Ions in solution interact with each other and with H2O molecules. In this way, ions behave chemically like they are less concentrated than they really are (or measured). This effective concentration, which is available for reactions, is called activity:
γi corrects for electrostatic shielding by other ions; hence, γi depends on the ionic strength (i.e. the concentration of electrical charge). There are several approaches to calculate the activity coefficients.
Here, ci and zi are the molar concentration and the charge of ion i. The sum is taken over all ions in the solution. Due to the square of zi, multivalent ions contribute particularly strongly to the ionic strength. [Note: In literature the ionic strength, I, is also abbreviated by the Greek symbol μ.]
Organic chelating agents, phosphates, and polyelectrolytes were studied to determine their calcium ion binding power (CBP) and binding mechanism. A calcium ion electrode was used to measure the equilibrium calcium ion concentrations from which stability constants were calculated. Except for EDTA and NTA, the binding power did not correspond to the formation of complexes with a simple stoichiometry. In most cases, more than one equilibrium is involved. Increase in the level of binding agent decreased CBP, especially for those agents which formed complexes with low stability constants. CBP calculated on the basis of weight should be used to measure the relative cost effectiveness of commercial calcium binding agents. At an amount lower than required to form stable stoichiometric complexes, poly(acrylic acid) had a higher CBP than traditional organic chelating agents. The electrostatic effect accounts for the excellent CBP of poly(acrylic acid) at low concentrations. To get maximum efficiency in detergent and water treatment applications, use of low levels is recommended. aPresent address: The Clorox Co. Technical Center, Pleasanton, CA 94566.
The differences in free fluoride: total fluoride ratio between analyzed mouthwashes reveal a need to develop a method for evaluation of free fluorides in mouthwashes for proper updating of national and international guidelines.
Mouthwashes are additional products for oral hygiene. Most of the currently available mouthwashes can be divided into four groups, depending on their therapeutic action: (1) mouthwashes reducing oral malodor; (2) mouthwashes decreasing dental plaque formation due to antibacterial action; (3) mouthwashes containing various concentrations of fluorine compounds and able to affect the mineralization of dental hard tissue; and (4) mouthwashes preventing or reducing gingivitis [12,13,14,15,16,17].
Mouthwashes containing fluorine compounds are divided into groups, depending on fluoride ion concentration: 0.05% sodium fluoride solutions can be used daily; and 0.2% sodium fluoride solutions are recommended for weekly or fortnightly application .
Rošin-Grget et al. reviewed various theories of the cariostatic action of fluorine. According to one of these theories, fluoride ions penetrate into the lattice of hydroxyapatite, Ca10(PO4)6(OH)2, resulting in the formation of fluorohydroxyapatite, Ca10(PO4)6(OH) F, which is more resistant to acids .
It should be noted that the equilibrium shift in both chemical processes is determined by the activity of fluoride ions in the oral fluid, which in turn depends on the concentration of free fluoride ions in the applied prophylactic and therapeutic preparations. Various ingredients of mouthwashes, such as colourants, flavouring agents, sweeteners, preservatives, and surfactants, can chemically bind fluoride ions. Such bound fluoride is not effective in preventing and reducing dental caries. Thus, the control and the determination of «active» fluoride can determine the quality and, hence, the therapeutic efficacy of mouthwashes.
The quality of the second type of preparations is in many cases determined only by the technical specifications in manufacturing and may not provide for the quantitative determination of the active substances in the final product.
To determine total fluoride concentration in mouthwashes, the method of standard addition was applied. Samples of mouthwashes were diluted in total ionic strength adjustment buffer (TISAB), which sets constant ionic strength during measurement and which also precomplexes interfering ions (e.g., aluminium ions Al3+). After the measurement of potential of each diluted sample a known amount of standard was added and the measurement was repeated again for further calculation of difference in the potentials and quantification of total fluoride. Since the results of quantitative determination of fluoride ions on standard mixtures prepared with aqueous solution and Albavit solution were slightly differentiated (i.e., the results obtained in the first case were systematically lower compared to those obtained theoretically, whereas in the second case the results were found to be higher), we attempted to modify the procedure by using the method of double increase in concentration without preliminary dilution with TISAB solution in order to determine possible binding of fluoride ions by components contained in the mouthwashes. Every sample of the mouthwash was measured five times and the mean of the measurements was used for further statistical estimation.
