbic acid in tablets of two pharmaceuticals. Pursued objectives consisted in the improvement and application of a iodometric titration method for pure ascorbic acid quantitative analysis. Another objective was to compare obtained results for the tablets of two producing companies one with each other and then referring to the Romanian Pharmacopoeia Rules, 10-th Edition and European Pharmacopoeia, 9-th Edition, with regard to permissible percentage deviations from the stated official amount of active substance on pharmaceuticals tablets [8]. -300 mL volumetric flasks; -graduated glass pipettes 1 mL, 5 mL and 10 mL; -Pellet Precisa 50 mL automated burette. A first step in volumetric quantitative analysis of ascorbic acid consisted in Na 2 S 2 O 3 , 0.1 N calibration on K 2 Cr 2 O 7 0.1 N standard solution.

Description of standardization methods
Calibration procedure of iodine I 2 , 0.1 N solution was carried out in two different steps. The first stage consisted in : Calibration of sodium thiosulphate Na 2 S 2 O 3 , 0.1 N on a standard solution of potassium dichromate K 2 Cr 2 O 7 0.1 N.
Potassium dichromate K 2 Cr 2 O 7 , in strongly acidic medium (H 2 SO 4 , 40 %), stored in a dark place during 25 minutes in iodometric brown vials , has oxidized existing iodide Ianion from potassium iodide KI and has released an amount of iodine I 2, equivalent to potassium dichromate quantity taken into experiment, in six titration beakers. Then, the iodine released was titrated with a 0.1 N sodium thiosulphate Na 2 S 2 O 3 solution from burette, until a yellowish-green coloration has appeared in titration beakers, in presence of 1 mL starch freshly prepared 1%. Titration process took place until the color of the solution has changed to transparent blue -greenish in equivalence point. Volumes of Na 2 S 2 O 3 , 0,.1 N (V) consumed from burette to equivalence point were measured (table 1). Chemical reaction has taken place as follows: Required volumes of 0.1 N potassium dichromate taken into beakers (V') and 0.1 N sodium thiosulphate consumed from burette to equivalence point (V), were written in table 1. A series of important improvements have been made to initial titration method: increased corresponding amounts of 1.0, 1.4, 1.8, 2.2, 2.6, 3.0 g KI solid salt (a greater excess) were weighed and brought progressively (table 1) with corresponding increased distilled water volumes into six iodometric brown glasses., compared to the basic method in which only 1 g KI solid salt was weighed constantly.
Sulfuric acid concentration was increased from 25% in classical method, to 40 % , absolutely necessary to create a much stronger acidity. Gradually, increasing volumes of H 2 SO 4 , 40% were added into each iodometric brown flask, over existing solutions (table 1). Storage time in a dark place was increased from 5 min in basic method, to 25 min nin curent proposed procedure, so the entire amount of existing iodide anions (I -) be oxidized to elemental iodine (I 2 ). Three titrations were made for each studied volume (table 1) of potassium dichromate (V').
The second step was consisted of iodine I 2 0.1 N calibration process and has been represented by: Standardization of iodine I 2 0.1 N on a calibrated solution of sodium thiosulphate Na 2 S 2 O 3 , 0.1 N Volumes of iodine I 2 0.1 N solution from six titration beakers were titrated with a standardized 0,1 N Na 2 S 2 O 3 from burette, until the solutions color has changed from reddish-brown to light-pale yellow. Titration process continued, in the presence of 1 mL starch 1% , until the color of the six solutions has changed from dark bluegray to complete colorless at equivalence point. Different volumes of Na 2 S 2 O 3 , 0.1 N (V 1 ) from burette consumed to equivalence, were measured and described in table 2. Calculation procedure (3) where: Tr 2 = real titer of iodine I 2 , 0.1 N; Tr average = average real titer of sodium thiosulphate Na 2 S 2 O 3 , 0.1 N; Eg Na 2 S 2 O 3 = gram-equivalent of Na 2 S 2 O 3 ; Eg I 2 = gram-equivalent of I 2 = M I 2 / 2; M I 2 = molecular weight of I 2 = 253.81 g; V 1 = sodium thiosulphate 0,1 N ( Na 2 S 2 O 3 ) volumes conumed from burette to equivalence point, V 2 = iodine 0.1 N (I 2 ) volumes taken for analysis in six titration beakers (table 2). Using ecuation (3), six real titers (Tr 2 ) of I 2 0.1 N solution were calculated and an average real titer (Tr 2 average ) of iodine I 2 , 0.1 N has been calculated. Thus, the normality factor of I 2 , 0.1 N was calculated:. f ''= Tr 2 average /Tt I 2 (4). Tt I 2 has represented theoretical titer of I 2 , 0,1 N solution, Tt I 2 = (Eg I 2 . N) / 1000 (g/mL). Normality factor f '' had to be between: 0.9 ≤ f '' ≤ 1.1.

