INGOLDLeading Process Analytics

Elevated temperatures, presenceof chlorineandhighsaltconcentrationthroughoutthechlor-alkaliprocessmakestableandreliablepHmeasurementwithconventionalpHelectrodesaconstantchal-lenge.Further,theconditionsmeanelectrodelife-timeisusuallyshort;therefore,costsintermsofsensormaintenanceandreplacementarehigh.Adual-membrane electrode that uses the processbrineasareference,isimmunetomeasurementproblems. In addition, digital transmission fromsensortotransmitterprovidesexceptionalsignalstability.

NoMoreTearsDurable Chlor-Alkali pH Electrode

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The chlor-alkali membrane cell process is highly dependent on accurate pH measurement in order to maximize production yield and ensure long lifetime of ion exchange membranes. High yield and reduced operating expenditure need accurate pH control at strategic points throughout production. However, process conditions, especially after electrolysis, are very tough on conventional pH electrodes, resulting in significant mainte-nance requirements and frequent sensor replacement. This white paper reviews why the chlor-alkali process is so damag-ing to pH electrodes and discusses a sensor design that has overcome all measurement issues.

MembranecellchlorineproductionIn the membrane cell chlor-alkali process, saturated brine enters an electrolysis cell where, at the anode, chloride ions are oxidized to chlorine gas which is collected at the top of the reactor. The sodium ions in the brine diffuse through an ion exchange membrane and enter the cathode side of the cell. Here, water from the solution is hydrolyzed, forming hydrogen

and hydroxide ions. The sodium and hydroxide ions combine to form caustic soda. Both the hydrogen and caustic soda are sold as by-products. The depleted brine from the anode compartment is resaturated with salt and is circulated back to the electrolysis cell (fig 1).

The costly ion exchange membranes are typically made of per-fluorinated polymers, and although both sides are exposed to a chemically aggressive environment, they can last for several years if treated correctly.

pH value is extremely important throughout the chlor-alkali pro-cess, particularly in the electrolysis cell. On the anode side, the reaction takes place under acidic conditions through the addition of hydrochloric acid. Though very low pH (< 3) leads to higher yield, it has a detrimental effect on the life of the cell membrane, leading to regular and expensive membrane replacement. Chlo-rate (ClO3–) formation during the process is unavoidable and undesirable as they reduce the solubility of salt and negatively impact chlorine yield. Over pH 4, chlorate formation increases significantly; therefore, to maintain a balance between chlorine yield and membrane life, the pH of the anolyte is usually controlled in the range 3 – 4.

The depleted brine leaving the membrane cell contains some dis-solved chlorine and chlorate, both of which need to be removed. In a dechlorination treatment the pH is lowered to 2 or less, which allows the remaining chlorine to become gaseous and be ex-tracted. Chlorate in the brine is converted to chlorine under acid-ic conditions or is reduced at high pH.

Impurities in the brine negatively affect electrolysis and mem-brane performance. Therefore, before entering the cell, the brine goes through an extensive purification process where undesirable components are removed via precipitation filtration. By adding various salts as precipitants and increasing the pH carefully in steps to pH 10 – 12, impurities such as sulfates, carbonates and hydroxides of calcium, barium, magnesium and other metals are allowed to precipitate. Following filtration, the brine passes through an ion exchanger for polishing to remove other impurities.

ProblemswithconventionalpHelectrodesThe chlor-alkali process is tough on standard pH electrodes. This is particularly true after electrolysis where chlorine gas is present. Chlorine diffuses through the electrode’s diaphragm and attacks D

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Fig 1: Schematic of membrane cell chlor-alkali process

pNa/pHtechnologyAn alternative to conventional pH electrodes is pNa/pH electrodes. The main difference between the two types is the reference system. In addition to pH-sensitive glass, a pNa/pH electrode uses a mem-brane glass that is sensitive to the sodium ion concentration. As the salt concentration during the chlor-alkali process remains fairly static, pNa/pH electrodes can use the brine itself as a reference. In the same way that pH-sensitive glass is charged by the oxonium ions (H3O+) in the measuring medium, the sodium-sensitive glass is charged by sodium ions (Na+). The difference in electrical po-tential between the charged sodium glass and pH glass is calcu-lated into the pH value by the transmitter according to the equa-tion below.

U = U0 + SpH (pH0 – pHi) + SNa (pNa0 – pNai)

Where:U: Measured electrical potential correlated to the solution’s pH

and sodium ion concentrations.U0: Offset depending on the sensor’s electrochemistry.

