Sensor for pH DeGrandpre.pdf

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A sensor for in situ indicator-based measurements of seawater pH

Matthew P. Seidel a,1, Michael D. DeGrandpre a,, Andrew G. Dickson b

aDepartment of Chemistry, The University of Montana, 32 Campus Drive, Missoula, Montana 59812, United Statesb Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States

Received 12 June 2007; received in revised form 6 November 2007; accepted 28 November 2007

Available online 15 December 2007

Abstract

Indicator-based spectrophotometric pH methods are now proven and commonly used for analysis of ocean samples; however,

no autonomous system for long-term in situ applications has been developed based on this method. We describe herein an

autonomous indicator-based pH sensor for seawater applications adapted from a design originally developed for freshwater pH

measurements (SAMI-pH). The new SAMI-pH uses a different pH indicator, flow cell design, detection system, and mixing

configuration to improve upon the freshwater performance. A new method was also tested that utilizes an indicator concentration

gradient in the sample to correct for the pH perturbation caused by the indicator. With these design changes, laboratory tests found

the precision improved from 0.004 to 0.0007 and the accuracy improved from 0.0030 to +0.0017 based on comparisons with

benchtop UV/Vis measurements. In situ testing of two SAMI-pH instruments was completed off the pier at Scripps Institution of

Oceanography. The average pH offset between the two instruments over the 22 d deployment period was 0.00420.0126 ( n =883),

with the precision primarily regulated by large spatial and temporal variability at the site. The results demonstrate that the SAMI-

pH can provide drift-free and precise pH measurements in adverse measurement conditions (extensive fouling and large tidal

variability). With the current battery power (18 alkaline D-cells), the system can be deployed for periods up to 2 months with a

0.5 h measurement frequency.

2008 Elsevier B.V. All rights reserved.

Keywords: pH sensor; in situ; autonomous; spectrophotometry; inorganic carbon; marine chemistry

1. Introduction

Quantifying and understanding the impacts of oceanic

uptake of anthropogenic CO2 is an important contempor-

ary problem that has brought intense focus on the ocean

inorganic carbon system. The rapid rise in atmospheric

CO2 has increased the concentration of CO2 in the oceans

(Peng et al., 2003). Surface water pH has already

decreased by

0.1 units and the continued uptake ofCO2 is expected to decrease surface ocean pH by 0.20.3

units over the next century (Orr et al., 2005). A decrease in

pH of this magnitude could have large-scale impacts on

marine ecosystems (Orr et al., 2005), will result in

decreased uptake of CO2 by the ocean (Sabine et al.,

2004), and will affect burial rates of organic and inorganic

carbon (Feely et al., 2004). Understanding the inorganic

carbon cycle is also key to understanding and predicting

net community metabolism, airsea gas exchange, and

export of carbon from the surface to depth. New and

Available online at www.sciencedirect.com

Marine Chemistry 109 (2008) 1828www.elsevier.com/locate/marchem

Corresponding author. Tel.: +1 406 243 4118; fax: +1 406 243 4227.

E-mail address: [email protected]

(M.D. DeGrandpre).1 Tel.: +1 406 243 4118; fax: +1 406 243 4227.

0304-4203/$ - see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.marchem.2007.11.013
mailto:[email protected]://dx.doi.org/10.1016/j.marchem.2007.11.013http://dx.doi.org/10.1016/j.marchem.2007.11.013mailto:[email protected]


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improved autonomous techniques are needed that can

collect high quality, high temporal resolution CO2 data for

characterizing both long and short term changes in the

marine inorganic carbon system.

Seawater pH has become a more appealing inorganic

carbon parameter because improvements in the spectro-photometric method have made it consistent with the

other CO2 system parameters (e.g., Byrne and Breland,

1989) and it is readily measurable on ships with commer-

cially available instrumentation. From our standpoint,

another advantage is that spectrophotometric pH methods

are, ostensibly, more easily adapted for in situ measure-

ments. If pH is used in inorganic carbon calculations, it is

highly desirable that the calculated dissolved inorganic

carbon (DIC), total alkalinity (AT) orpCO2be comparable

in precision and accuracy to measured values, i.e.,

1.5 mol kg

1

(Byrne et al., 1999), 2.0 mol kg

1

(Dickson et al., 2003),and1atm (Murphy et al., 1998),

respectively. To illustrate the sensitivity of calculated DIC

andpCO2 to pH uncertainty, we used the CO2 equilibrium

model CO2SYS (Lewis and Wallace, 1998). The carbonic

acid constants in Mehrbach et al. (1973) refit by Dickson

and Millero (1987) were used because these constants

give the most accurate calculations of inorganic carbon

parameters from spectrophotometric pH measurements

(Clayton et al., 1995; Byrne et al., 1999). Calculations

were made using a constant alkalinity of 2250 mol kg1

over the pH range 7.78.3. For a pH uncertainty of

0.005, the error in DIC ranges from 2.03.5 molkg1, lowest at the lower pHs typical of deep water. If the

AT precision stated above is included in the calculation,

the pH accuracy needs to be 0.002 to estimate DIC

within 1.5 mol kg1. This level of uncertainty is

adequate for most applications. Similar calculations for

pCO2 reveal that the pH error is more critical at lower pH

values wherepCO2 is higher. A pH uncertainty of 0.002

gives a pCO2 error of 5.0 atm at pH 7.7 (pCO2=980

atm) and an error of 1.1 atm at pH 8.3 (pCO2=182-

atm). Previous shipboard studies have obtained pH

accuracy (which typically limits the pH uncertainty) from0.0010.005 and precision from 0.0010.0004 (Byrne

and Breland, 1989; Clayton et al., 1995; Tapp et al., 2000;

Bellerby et al., 2002).

