PH of tears

Buffer solutions are widely used in pharmacy to adjust the pH of aqueous solutions to that required for maximum stability or that needed for optimum physiological effect. Solutions for application to delicate tissues, particularly the eye, should also be formulated at a pH not too far removed from that of the appropriate tissue fluid, as otherwise irritation may be caused on administration. The pH of tears lies between 7 and 8, with an average value of 7.4. Fortunately, the buffer capacity of tears is high and, provided that the solutions to be administered have a low buffer capacity, a reasonably wide range of pH may be tolerated, although there is a difference in the... 

Ideally, every product would be buffered to a pH of 7.4, considered the normal physiological pH of tear fluid. The argument for this concept is that the product would be comfortable and have optimum therapeutic activity. Various experiments, primarily in rabbits, have shown an enhanced effect when the pH was increased, owing to the solution containing a higher concentration of the nonionized lipid-soluble drug base, which is the... 

Co-administration of ofloxacin and chitosan in eyedrops increased the bioavailabUity of the antibiotic [290]. Trimethyl chitosan was more effective because of its solubility (plain chitosan precipitates at the pH of the tear fluid). On the other hand, N-carboxymethyl chitosan did not enhance the corneal permeability nevertheless it mediated zero-order ofloxacin absorption, leading to a time-constant effective antibiotic concentration [291]. Also W,0-carboxymethyl chitosan is suitable as an excipient in ophthalmic formulations to improve the retention and the bioavailability of drugs such as pilocarpine, timolol maleate, neomycin sulfate, and ephedrine. Most of the drugs are sensitive to pH, and the composition should have an acidic pH, to enhance stability of the drug. The delivery should be made through an anion exchange resin that adjusts the pH at around 7 [292]. Chitosan solutions do not lend themselves to thermal sterilization. A chitosan suspension, however. 

These are used to hydrate the surfaces of hard lenses after disinfection. Since they must also cope with chance contamination, they mnst contain a preservative as well as a wetting agent. They may be isotonic with lacrimal secretions and be formulated to a pH of about 7.2 for compatibility with normal tears. 

Alkyl monoesters of poly(vinyl methyl ether-maleic anhydride) (PVM-MA) are bioerodible acidic polymers that are used to control drug release. In biological fluids with poor buffering capacity, drug release from the polymers and their dissolution are slowed owing to the lower pH on the polymer surface. We studied whether the release of timolol from matrices of monoisopropyl ester of PVM-MA in vitro and in vivo in rabbits eyes could be affected by disodium phosphate in the matrices. Addition of disodium phosphate to the matrices doubled the release rate of timolol in vitro, but it did not affect the bulk pH of the dissolution medium. On the basis of the timolol concentrations in the tear fluid and in systemic circulation, disodium phosphate seems to accelerate drug release in vivo also. Disodium phosphate probably affects the rate of dmg release by increasing the microenvironmental pH on the polymer surface. 

The pH of the normal tears has been determined by many investigators to have a mean value of 7.4, but there are wide variations. It is lowest on awakening as a result of acid by-products associated with relatively anaerobic conditions in prolonged lid closure and increases because of loss of carbon dioxide as the eyes open. The tears are more acid in contact-lens wearers due to the impediment of the efflux of carbon dioxide, and more alkaline in the case of diseases such as dry eye, severe ocular rosacea and lacrimal stenosis. 

Cellulose acetate phthalate forms a pH-triggered phase transition system, which shows a very low viscosity up to pH 5. This system will coacervate in contact with the tear fluid (pH 7.4), forming a gel in few seconds and releasing the active ingredients over a prolonged period of time. The half-life of residence on the rabbit corneal surface was approximately 400 seconds compared to 40 seconds for saline. However, such systems are characterized by a high polymer concentration, and the low pH of the instilled solution may cause discomfort to the patient. 

Flarada M. 1977. Photochemical smog and tear fluid. [Effects [of smog] on pH, volume, and lysozyme activity of tear fluid.] Nipon Ganka Gakkai Zasshi 81.215- 286. (Japanese)... 