Study of proposed dosing method was described by:
Volumetric quantitative analysis of pure ascorbic acid in two brands of pharmaceutical tablets Pure ascorbic acid in samples volumes (V 4 ) added into six titration beakers of each of two pharmaceuticals, was oxidized to dehydroascorbic acid by a standardized I 2 0.1 N solution from burette (V 5 ) in strongly acidic medium (H 2 SO 4 , 40%), while elemental iodine (I 2 ) was reduced to iodide (I -) anion. Titration process was carried out in presence of starch, 2% with color change from colorless to persistent dark blue-gray to equivalence point. Chemical reaction took place as follows ( fig. 3).
Vitamin C volumes of two samples solutions taken into beakers (V 4 ) and iodine I 2 , 0.1 N standardized solution volumes consumed from burette to equivalence point (V 5 ), were written in table 3. Three titrations have been done for each studied volume of ascorbic acid sample.
Samples preparation. Average tablet weight of each of two pharmaceutical products was m c = 0.5977 g. Active substance (pure ascorbic acid) content in pharmaceutical tablet was reported to be 180 mg, the same value in both pharmaceuticals.
Two samples made up of a = 0.5926 g vitamin C powder has been obtained from each of two pharmaceuticals, which were weighed and completely dissolved with 15 mL of 1,4-dioxane p.a. into two tightlyclosed volumetric flasks V 3 = 500 mL each of them. Both contents were stirred well about 3-4 min, until complete dissolution of vitamin C powder.
The volumetric flasks were filled out with distilled water to the mark. Different sample volumes of vitamin C were added in six titration beakers for each of two pharmaceuticals (V 4 ), according to table 3. In equivalence point (dark blue-gray color of solutions), titration processes were stopped and volumes of iodine I 2 , 0.1 N consumed from burette to equivalence point (V 5 ) were measured. Six titrations were performed for each pharmaceutical product powder sample and mean ascorbic acid (Z 1 %) percent concentration was determined. Then, average concentration of ascorbic acid calculated on pharmaceutical tablet (T%) and corresponding ascorbic acid amount measured in tablet of two pharmaceuticals (X 1 mg) were calculated. Some important improvements have been made to the original volumetric dosing method: both solid vitamin C powder samples were first completely dissolved in a small volume (15 mL each) of 1,4-dioxane p.a. as a polar solvent, compared to the original method. Then, the volumetric flasks were filled out to the mark with distilled water. During titration, 2% freshly prepared starch and H 2 SO 4 40% solutions were used, two and four times more concentrated respectively, than those applied in the initial dosing procedure. Appropriate increasing volumes of starch 2% and concentrated H 2 SO 4 40% solutions were measured into six titration beakers, as shown in table 3, compared to the original dosing method in which the volumes of added reagents were constant for all measurements ( 1 mL starch 1% and 5 mL H 2 SO 4 2 N which has corresponded to 9.8 %. solution ).

Calculation procedures
A.Percentage concentrations (Z %) of pure ascorbic acid from two pharmaceutical products samples and its average concentration (Z 1 %).
From chemical reaction (3), it was observed: Fig. 3 Titrimetric analysis of ascorbic acid -chemical reaction Romanian Pharmacopoeia, 10-th Edition according to European and International in force rules, has confirmed the following percentage deviations from the declared active substance content [8].
Standard Deviation (SD). This parameter was a measure of how widely individual values (measured V 5 volumes of iodine I 2 from burette) are dispersed from the average value (the mean), how closely the data are clustered about the mean. Standard Deviation was a measure of the width of the distribution .
Standard Error (SE), has represented a measure of errors amount in y values prediction (measured V 5 iodine I 2 0.1 N from burette) for an individual x (V 4 mL acid ascorbic samples taken into titration beakers) [9][10][11].
Sample Variance (SV). expressed the relative distance between each data value and the mean and it has measured how the data (measured volumes V 5 of iodine I 2 , 0.1 N) distributes itself about the mean (expected value). It has represented the square of the standard deviation [13][14][15].
Relative Standard Deviation (Coefficient of variation or homogeneity) RSD % . Relative standard deviation (standard deviation divided by the mean), consisted in the deviation measurement about how different numbers in a particular data set (measured volumes V 5 of iodine I 2 , 0.1 N), have been scattered around the mean. This formula showed the spread of data in percentage.. It is also known as measure of variability of a series of numbers (Iodine I 2 ,0.1 N consumed volumes from burette) , independently of the unit of measurement used for these numbers. Formula was given as: RSD % = (SD / X mean ) . 100 (10) , whereas SD = standard deviation and X mean = average of the measured values. In this case, maximum range of allowed values was between 30 -35% [9][10][11][12][13][14][15].
Confidence Level (95% degree of freedom) has been described by the expression stating that the true mean was likely to lie within a certain distance from the measured mean of values.
All statistical parameters described above were calculated using Data → Data Analysis →Descriptive Statistics menu in Microsoft Office Excel 2016. P value, probability value or asymptotic significance used in context of null hypothesis, has been performed in order to exactly quantify, to assess statistical significance of measured values. This parameter highly expressed the veracity of obtained results. If P < 0.001 , than the results were highest statistically significant (less than one in a thousand chance of being wrong). For values ranged between 0.001 < P < 0.05, the obtained results has presented a high statistically significance . If 0.05 < P < 0.1, than the result values had a much lower statistical significance. For P values ranged within P > 0.1 , the resulting values presented lowest statistical significance [16][17][18][19]. P value was established using Data →Data Analysis →Anova Single Factor test.