Some work on in situ spectrophotometric pH sensors

has been undertaken. An instrument for obtaining vertical

depth profiles of pH and pCO2 from a ship has been

developed (Nakano et al., 2006). The instrument collected

data to 1000 m with accuracy of 0.0020.005 pH units

when compared to discrete samples. The SEAS pH

instrument, designed for in situ spectrophotometric pH

and other inorganic carbon measurements, was tested in

seawater and freshwater (Liu et al., 2006). Shipboard

depth profiles in the Equatorial Pacific and Gulf of

Mexico were collected. The in situ precision was 0.0014

while accuracy was not reported. Both the Nakano et al.

(2006) and Liu et al. (2006) designs require high power

(67 W) light sources, which limit the duration they can

be deployed in the field.We have also previously presented the design and

performance of an in situ spectrophotometric pH system,

optimized for freshwater measurements (Martz et al.,

2003). The Submersible Autonomous Moored Instrument

for pH, or SAMI-pH, accuracy and precision was 0.003

and 0.004 pH units (n = 16), respectively, when deployed

in a river over an 8 d period. No in situ indicator-based pH

sensors, however, have been deployed for extended

periods (weeks to months) in a marine environment. In

this paper, we describe the optimization of the Martz et al.

(2003) design for seawater applications. The new SAMI-pH uses a different pH indicator, flow cell design,

detection system, and mixing configuration to improve

upon the freshwater performance. A 22 d in situ seawater

pH time-series is presented and demonstrates that the

SAMI-pH sensor is capable of collecting accurate and

precise long-term in situ data on ocean moorings and other

unmanned platforms.

2. Principle of operation

A theoretical basis for the spectrophotometric deter-

mination of pH is given in Clayton and Byrne (1993)and readers should refer to that article and other articles

by Byrne et al. for a more in depth discussion. The

method is based upon the equilibrium reaction of a pH

indicator, usually a diprotic sulfonephthalein indicator,

HLpKa

H L2 1

where HL and L2 are the protonated and deprotonated

forms of the indicator and pKa is the second apparent

dissociation constant. The acidic H2L form is not present

at seawater pHs. The sulfonephthalein indicator m-cresolpurple (mCP) was selected because it has been exten-

sively used in past studies (Clayton and Byrne, 1993;

Clayton et al., 1995; Byrne et al., 1999; DOE, 1994).

Combining the log form of the indicator equilibrium

expression and Beer's Law yields:

pH pKa logR e1

e2 Re3

2

whereR is the absorbance ratioA2/A1 corresponding to the

peak absorbance wavelengths 2

and 1

of the L2 and

HL forms, respectively, and the eis are the temperature-

19M.P. Seidel et al. / Marine Chemistry 109 (2008) 1828


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dependent ratios of the molar absorptivities () of the two

species at both 1 and 2,

e1 HLe578

HLe434

e2 Le578

HLe434

e3 Le434

HLe434

: 3

For mCP, the s are measured at the peak absorbance

wavelengths for L2 (578 nm) and HL (434 nm).

The adaptation of this methodology for autonomous in

situ applications requires special considerations to retain

the accuracy and precision of the benchtop method. Most

lab measurements are performed at a fixed temperature,

typically 25 C (DOE, 1994). For in situ applications,

temperature is not controlled and the temperature de-

pendence of pKa and eis must be accurately known. The

eis also depend upon the detection system optical band-

pass and must be determined for that bandpass andliterature values cannot be used. In situ instruments, in

particular, may use a larger bandpass to increase light

throughput to reduce the light source power requirements.

Moreover, when using a short pathlength optical cell

(for reasons described below), the pH indicator solution

can significantly alter the sample pH thus requiring some

method to correct for the pH perturbation. Most benchtop

methods use 10 cm pathlength cuvets with5106 mol

kg1 indicator (e.g., Clayton and Byrne, 1993). At this

concentration, the magnitude of the perturbation is

typically b0.001 pH units (Chierici et al., 1999). Even

so, various methods have been used to account for the pHperturbation (Zhang and Byrne, 1996; Bellerby et al.,

2002). In the design presented here, we use a custom-

made 1.1 cm pathlength flow cell to maximize light

throughput and minimize the light source power require-

ments and flush volume. Light throughput with a 0.5 W

tungsten lamp gives sufficient signal-to-noise to achieve

the desired level of absorbance precision. From our ob-

servations, throughput drops off roughly proportionally topathlength, so a 1 W lamp is required to achieve the

same signal-to-noise for a 2 cm pathlength cell. The short

pathlength also reduces the pumping required to flush the

system completely for blank measurements. These power

savings allow for longer deployments, critical for an au-

tonomous system. However, because the 1.1 cm cell

requires indicator concentrations 10 times greater than

10 cm cells, the pH perturbation becomes significant. To

address this problem, a perturbationcorrection metho-

dology suggested by Martz et al. (2003) has been

thoroughly evaluated here, as described below.

