Optical Sensing for a Changing Planet. Smart Cuvettes!

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1 Optical Sensing for a Changing Planet Smart Cuvettes! phuvettes for optical ph measurements foxyvette for optical po 2 measurements phoxyvettes for both ph and po fax info@spectrecology.com The phuvette is an optical ph sensor embedded in the wall of a standard 1 cm cuvette. The sensor is a proprietary nano-material that traps bromcresol green ph indicator ions in a hydrophilic sol-gel cage. The phuvette can be used with any spectrophotometer to provide accurate stable measurements of the ph of the fluid contained in the cuvette. Bromcresol green colorimetric indicator dye embedded in sol gel matrix Useful ph range is 5 < ph < 9 Can be used with any VIS spectrophotometer (measurements are made at 445, 618 and 750nm) Typical precision at pk OD = ph units Long lasting patches last months in water. However, strong oxidizers will bleach the patch. SC-PH-CVFL SC-PH-VIS1M SC-PH-VIS1M-SAM Item code description price SC-BLANK-VIS1M-SAM BCG patches in quartz cuvette, 3.5ml volume, 4 clear sides, Teflon stopper, 1 cm path, patch centered at 15mm (standard z height) BCG patch in polystyrene cuvette with cover, 2 clear sides, 4.0ml volume, patch centered at 15mm (standard z height). Pack of 100 (one is a blank) BCG patch in polystyrene cuvette with cover, 2 clear sides, 4.0ml volume, patch centered at 15mm (standard z height). Blank patch in polystyrene cuvette with cover, 2 clear sides, 4.0ml volume, patch centered at 15mm (standard z $ each $ pack of 100 $20.00 each $10.00 each

2 SP-pH height). Used as a reference for absorbance measurements Smart patches for ph. ¼ diameter self adhesive patch, can be installed on any clean clear surface. 6 per pack (5 sensor patches and 1 blank) $59.00 Application Notes The phuvette is a standard 1cm square cuvette designed for absorbance spectrophotometers, with a special sensing patch fixed to the inside wall. The patch is a special hydrophilic sol gel doped with a ph indicator dye bromcresol green. The large dye molecules are free to interact with hydronium ions, which readily diffuse into and out of the sol gel, but are trapped by the cage-like structure of the sol gel. The patch behaves much like a dye in solution, changing color in response to the ph of the sample (see absorbance plots below). A spectrophotometer is used to quantify the color change. The ph is calculated from the change in absorbance of the unprotonated and protonated forms of the dye molecule. Absorbance phuvette Titration wavelength (nm) ph 1 ph 2 ph 3 ph 4 ph 5 ph 6 ph 7 ph 8 ph 9 ph 10 ph 11 ph 12 model

3 The indicator anion (abbreviated A - ) exists in two forms that are in equilibrium with hydronium ions: (1) H + + A - = HA The A - species (immobilized bromcresol green) has a strong absorbance with a peak at 618nm At high ph values all of the ph. The equilibrium equation for equation (1) expressed as logs is: (2) ph = pk + log (A - )/(HA) There several methods to use the spectrophotometric data obtained from the patch to calculate the ratio (A - )/(HA). The easiest is based on the values of absorbance at the absorbance peak of the unprotonated form: Method 1 1. The absorbance of the phuvette film is measured against a blank film (the substrate without bromcresol green). Use the blank cuvette filled with water, or a sample blank (if the samples are colored or turbid, this will remove that effect). 2. Fill the phuvette (with the BCG patch) with ph=12 buffer, or with 0.01 M NaOH solution. Record the spectra (or alternatively the absorbance values at 618nm and 750nm). The value at 750nm is used to correct for any baseline shifts, including cuvette to cuvette differences between samples and blanks. Calculate the maximum absorbance possible at 618nm (Amax) as: (3) A max = A 618 A Rinse the phuvette with DIW, and then introduce the samples. Allow the samples to come into equilibrium (response is usually complete in a few seconds, but this depends on mixing and ionic strength of the samples. The system is in equilibrium when the absorbance values are stable). 4. Record the spectra (or absorbance values at 618nm and 750nm. Calculate the sample absorbance as: (4) A sam = A 618 A 750

4 Calculate the sample ph as: (5) ph = pk + b * log(asam/amax)/(1-asam/amax) The values of pk and b are determined experimentally by measuring the spectra of the film at 2 or more ph levels. The spectra presented above were obtained using NIST dilute buffers. The data for ph = 5.00, 6.00, 7.00, 8.00 and 9.00 were used to calculate pk and b from regression analysis. ph vs log(a/ha) of BCG patch ph y = x R 2 = log(asam/amax)/(1-asam/amax) The pk and slope are properties of the immobilized indicator and are expected to be stable. These values are provided with each lot of BCG patches at one temperature. Please note that the ph of samples, buffers and the pk and slope of the BCG patch are all temperature sensitive. For accurate answers, the calibration and sample analysis must be performed at a constant temperature.

5 Method 2 Method 1 provides the best sensitivity, as the absorbance change with ph is maximized at 618nm. However, bleaching or leaching of dye could occur, especially with long duration experiments. Method 2 gets around this issue by looking at the ratio of two wavelengths. Method 2 is also recommended if it is difficult to measure the same portion of the film for each sample, for example if the patches are installed in bottles, flasks or other oddly shaped vessels. Inspection of the spectra presented above shows that at 618nm, virtually all of the absorbance is due to the A - ion. In contrast, the absorbance at 449nm includes contributions from HA and A -. The ratio of absorbance at 449 to that at 618 (R) is related to the concentrations (A) and (HA) and the extinction coefficients of these species (e) (6) R = { 449 e A (A)+ 449 e HA (HA)}/ 618 e A Equations (1), (2) and (6) can be combined to give ph as a function of the measured absorbance ratio, R, and easily determined ratios of extinction coefficients, the pk. (7) ph = pk + log{( 449 e HA / 618 e A )/(R e A / 618 e A )} The extinction coefficient ratios are calculated from the absorbance values at ph = 12 and ph = 1. These ratios are provided with each batch of sensor patches. However, they should be checked as absorbance values depend on the sampling optics and linearity of your spectrophotometer (absorbance values among different styles and brands of spectrometers typically vary by as much as 5%). Method 3 The absorbance spectra presented so far represent as close as possible the absorbance due to the immobilized indicator only ( true absorbance). The absorbance of the sol gel and patch are removed by using a separate cuvette with a blank or dye free substrate. This technique is subject to errors from the extra handling of a separate cuvette and differences between the blank cuvette and sample cuvette. Another approach is to use just one cuvette. The cuvette filled with ph=1 buffer is used as the blank. The resulting sample spectra (absorbance difference spectra) are equal to the true spectra minus the spectra at ph = 1.

6 BCG Titration: Absorbance vs Absorbance at ph delta A wavelength (nm ph 1 ph 2 ph 3 ph 4 ph 5 ph 6 ph 7 ph 8 ph 9 ph 10 ph 11 ph 12 The absorbance difference curve is zero at ph = 1. As ph rises, appearance of A - causes the absorbance to increase at 618nm, while disappearance of HA causes the absorbance to go negative at 449nm. The extinction coefficient of HA is essentially zero at 618nm, and the data can be analyzed with equation (5). The ratiometric method (equation 7) is not valid for this type of data.