Results and discussions
Standardization of sodium thiosulphate Na 2 S 2 O 3 0.1 N by titration on a potassium dichromate K 2 Cr 2 O 7 0.1 N pure standard solution In close correlation with equations (1) and (2), individual real titers, average titer value and normality factor of Na 2 S 2 O 3 0.1 N solution were calculated. The obtained results were described in table 1.
Calibrating iodine I 2 0.1 N solution by titration on standardized sodium thiosulphate Na 2 S 2 O 3 , 0.1 N According to relations (3) and (4), six real titers, average titer value and normality factor of iodine I 2 , 0.1 N solution were investigated. The obtained results were written in table 2.
Volumetric quantitative analysis of pure ascorbic acid in two pharmaceuticals Ascorbic acid that came from Fiterman® and Labormed® tablets , was titrated with exactly the same volumes of iodine I 2 , 0.1 N solution from burette. Thus, the calculations values presented in table 3 were identical for vitamin C tablets which has belonged to both producing companies.
According to equations (5)-(9), percentage concentrations (Z %) of pure ascorbic acid from powder samples (a = 0.5926 g) of two pharmaceuticals and its related average value (Z 1 %) were calculated, as well as ascorbic acid pure content (X 1 mg) assigned to tablets of two pharmaceutical products. The obtained results were highlighted in table 3.
Mean percent concentration of pure ascorbic acid in weighted sample (a = 0.5926 g) was 95.76 % and average percent concentration of ascorbic acid in vitamin C tablet ( m c = 0.5977 g.) has been assigned to 96.58% in both pharmaceuticals, with a maximum percent allowed deviation of 3.42 %, to the declared active substance content (180 mg).

Statistic study
Following statistical analysis, the obtained results were presented in table 4. All studied parameters ranged within normal limits.
Average of studied values, which were represented by volumes V 5 of iodine I 2 , 0.1 N consumed (table 3) from burette to equivalence point , was found to be 2 .
Standard Deviation SD = 0.525357 and Sample Variance SV = 0.276 (table 4), as direct measures of dispersion, were very low which meant that the experimental individual obtained values were tightly close one to each other and by the mean (expected value).

Conclusions
Following the iodometric dosing method of ascorbic acid in two pharmaceuticals, it was found, in both cases, that the average percent concentration of ascorbic acid in tablets was 96.58% , the same in both pharmaceutical products. The amount of pure ascorbic acid in tablets was 173.84 mg, an identical value for both branded pharmaceutical products, which was very close to the stated amount of active substance by the producing companies (180 mg). Maximum percent deviation from declared active substance content in both pharmaceuticals has been only 3.42 %, which was located below maximum allowed value of ± 5% imposed by Romanian Pharmacopoeia X-th Edition, according to European and International Standards.
Performed statistical analysis has confirmed the experimental obtained results. Relative standard deviation RSD = 26.268 % was within the normal range of values, below maximum range of accepted values (30-35%).. Standard error SE = 0.214476, was found to be within the normal limits, for a confidence level of 0.551328. Standard Deviation SD = 0.525357. and Sample Variance SV = 0.276 were also in normal range of values.
Statistical P parameter P = 7.44.10 -6 has reported a very small value. It has been observed that P< 0.001, so all the experimental obtained results has shown highest statistical significance.
The iodometric dosing method can be successfully applied in quantitative analysis of ascorbic acid from different samples.