3. Methods

3.1. Instrument modifications

Two instruments, SAMI-47 and SAMI-51, with 47 and 51

corresponding to their serial numbers, were manufactured by

Sunburst Sensors, LLC with modifications to the Martz et al.

(2003) design. The new design uses mCP instead of cresol red

indicator because the mCP response better matches the full pH

range of seawater. A commercially available static mixer (de-

scribed below) replaced a coiled tube mixer to more effectively

mix the indicator and sample. A custom made z-configuration

flow cell (Fig. 1) replaced the 90o flow path optical cell used in

the original SAMI design (DeGrandpre et al., 1995; Martz et al.,

Fig. 1. Instrument design for the seawater SAMI-pH modified from the SAMI-pH for freshwater ( Martz et al., 2003). NO and NC refer to thenormally open and closed valve settings. Dotted lines represent fiber optics.

20 M.P. Seidel et al. / Marine Chemistry 109 (2008) 1828


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2003) to reduce bubble trapping that occasionally occurred in the

90 flow cell.Because of the orientation of the inlet relative to the

optical channel, bubbles are more easily swept through the z-

configuration. To improve light throughput, the spectrograph

detection system was replaced with a fiber optic splitter con-

nected to three silicon photodiode detectors with discrete inter-ference filters (Yuan and DeGrandpre, 2006).

3.2. Flow design

The sample and indicator solutions are connected to the

normally open and normally closed ports, respectively, of a

three-way solenoid valve (161K031, Neptune Research Inc.)

(Fig. 1). A low power 50 l per pulse solenoid pump

(LPLA1210050L, The Lee Co.) draws sample or indicator

through the valve and dispenses it into the downstream

plumbing. The pump and valve are enclosed in a silicone oil-

filled housing with a diaphragm for pressure equilibration. The

indicator solution, described below, is contained in a 150 ml gas-impermeable bag (Pollution Measurement Corp.). The static

mixer, developed for HPLC, is a 350 l internal volume

convoluted flow path encased in a stainless steel housing (421-

0350B, Analytical Scientific Instruments). The connecting and

exit tubing consists of 1.6 mm o.d., 0.76 mm i.d. PEEK.

3.3. Optical design

Light from a 2.3 mm diameter 0.5 W tungsten lamp (4115-B,

Gilway Technical Lamp) is collected by butt coupling to a 0.37

numerical aperture 1000 m fused silica fiber (ThorLabs Inc.).

The fiber is fitted into the custom made PEEK z-cell (Fig. 1)using a nut (P-331, Upchurch Scientific Inc.) andferrules (P-359,

Upchurch Scientific Inc.). The cell pathlength is 1.1 cm with a

17.7 l internal volume. The fiber input and output ferrule ends

create a watertightseal by compressing Teflon gaskets onto fused

silica windows. The transmitted light is collected by another

1000 m fiber optic coupled to a 1 3 fiber optic bundle splitter

(FiberTech Optica Inc.) (Yuan and DeGrandpre, 2006). The light

from each bundle channel passes through a series of optical

filters, which isolate the three wavelengths of interest (434, 578,

and 780 nm). Measurements at 780 nm, where mCP has no

absorbance, serve as a correction for light throughput changes

between blank measurements. Additional filters are used tominimize out of band (stray) light. For detection at the 780 nm

referencechannel, two 780 nm bandpass filters (780-10-50; Intor,

Inc.) are used. At the two analytical wavelengths, optical

interference filters (435.8-10-45 and 577.7-10-50; Intor, Inc.) are

combined with heat absorbing filter glass (040FG11 at 434 nm

and 005FG13 at 578 nm, Andover Corp.). The combined filters

reduced stray light to b0.15% (Yuan and DeGrandpre, 2006)

which is critical for good absorbance and pH accuracy. All

interference filters have a 10 nm bandpass at full width half

maximum; however, the center wavelength can vary slightly

between filters with the same part number, requiring thateis (Eq.

(3)) be measured for each SAMI. The filtered light is detected bysilicon photodiodes (S2386-45K, Hamamatsu Corp). The

detector photocurrent is converted to voltage in an amplifying

circuit, digitized into 12 bits and stored using a data logger (TFX-

11, Onset Computer Corp.).