SpH: Electrode slope for the pH glass membranepH0: pH value in measuring solutionpHi: pH value in the electrode’s inner buffer solution

SNa: Electrode slope for the Na+ glass membranepNa0: Negative logarithm of the Na+ concentration in the mea-

suring solutionpNai: Negative logarithm of the Na+ concentration in the elec-

trode’s inner buffer solution

the Ag/AgCl reference system. The silver is oxidized to silver ions by the chlorine which results in a large shift in zero-potential and is exacerbated by the high process temperature (70 – 90 °C). To an extent, the drift can be compensated for with frequent electrode calibrations; however, over time the effect on the electrode mani-fests as increasingly unstable values, low measurement accuracy and reduced operating life.

At measurement points where suspended solids or particles are present (e.g. in purification), conventional pH electrodes are sub-ject to clogging of their diaphragm. Particles diffuse into the dia-phragm or form a layer on the surface, causing measurement error and sluggish response. Solids that penetrate through the dia-phragm are difficult to remove and the electrode’s performance will never be as good as when first installed. Regular cleaning with acid or detergent helps to increase electrode performance and prolong lifetime, but is time-consuming for maintenance personnel.

A third challenge for pH electrodes in brine processes is alkali er-ror. This occurs most noticeably above pH 10 when sodium ions are present. The sodium ions contribute to the charge on the outer gel layer of the electrode’s pH-sensitive membrane glass. This causes the electrode to report a lower pH than the correct value. As alkali error increases with temperature, the effect in the hot chlor-alkali processes can be significant. All conventional pH electrodes exhibit temperature dependence according to the Nernst equation, which means that the electrode’s slope value increases with tem-perature. This dependency can be compensated for by modern pH transmitters. However, the temperature effect of the alkali error is unpredictable and cannot be compensated for.

As mentioned, to obtain high measurement reliability under such conditions necessitates significant sensor maintenance and fre-quent calibration. Each measurement system needs to be checked regularly and it is common to bring a calibrated handheld pH meter to the measurement points and, when needed, do a 1-point adjustment. This procedure requires substantial manpower effort and skilled working staff.

Up until now, the most reliable pH electrodes for chlor-alkali use have been those with pressurized reference electrolyte. The over-pressure prevents clogging of the diaphragm and inhibits chlorine diffusing into the electrode’s reference system. However, these elec-trodes require installation accessories such as special housings, and entail more maintenance efforts due to the additional handling.

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3.51 M NaCl, pH 7

Na membrane = – 2.7 mV

pH membrane = 0 mV

∆ potential = – 2.7 mV

∆ pH = 0.05 pH units

Na membrane = 0 mV

pH membrane = 0 mV

∆ potential = 0 mV

3.9 M NaCl, pH 7

Fig. 2: A 10 % change in sodium concentration will only cause a shift

of 0.05 pH units.

As mentioned, pNa/pH electrodes need a stable sodium concentra-tion during measurement, and also for calibration. Changes in sodium concentration result in a shift of the electrode’s baseline and hence also the measurement value. However, small variations are not significant as the sensitivity is logarithmic. To keep the accuracy in a limit of ± 0.05 pH units, the variation in sodium concentration should be maintained in a range of ± 10 % (fig 2).

For calibration purposes, the sodium concentration in the pH buffer should be as close as possible to the actual concentration in the process. A difference between the two will result in a measurement offset that needs to be compensated for with an additional 1-point process calibration.

As long as the concentration of sodium ions in the electrode’s inner sodium buffer solution is equal to the concentration in the process (pNa0 – pNai = 0), the measured electrical potential is only depen-dent on the electrode’s general offset and the contribution from the pH membrane glass. When pNa0 ≠ pNai the potential from the so-dium reference is added to the sensor’s offset. Na+ ions increase their activity with temperature with the same characteristic as oxo-nium ions; therefore, SNa = SpH, and temperature compensation can be made in the transmitter.

What makes pNa/pH electrode technology so suitable for the chlor-alkali industry, is that the pNa reference system is hermetically sealed: pNa/pH electrodes do not have a diaphragm, therefore no oxidants or other ions can enter and damage the inner part of the electrode. Like-wise, diaphragm clogging is not possible. Compared to conventional pH electrodes that suffer from unstable and drifting measurement values caused by the process, pNa/pH electrodes are extremely stable.