Significant differences in concentrations determined by the method of standard addition and by the modified method of quantitative assessment were observed for such solutions as Oral B®, Reach®, and Foramen®. The fact that an increase in total fluoride content was observed with the modified method may be explained by a change in the ionic strength of the solution during the measurement; or by partial binding of fluoride ions by certain components of the mouthwashes; or possibly by polymeric compounds such as poloxamers and polyphenylene oxides. Analogous observations were made for the Sensodyne® solution, which does include polymeric compounds. This assumption is supported by the fact that in standard solutions prepared with Albavit solution containing polymeric compounds, the determined concentration was systematically higher than the theoretical concentration; whereas systematically lower results were obtained with model solutions in the procedure of fluoride ion determination using TISAB solution.
In those preparations lacking polymeric compounds, results of fluoride ion determination were identical at the constant ionic strength and modified method within the error of experiment. To confirm these assumptions, we determined concentration and activity of the test solutions following 10-fold dilution. The behavior of investigated values of activity and concentration also differs in both groups of preparations: while concentration for the preparations not containing polymeric compounds showed on average a 10-fold change following dilution, the 10-fold dilution of preparations containing polymeric compounds resulted in an 18-to-20-fold change of these values.
These trends are not observed for mouthwash containing AmF (Lacalut Sensitive®) as a source of active fluorine, even though it contains polymeric compounds (polyethylene glycol) in its formulation; and the mode of change of both activity and concentration following dilution corresponds to that of mouthwashes that do not contain polymeric compounds. It is noteworthy that after each measurement of AmF mouthwash the electrode continued retaining the potential when being immersed in distilled water and even required additional washing or mechanical cleaning of membrane surface. Therefore, one could presume that high free fluoride to total fluoride ratio (94%) could be due to adsorption of AmF on the surface of the electrode membrane.
The determinable concentrations are almost identical for the Colgate Total Plax® mouthwashes, which only vary by type of coloring agent. However, for the Colgate Total Plax (Classic Mint®) mouthwash, the decrease in the activity of fluoride ions can be explained by the difference in the type of colorant added at approximately the same concentration of total fluoride. Therefore, it is reasonable to assume that the coloring agent added to Colgate Total Plax (Classic Mint®) mouthwash could bind free fluorides.
The analysis of the results, obtained for different series of the same mouthwash, revealed differences in the content of active fluorine for the Oral B® rinses and almost identical values for the Reach Fresh mint® solutions. The comparative analysis of test solutions containing NaF or AmF as a source of fluoride ions showed that there is no significant difference between the measured values of active fluorine and those calculated theoretically, in both procedures. Thus, both organic and inorganic sources of fluoride have similar potential against tooth decay, which can be confirmed by clinical data .
The present study has some limitations since it does not reproduce in vivo conditions, i.e. dilution by saliva and influence of saliva components on the release of fluoride ions. It is known that phosphatase, one of saliva components, hydrolyzes Na2PO3F, releasing fluoride ions [25, 32]. Thus, it may be necessary to add components reproducing artificial saliva during sample preparation in order to avoid underestimated results when measuring Na2PO3F mouthwashes. The method of potentiometry itself does not account for the influence of mouthwash components on the work of the electrode.
Mouthwashes are oral hygiene products aimed at enhancing the remineralization process and reducing demineralization due to the presence of fluoride ions; however, mouthwashes should also contain an appropriate amount of free fluorides to provide bioavailability. Ten different mouthwashes analyzed in the present study they revealed various free fluoride:total fluoride ratios due to binding of fluoride ions by mouthwash components or ability of fluoride source itself to form its complexes. The lower concentration of free fluoride in comparison with total one could lead to a decrease in the caries-preventive effect. However, the methods to quantify free fluoride have some disadvantages since they do not reflect in vivo conditions and, therefore, may result in distorted or underestimated values of fluoride content. Thus, there is a need to develop a method for evaluation of free fluorides in mouthwashes for proper updating of national and international guidelines. 2b1af7f3a8