3.4. Indicator solution

The SAMI-pH and benchtop UV/Vis indicator solutions

are comprised of1.0103 mol kg-soln1 mCP (211761-

10G, 90% dye content, Aldrich) and 0.70 mol kg-soln1 NaCl

(S671-3, Fisher Chemicals) adjusted to a pH of 7.5 with

1.0 M HCl. The indicator concentration was tested over the

range 0.020.0005 mol kg-soln1 and it was found that

1.0103 mol kg-soln1 mCP gave the best balance between

pH precision and flush times (results not shown). The

influence of indicator pH is discussed below.

3.5. Measurement sequence

Each measurement begins by flushing 3.75 ml of sample(75 pump pulses) through the normally open (NO) port on the

valve (Fig. 1). This flushing is done to completely clear the

system of indicator from the previous measurement so a blank

measurement can be taken. Next, a 50 l pulse of indicator is

injected into the seawater sample through the normally closed

(NC) port on the valve (Fig. 1) followed by an additional 25

pump pulses (1.25 ml). Light intensity is recorded for each of the

25 sample pump pulses as the indicator peak flushes through the

optical cell. The first four of these measurements are prior to

arrival of the indicator and are averaged to obtain a blank value,

i.e., 100% transmittance. Optical blanks are therefore recorded

for every pH analysis rather than every 12 h as done in Martzet al. (2003). The standard deviation of the four blank mea-

surements is typically b0.0005 absorbance units. The full

measurement sequence requires four min to complete. The

SAMI 18 D-cell battery pack provides sufficient power for 2448

measurements or50 d for a 0.5 h measurement frequency, 100

d for a 1 h measurement frequency, etc.

3.6. Molar absorptivity ratios (eis)

To calculate the eis (Eq. (3)), molar absorptivities for the

acidic and basic forms of mCP were determined at both

434 nm and 578 nm for the SAMI-pHs and the reference

spectrophotometer (Varian Cary 300 Bio UV/Vis). A 0.68 molkg1 NaCl solution with 0.02 mol kg1 NaOH was used for

the determination of the basic L2 form of mCP. In this

solution, greater than 99.995% is in the L2 form. A solution

with pH of 5.5 was prepared for the HL form consisting of

0.018 mol kg1 sodium acetate, 1.0 mol kg1 HCl, and

sufficient NaCl to set the ionic strength at 0.70 mol kg1

(Clayton and Byrne, 1993). At pH 5.5, the presence of H2L,

which has a poorly documented pKa and cannot be accurately

quantified, is minimized. L2, which is present at pH 5.5, was

accounted for by calculating its concentration and correcting

for its molar absorptivity contribution to the final absorbance

value (Martz, 2005). The correction was approximately 53%for HL578 due to the much larger L578. However, because



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HL578 is small, pH is relatively insensitive to errors in this

parameter (Table 1). Corrections for the other three molar

absorptivities were all less than 0.3% of the measured value.

It is critical to account for the ei temperature dependence

(Zhang and Byrne, 1996; Gabriel et al., 2005). For example,

using constanteis for a 10 C temperature change creates a pH

error of0.006 pH units. The molar absorptivity temperature

dependence from 525 C was determined for each SAMI and

the Cary UV/Vis. Replicate measurements were used to

determine the precision and resulting uncertainty in

calculated pH (Table 1). As can be seen in Table 1, uncertainty

of the limits the pH accuracy to 0.0010.002 pH units. The

measurement uncertainty is primarily controlled by indicator

solution preparation.

3.7. Indicator pKa

The pKa for mCP, determined on the total hydrogen ion

concentration scale (Clayton and Byrne 1993; DOE, 1994) is:

pKa 1245:69

T 3:8275 0:00211 35 S ; 4

where Tis temperature in Kelvin and S is salinity. The pKa in

Eq. (4) was determined for temperatures ranging from

293 KT303 K and salinities from 30S37. It should

be noted that in situ deployments may have temperatures

outside of this range. We used the van't Hoff equation to

extrapolate the pKa temperature dependence. The change in

entropy and enthalpy were calculated using the constants A, B,

and C reported by Millero (1995).

3.8. pH perturbation correction

A gradient of indicator passing through the flow cell is used

to correct for the indicator pH perturbation. The indicator and

sample mix as they move toward the flow cell, generating a

dilution curve (Fig. 2). The gradient of indicator absorbances is

recorded and the pH is calculated at each point in the dilution

curve using Eq. (2). The total indicator concentration [HL]T,

which is the sum of the two indicator species (Eq. (5)), is

calculated from absorbances and molar absorptivities using

Eqs. (6) and (7),

HL T HL L2

5

HL A434 Le578b A578 Le434b

HLe434 Le578b2 HLe578 Le434b

26

L2

A578 HLe434b A434 HLe578b

HLe434 Le578b2 HLe578 Le434b2:

7

Table 1

The temperature dependence of the mCP molar absorptivities

Species SAMI-47 SAMI-51 UV/Vis SAMI

precision

SAMI-pH

error

HL434 17104

26(t25)

16911

26(t25)

17340

26(t25)

52 0.0012

HL578 85 +

(t25)

110+

(t25)

79 +

(t25)

11 0.0010

L434 2301+

12(t25)

2238+

11.7(t25)

2151+

12(t25)

27 0.0009

L578 38040

71(t25)

37480

71(t25)

37973

71(t25)

170 0.0020

Molar absorptivity values are given in kg mol1 cm1 and temperature (t)

is in C. These values are only applicable to mCP dye lot 11517KC from

Sigma Aldrich. The average SAMI precision is calculated as the standard

deviation of replicate measurements (n =9) from both instruments. The

SAMI-pH error is estimated by adding or subtracting the SAMI precisionfrom the molar absorptivity value and recalculating the pH value.