Buffer Capacity and pH The normal pH of the tear fluid is 7.4. Ocular formulations should ideally be formulated between pH 7.0 and 7.7 to avoid irritation of the eye [31], However, in most cases the pH necessary for maximal solubility or stability of the drug is well outside this range. The tear fluid has only a limited buffering capacity, which is mainly due to the dissolved carbon dioxide and bicarbonate. It is therefore recommended to formulate using buffers with a low buffering capacity to allow the tears to regain their normal pH more rapidly [31],... 

The pH of the tears is approximately 7.4, and the tear layer contains small amounts of protein, including lysozymes, lactoferrins, gamma globulins, and other immune factors. The tears are primarily responsible for supplying the oxygen requirements of the corneal epithelium. 

Buffers. The normal tear pH of 7.5 depends on bicarbonates and, to a lesser degree, proteins, phosphates. 

Ophthalmic Sterile, isotonic, pH close to that of tears viscosity builder Eye treatment... 

The ocular bioavailability of a topically applied drug (i.e. the percentage of effective drug available at the ocular surface or in the anterior segment) depends upon a number of factors. The acidic dissociation constant (pRa) of the drug and the pH of the vehicle determine the differential solubility of the drug on the ocular surface and its intraocular bioavailability. Increasing the concentration of buffer in ophthalmic solutions extends the time taken for the tear film to return... 

Marked deviation of pH and tonicity from that of tears provokes reflex lacrimation, diluting the drug and accelerating its elution from the eye. H) ertonic solutions may be further diluted by fluid drawn from subconjunctival tissues. Animals will tolerate a pH range of 3.5-10.5 and tonicity from 0.5% to 2% sodium chloride concentration equivalents (Miller 1992). 

Which is more basic, a soap solution with a pH of 10.0 or human tears with a pH of 7.4 ... 

Dr. Emanuel Revici s approach is described in a letter from Harold J. Kristal, DDS, based on meeting Dr. Revici s niece Elena Avram, who is carrying on his work, and on William Kelly Eidem s book The Doctor Who Cures Cancer. Dr. Revici s analysis is aimed at striking an optimum balance between anabolic and catabolic cellular processes, the one building and the other tearing down. Called metabolic typing, the pH of the blood can be used as a marker, 7.46 being called the ideal, and which can be adjusted by nutrition. 

Atropine itself is a relatively strong base with a pfc, at 15-20° C. of about 10 (31), i.e., its mineral salts are about 99.9 % ionized at pH 7. However, when an atropine salt is applied locally to the eye, it mil be somewhat less well ionized, because the eye temperature will be near 37° C. and bases become weaker as the temperature rises, and because tears, which normally bathe the cornea, are slightly alkaline (pH 7-8), so that we shall probably not be far wrong in assuming that in the eye about 99% of atropine is present as atropinium cations. If un-ionized atropine base is able to penetrate cell membranes much more easily than the atropinium cation, and only 1 % is present as such, we should expect absorption to be slow, and, as already mentioned, atropine does not achieve its full effect in the human eye under an hour. In conformity with this idea, alkalization of atropine salt solutions increases both the speed and the intensity of their mydriatic activity. Thus, whereas a 1 10,000 solution of atropine sulfate produced no effect in a cat s eye in 10 min. and a barely perceptible dilatation in 50 min., a similar solution made 0.5 N with respect to sodium bicarbonate began to act in 10 min. and had produced full dilatation in 50 min. (32). The alkalinized solution was judged by indicators to have a pH of 8-9, so that the concentration of un-ionized atropine was probably increased about tenfold by the addition of sodium bicarbonate. 

If the offending substance is an acid or a base, check the pH of the victim s tears after irrigation and continue irrigation if the pH remains abnormal. 

Aqueous ear drops intended for the middle ear should have a pH between 6 and 8. However a different pH may be needed because of stability, efficacy or tolerability of the active substances. When the pH of the vehicle is not 7.4, the buffer capacity should be as low as possible. These requirements are similar to those for eye drops, but they are more strict, as there is no physiological correction by tears. Thus, formulations of eye drops are not always suitable for the middle ear. 

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