Measurement of pH in the eye

Fig. 6.3.11 Chromatography of an extract of the eye light organs of Symplecto-teuthis luminosa on a column of Superdex 200 Prep (1x27.5 cm) in 20 mM phosphate buffer, pH 6.0, containing 0.6 M NaCl, at 0°C (monitored at 280 nm). Each fraction (0.5 ml) is measured for the initial intensity of H202/catalase-triggered luminescence and the content of dehydrocoelenterazine measured as coelenterazine after NaBH4-reduction 1LU = 6 x 108 photons.

Fig. 5.10 Voltage-time curve (solid line) for an n -type silicon electrode (3 mficm) anodized with a constant current density of 6.25 mA crrf2 for t> 0 (sample at OCP for t<0) in 0.3 mol kg- NH4F (pH = 3.5). The thickness of the anodic oxide was measured by ellipsometry (open circles, broken line fitted as a guide to the eye). The etch rate of the anodic oxide in the electrolyte was measured (values above arrows) at different...

The curves represent the intraocular pH over time measured on real rabbit eyes that are burnt with 50 pL of 2M NaOH for 20 min and on the cornea for 20 s with a filter paper. The rinsing solutions were applied for 15 min with a flow of 66 mL/min. All curves represent five repeated experiments. Original data were published by Rihawi et al. in 2005. There is a considerable later onset of pH rise with low osmolarity rinsing and a lower extent of pH rise with tap water rinsing and an earlier onset of pH rise in all solutions containing ions like saline, Diphoterine , phosphate buffer, and RINGER lactate. 

Acidity is measured by the concentration of hydronium ions using a measure called pH. We have become accustomed to seeing references to pH in advertising, product labeling, and swimming pool maintenance, for example. Shampoos are said to be pH-balanced, which certainly sounds like something that we would want. It means that the pH of the shampoo is the same as the pH of your scalp. As you teach yourself about pH in this section, keep an eye out for the mention of pH as you shop and watch television. 

Accuracy of colorimetric measurements. An accuracy of 0.06-0.1 pH unit is possible in routine work when buffer solutions differing by 0.2 pH are employed. Estimates to 0.01 unit, as is customary when measuring the pH of blood serum, are significant only if the pH difference between comparison solutions is 0.05-0.1 pH unit. The attainment of such precision is facilitated by the colorimeter or spectrophotometer, although an experienced worker may obtain equal precision with the naked eye. In no event should the accuracy of the measurement be exaggerated, for a number of other factors influence the results (differences in ionic strengths of unknown solution and buffer). An accuracy... 

The osmolality of the tear film equals 310-350 mOsm/kg in normal eyes and is adjusted by the principal inorganic ions Na K+, CD, HC03 , and proteins. The mean pH value of normal tears is about 7.4. Depending on age and diseases, values between 5.2 and 9.3 have been measured. 

A 4.1 % w/v solution of pilocarpine hydrochloride is iso-osmotic (293 mosmol as determined by measuring its freezing point depression). A 2 % wA pilocarpine hydrochloride thus contributes to about 50 % of the iso-osmotic value (the osmolarity as determined at pH 6 is 147 mosmol). The other 50 % should be provided by the excipients. Iso-osmotic stock solutions (see also Sect. 10.7.1) for the preparation of eye drops could be very useful for the purpose of easy calculation. The stock solution Boric acid-benzalkonium solution FNA (see Table 10.10) is nearly iso-osmotic (boric acid is iso-osmotic at a concentration of 19 mg/mL). The contribution to the osmotic value of the benzalkonium chloride 100 mg/L is too small and can be neglected in the calculations. Without adjusting the pH, 50 % v/v of Boric acid-benzaUconium solution would be needed. But because the stability of pilocarpine is optimal at pH 6.5, the pH is adjusted with 3.75 mg/mL borax to 6.5. As 3.75 mg/mL borax contributes 15 % to the osmotic value, 35 % is left for the Boric acid-benzalkonium solution. 

Fig. 2.35 Force measured between a flat silica surface and a silica sphere (R = 2.2 pm) in the presence of 1.5 vol% Ludox spheres (radius 11 nm) as depletants at pH 5.6. The ionic strength was 0.76 mM. Redrawn from Piech and Walz [80]. Curve guides the eye...

Figure 2. A direct comparison of the coefficient of friction (y-axis on the left-hand-side) and the lubricant film thickness (y-axis on the right-hand-side) for both polymer-free and PLL(20)-g[3.4]-PEG(2)-containing HEPES buffer solution, measured by MTM and ultra-thin-fihn interferometry. Squares are for the coefficient of friction and circles for the lubricant film thickness. The lines between data points serve as a guide for the eye. Ball = stainless steel (19 mm in diameter), substrate = silica, buffer solution = 10 mM HEPES (pH 7.4), polymer concentration...

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