Fig. 2. (Top) Absorbances calculated from intensity measurements taken

for 25 pump pulses as the indicator slug passes through the flow cell

(1 absorbance measurement obtained per pump pulse). Absorbances

between 1.5 and 0.2 absorbance units are used to calculate the pH

(symbols). Filled circles are A434 and open circles are A578. (Bottom) pH

versus total indicator concentration [mCP]T for the SAMI data from the

top figure (filled circles) and theoretical equilibrium model calculations

(open circles). A linear extrapolation through each data set gives an R2

value of 0.9998 for the SAMI data and N0.9999 for the pH perturbation

model. The y-intercept (7.9169) represents the perturbation-free pH.



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The are in kg-soln mol1 cm1 and pathlength (b) in cm. A

plot of pH versus [HL]T is extrapolated to zero indicator

concentration to determine the perturbation-free pH (Fig. 2).

Mixing occurs at both the leading edge and tail end of the

indicator slug as shown in Fig. 2. The leading edge of the

indicator pulse has only 23 absorbance values in the optimal

absorbance range (0.21.5 a.u.) (Ingle and Crouch, 1988);

however, the tail end of the dilution curve is more dispersed.

Approximately 9 points in this portion of the dilution curve are

used to create the perturbation correction plot (Fig. 2). Because

in this case the indicator pH is 7.5, the pH decreases with

increasing indicator concentration.

It was proposed in Martz et al. (2003) that a linear ex-

trapolation could be used to determine the pH with no indicator

present. We examined the linearity of the pH perturbation using

an equilibrium model. The model includes the CO2 and indicator

equilibrium expressions, equilibrium constants, the original DIC

andATof the sample and indicator, and dilution factors (calculated

from the total indicator concentration). The program calculates apH value for a range of points with varying indicator con-

centration and dilution factors. An example of a theoretical pH

perturbation curve is shown in Fig. 2. TheR2 value for a linear fit

to the theoretical data is N0.9999 over the [HL]T range used.

Linear relationships are also found over the entire pH range tested

(7.58.5) (not shown). Therefore, a linear fit is used with the

dilution curve data from each sample measurement (Fig. 2) to

determine the pH perturbation.

3.9. Laboratory tests

Laboratory pH measurements were made on certifiedreference materials (CRMs) (Dickson, 2001) or seawater

collected in Hood Canal, Washington (USA). To prevent CO2exchange during replicate analyses, samples were stored in

gas-impermeable bags (Pollution Measurement Corp.) and the

bags were connected directly to the input line of the sensor

(Fig. 1). All laboratory measurements were made at20 C.

Temperature was controlled to 0.05 C using a circulating

water bath (Thermo Neslab RTE 7) and a water chamber

placed over the top of the SAMI-pH. The sample bags were

also placed inside the chamber so that the sample and optical

cell of the SAMI-pH were at the same temperature. All tem-

perature probes and water baths were calibrated to a certifieddigital thermometer (4400, Eutechnics).

Comparison measurements were obtained using the Cary

double beam UV/Vis spectrophotometer. Dual 10-cm jacketed

cell holders were used for temperature control. The 10 cm

sample cuvet was mixed using a magnetic stirrer (Spectrocell,

Inc.). Each pH measurement took sample directly from the same

bag used for the SAMI-pH measurements. The outlet tube from

the bag was placed inside the cuvet and sample was allowed to

overflow the cuvet 10 volumes to insure the cuvet was

thoroughly rinsed. The cuvet was then placed inside the UV/Vis

and allowed to reach the desired temperature (5 min). Each

measurement used three 40 l additions of 1103 mol kg1

indicator with absorbance measurements taken for each addition.

As in the SAMI-pH, the resulting plot of pH versus total

indicator concentration was used to linearly extrapolate to a zero

indicator concentration.

3.10. Field tests

In situ testing of SAMI-47 andSAMI-51 wasperformed in a

seawater tank and off the SIO pier. Comparison measurements

were made on a Cary 1E UV/Vis spectrophotometer. Samples

were taken from the tank or pier in glass bottles with minimal

head space. Samples were first placed in a water bath for 20 min

set to the in situ water temperature and then a 10 cm cuvet at the

same temperature was filled with sample, minimizing the head

space. The cuvet was inserted into the Cary 1E for a blank

measurement and indicator was added as described above but

the cell containing sample and indicator was shaken by hand.

All discrete pH samples were measured immediately after

collection using the procedure described above.

Preliminary tests were made in a 100-gallon tank filled withseawater collected from a pipeline that extends 400 m into the

Pacific Ocean. A re-circulating pump was used to keep the water

uniformly mixed. The SAMI-pH instruments were programmed

to take measurements every 15 min. Discrete samples were

collected at 1 h intervals.

Immediately after these tests, the SAMI-pH instruments

were deployed from the SIO pier for a three week period.

Salinity, temperature, and depth were measured using a CTD

(Microcat, Seabird, Inc.). Discrete samples were collected for

DIC, AT, pH, and salinity. The SAMI-pHs were programmed

to measure pH every 30 min.

The SAMI-pH instruments went through a number ofpreparation steps in order to safely package them for deployment

off the pier. A copper cage was placed around the exposed SAMI

flow cell, fiber optics, temperature probe, and pump-valve

housing in order to protect the SAMI and provide fouling

resistance. The reagent bag was placed in a PVC box clamped to

the SAMI housing. The SAMI was thenplacedin a stainless steel

cage for protection and to provide solid anchor points. A 100-

foot serial communication cable was attached to the SAMI to

monitor the instrument performance. The deployment took place

from April 27, 2006 through May 25, 2006. Water depth at the

end of the pier is 8 m and the instruments were placed at 4 m

depth on opposite sides of the same piling (2 m apart), with anorth and south orientation to obtain the same exposure to east

west tidal and wave motion.

4. Results and discussion

4.1. Indicator pH

The pH perturbation is strongly dependent upon the

difference between the sample and indicator pH. To determine

the performance sensitivity to indicator pH, a 5.0103 mol

kg1 indicator solution was adjusted to pHs of 8.2, 8.0, 7.8, and

7.5 (0.1) using 1 M HCl or NaOH. Over this range, indicatorpH did not significantly influence accuracy and precision using



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the linear extrapolation method; however, the best fits (highest

R2) were found with the lowest pH solution (Table 2). Based on

these results, we continued to use an indicator solution pH of

7.5, the pH chosen during the early stages of testing. Although

it makes intuitive sense to use a pH close to the sample pH to

minimize the perturbation, the results here suggest relative

insensitivity to this parameter.

4.2. Flow cell gradient

Initially, we were concerned that the gradient of indicator

concentration in the SAMI-pH optical flow cell could lead to

systematic errors because Beer's Law assumes a homogeneous

concentration. The validity of this assumption with the 17 l

optical cell was examined. The absorbance gradient between two

consecutive 50 l pump pulses is estimated from data in Fig. 2.

We split thisabsorbance gradient into30 separate slices along the

flow cell and converted the absorbances to transmittance. A

corrected absorbance and pH were then calculated from theaverage of these thirty transmittance values. This procedure

estimates a +0.0004 pH unit error in the actual pH of the sample.

Therefore, the absorbance gradient is nota significant problemin

the SAMI-pH; however, it could be more important for systems

that use long (10 cm) pathlength cells such as liquid core

waveguides (Liu et al., 2006) if the sample and indicator are not

well mixed.

4.3. Laboratory precision and accuracy

In the Martz et al. (2003) design, precision was found to

be 0.004 pH units and was controlled by blank drift, lampstability, and microscopic bubbles. The new design precision

was quantified over a wide range of sample pH (Table 3). The

average precision is 0.0007 pH units (n =165). We attribute

the improved precision to reduced bubble trapping in the z-

cell, a broader mixing curve due to the static mixer, and

implementation of frequent blank measurements.

The accuracy for the two SAMI-pH instruments was

determined by analyzing Hood Canal seawater (S=31, t=25 C)

every 20 min for an 8-hour period followed by analysis of the

remaining sample on the Cary UV/Vis (n=7). The average pH

measured by SAMI-47 and SAMI-51 were 7.91930.0005 and

7.91590.0006, respectively. The average pH measured on the

Cary spectrophotometer was 7.9180 0.0008 resulting in an

accuracy, relative to the Cary, for SAMI-47 and SAMI-51 of

+0.0013 and 0.0021, respectively (n=7).

4.4. SAMI-pH in situ testing

During the initial in situ tests in the laboratory seawater tank,

the average offset between SAMI-47 and SAMI-51 for 45tankmeasurements was 0.0017 units over a pH range of 8.19

8.21 and temperature from 22.1 to 22.2 C (data not shown).

Benchtop spectrophotometric pH measurements agreed to within

+0.0005 for SAMI-47 and 0.0015 for SAMI-51 (n =12).

The data collected during the 22 d deployment off Scripps

pier are shown in Fig. 3. A total of 1005pH measurements were

made by each SAMI. The pH was calculated using the linear

extrapolation method described above and shown in Fig. 2.

Sample regression plots for SAMI-47 and SAMI-51 at two

different pH values are shown in Fig. 4. All in situ and sample

data were corrected to the same in situ temperature using a

temperature coefficient of 0.0015 pH units per 0.1 C

calculated using the salinity and DIC/AT found at the site. The

resulting pH ranged from 8.42 to 7.74 with changes as large as

0.6 pH units over diurnal and tidal cycles (Fig. 3).

The time-series show large tidal swings and associated

temperature and salinity changes (Fig. 3). The depth ranged from

3.5 to 5.5 m and temperature changes N6 C were observed

during the tidal oscillations. The temperature alsoincreased from

a daily high of 1516 C at the start of the deployment to 18

19 C at the end of the deployment. The presence of large lateral

and vertical temperature gradients was confirmed using the

temperature records from SAMI-47 and SAMI-51. The two

SAMI-pH temperatures differed by up to 2 C during periods of

rapid temperature fluctuations. These times correspond to

Table 2

Evaluation of different indicator pHs

[mCP]

(mol

kg1)

Indicator

pH

Average

pH

pH

precision

Slope

(pH kg

mol1)

R2 pH

perturbation

0.005 8.2 7.8890 0.0004 222 0.98 0.011

0.005 8.0 7.8884 0.0004 40 0.79 0.0020.005 7.8 7.8886 0.0004 166 0.97 0.008

0.005 7.5 7.8886 0.0007 366 1.00 0.018

The average seawater sample pH was determined using the pH

perturbation method described in the text (n =5). Slope is the average

of the five slopes generated from the pH versus total indicator plot. The

R2 value is the average for the five measurements. As expected, the pH

perturbation is larger (larger positive or negative slope) when the pH of

the indicator is further away from the sample pH (to within the

tolerance of the uncertainty in the indicator pH). The perturbation is the

change in pH of the sample due to 5105 mol kg1 total indicator

concentration (the indicator concentration in the indicator reservoir

was 5 103 mol kg1). The indicator pH was determined using a pH

electrode and is only accurate to within 0.1 pH units. Measurementtemperature was 25 C.

Table 3

The pH precision for measurements made over the typical seawater

range

pH Precision 95% confidence Measurements

7.6862 0.0005 0.0002 23

7.7833 0.0009 0.0003 507.9159 0.0005 0.0002 25

7.9193 0.0006 0.0002 25

8.0260 0.0007 0.0003 17

8.2029 0.0009 0.0004 25

Samples are CRMs and Hood Canal seawater adjusted to varying pH

values using concentrated HCl or NaOH. Measurement temperature

was 25 C.



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significant pH differences between the SAMIs, presumably due

to large pH gradients associated with the temperature (water

mass) differences. Therefore, any time the measured temperature

difference between SAMI-51 and SAMI-47 wasN0.2 C, the pH

measurements were removed from the intercomparison between

the two instruments (122 measurements were removed). Using

the remainingdata, the agreementbetween SAMI-47 andSAMI-

51 was 0.00420.0126 (n =883) for the 22 d period (Fig. 5).

Tightening the temperature difference criteria to 0.1C improved

the comparison to 0.00400.011 and removed another 116

measurements. Filtering both time-series using a 2 h average

further reduces the scatter between the two SAMIs to 0.0084.

The lab measurements were more precise than this (Table 3) and

it is likely that the large spatial gradients, particles in the flow cell

(see below), and small response time differences, e.g., due to

different flushing times, degraded the precision between the two

instruments. The between-instrument precision is notably larger

during periods when pH was rapidly changing, i.e., at the

beginning and end of the deployment (compare Figs. 3 and 5).

Early on, the pH rate of change was as large as 0.016 min1 and

often largerthan 0.005 min

1

. Looking at the pH dilution curves,SAMI-51 flushed indicator1530 s more slowly than SAMI-

47 due to flow rate differences (pump efficiency), which, based

on the pH rates of change, could account for a significant portion

of the pH differences.

Importantly, the offset is small and there is no apparent drift

between the instruments during the deployment (Fig. 5). There

was also no systematic trend in the SAMI difference over the

pH and temperature range during the deployment (Fig. 6).

These results were further verified by comparison with

Fig. 3. In situ SAMI-pHs, salinity, temperature and depth during the 22

d deployment off SIO pier.

Fig. 4. pH versus total indicator concentration [mCP]T for two measurements from SAMI-47 and SAMI-51 during deployment off SIO pier. Theresulting pH (y-intercept) and R2 values are given in each plot.

Fig. 5. Comparison between SAMI-47 and SAMI-51 during the 22

d deployment calculated from Fig. 3 data. The average offset is

0.0042 0.0126 (n =883). See text for more discussion.



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measurements of pH on discrete samples collected off the pier

on April 30 and May 3. SAMI-47 was deployed for both of

these days while SAMI-51 was deployed on May 2 and was

only compared to the pH values on May 3. Good agreement

was achieved between the SAMI-pH instruments and bench-

top spectrophotometric pH measurements with an average pH

offset of +0.0013 (n =8) for SAMI-47 and 0.0043 (n =3) for

SAMI-51.

After May 3, pH values were calculated from DIC and ATfrom discrete samples using CO2SYS (Lewis and Wallace,

1998) and the dissociation constants of Mehrbach et al. (1973)

refit by Dickson and Millero (1987). Using these pH values, the

average offset for both SAMI-47 and SAMI-51 are nearly

identical at 0.0348 and 0.0330 pH units (n =14). Use of other

CO2 equilibrium constants in CO2SYS (Lewis and Wallace,1998) did not improve the offset. Examination of the discrete

sample data pointed to a possible error in the AT values. In

surface waters, AT often follows a conservative relationship with

salinity (Millero et al., 1998); however, AT of the discrete

samples varied by 40 mol kg1 while the sample salinities

were very similar (33.54 0.05). Using an approximate relation-

ship between salinity and AT for California coastal waters

(AT=2150+44 [S31.25]) (G. Friederich, pers. comm., 2006),

the expected AT for the salinity range observed is 2249

2252 mol kg1 compared to the measured range of 22162254

(14) mol kg1. Using the salinity-predicted AT and the

measured DIC, the pH errors reduce to 0.014 and 0.012 pH unitsfor SAMI-47 and SAMI-51, respectively. Uncertainty in the pKa

(Eq. (4)), due to extrapolation to the in situ temperatures, may

also contribute to the pH uncertainty.

Although the instruments were only in the water for a short

time, barnacle and algal growth completely covered the

instruments. One concern in such an intense fouling environ-

ment is that, in drawing in a sample for analysis, particles can

occlude the optical path or clog the plumbing. The 780 nm

reference channel light throughput for the two instruments

during the 22 d deployment suggest that particles may have been

drawn in at times (Fig. 7). The fluctuating intensity for SAMI-51

from May 8

9 may be due to small particles entering the tubingand optical cell. The return to the original intensity value shows

that the particles eventually flushed out. SAMI-47 shows a

different trend with the intensity slowly drifting downward with

only 40% of the original intensity at the end of the deployment.

The resulting behavior was most likely due to lamp drift or

particulate matter entering the flow cell and slowly reducing the

light throughput. No particles were evident upon inspection of

the inside of the flow cell, however. The change in light

throughput shown in Fig. 7 was accounted for by obtaining blank

measurements for each pH analysis, i.e., for one set of data as

shown in Fig. 2. To verify the need for frequent blanks, pH

values for both instruments were calculated using individual

blank values and an initial blank value over a 32-hour section of

the field deployment data (May 2022). Using individual blank

values over this time-period, the average difference between

SAMI-47 and SAMI-51 pH valueswas 0.0017 0.0149pH units(n =64), while using a single initial blank value gave an average

offset of 0.0070 0.0246 pH units. No drift was observed

between the two instruments (Fig. 5), even with the large

changes in light throughput (Fig. 7), demonstrating the

robustness of the technique.

Fig. 6. pH difference between SAMI-47 and SAMI-51 versus temperature (A) and pH (B). The plot shows that the SAMIs respond very similarly over

the range of temperature and pH found during the deployment.

Fig. 7. % transmittance (relative light throughput) at 780 nm. This

wavelength is used for the correction of lamp drift, bubbles, particles,

and any other factors that affect light throughput between blank

measurements. The % transmittance is calculated relative to the initialthroughput at the start of the deployment.



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5. Conclusions

Multiple steps were undertaken to improve the SAMI-

pH performance. The filter-based detection system im-

proved the signal-to-noise and consequently, the absor-

bance precision. Molar absorptivities were corrected forthe presence of other indicator forms by using the pH,

concentration, and absorbance values of the acidic and

basic solutions. With these changes, the agreement be-

tween the Cary UV/Vis and SAMI-pH instruments in a

laboratory setting was improved from 0.003 to +0.0017.

Optimization of various procedures, including the pH

perturbation correction and mixing of the indicator and

sample, improved the precision to 0.0007 compared to

the previous precision of 0.004 of the Martz et al. (2003)

design.

The 22 d deployment off Scripps pier collected fromSAMI-47 and SAMI-51 collected the longest known in

situ spectrophotometric seawater pH time-series to date.

The excellent agreement between the two SAMI-pH

instruments and the Cary UV/Vis is very promising;

however, further tests are needed to determine internal

consistency with other inorganic carbon parameters.

The SAMI-pH can be used for studies related to

pressing contemporary carbon cycle questions such as

the oceanic uptake of atmospheric CO2 and the impacts

of ocean acidification. The instrument can be deployed

on many different platforms including Volunteer Obser-

ving Ships, permanent moorings, drifters, autonomousprofilers, and AUVs. Development of sensors for DIC

and AT is needed to make possible fully autonomous

characterization of the inorganic carbon system, i.e.,

through combined measurements of pH or pCO2 with

DIC orAT.

Acknowledgments

We thank Cory Beatty (University of Montana) for lab

and field assistance, Todd Martz (formerly UM, now at

MBARI) for discussions and Charles Coughran (SIO) fordeployment of instruments. Financial support was pro-

vided by NSF grant OCE-0327763 and a NSF-EPSCoR

fellowship to M. Seidel.

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