In addition, unlike pH electrodes with pressurized electrolyte, no special housings or accessories are necessary. Also, maintenance of pNa/pH electrodes is low, as refilling of electrolyte is not possible and pressurization is not required.

MeasurementchallengesDespite all their advantages, pNa/pH electrodes also have their problems. Firstly, they can suffer from measurement error that ap-pears at low pH in combination with a low sodium ion concentra-tion. In the same way that alkali error occurs at high pH, acid error transpires at low pH (below 2) when the high concentration of oxo-nium ions interferes with the functioning of the pNa-sensitive glass. As is the case with alkali error, acid error increases with tempera-ture and can exceed 1 pH unit. With good glass technology it is possible to minimize acid error and measure pH below 2 with ac-curacy better that 0.1.

A second, and more significant downside to pNa/pH electrodes is the high impedance signal they output, necessitating special transmit-ters. Outputs of both the pNa and the pH side of the sensor are high impedance and require a suitable transmitter plus cabling with very good shielding. The transmitter reference input must be able to handle high ohm signals, typically 1012 ohm, in comparison to the 106 ohm from conventional pH electrodes.

The main problem with high impedance signals is that they are very sensitive to electrical interference, and signal drops because of cor-rosion on the electrical contacts. High-frequency interference from equipment such as pumps can cause signal spikes and unstable values. Even approaching or touching the sensor/cable causes the signal to fluctuate enormously and affects the pH output of the

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Acid error

Alkali error

pH

Fig. 3: Effect of acid and alkali error on pH measurement, 25 °C and 1 mol / L brine.

METTLER TOLEDO InPro 4850 i

Typical pNa / pH electrode

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transmitter. This increases the requirement on cable shielding and limits the distance between sensor and transmitter. (These prob-lems also occur with conventional pH electrodes but the effect is far smaller.) Therefore, a pNa/pH electrode that does not output a high impedance signal would be the perfect solution for the chlor-alkali industry.

AdvancedpHelectrodeMETTLER TOLEDO’s InPro 4850i is a pH electrode with a pNa reference system, especially developed for chlor-alkali use. Draw-ing on METTLER TOLEDO’s long experience in membrane glass technology, both glass membranes are designed for high accuracy measurement over the full pH range. The pNa membrane glass features strong chemical resistance and negligible acid error in low pH solutions. To minimize alkali error, METTLER TOLEDO’s unique High Alkaline glass composition is used for the pH-sensi-tive glass (fig 3).

The “i” at the end of the electrode’s name denotes that it features METTLER TOLEDO’s unique Intelligent Sensor Management (ISM) technology. ISM greatly simplifies sensor handling, enhanc-es measurement reliability and reduces sensor lifecycle costs.

One particular feature of ISM is of great benefit to the chlor-alka-li industry: a digital signal. ISM sensors include a microprocessor in the sensor head that converts the analog measurement signal into a digital one which it exchanges with the connected transmit-ter. Not only is this low impedance digital signal more precise than an analog signal, it is completely unaffected by moisture and electrical interference and is therefore 100 % stable.

The combination of pNa reference system, advanced glass technol-ogy and digital signal means the InPro 4850i provides incompa-rable measurement reliability, long term stability, extended cali-bration intervals, plus the flexibility to position the transmitter away from the sensor.

PlugandMeasure–fastandsimplestartupWhen sensor replacement is required, it may take a skilled main-tenance engineer up to one hour to calibrate and configure a new electrode. With ISM, sensors can be pre-calibrated then stored until they are needed. Calibration can be done on a standard computer or laptop in the comfortable environment of a mainte-nance shop. This is achieved through a direct USB connection and METTLER TOLEDO’s iSense Asset Suite software*, or by using an ISM transmitter. Further, when connected to the transmitter at the measurement point, due to the configuration data held on an ISM sensor, the new probe is instantly recognized. Now when an exchange of sensor is needed, Plug and Measure functionality means a pre-calibrated sensor can be installed and be ready to measure in under a minute, there-fore substantially in-creasing production capacity by avoiding longer interruptions, and allowing operators to concentrate on more critical tasks.

* Coming soon

InPro 4850i dual-membrane pH electrode

pH-sensitive glass

Sodium-sensitive glass

Solution ground and shielding

ISM sensors can be conveniently calibrated in a maintenance shop, using a standard PC and iSense Asset Suite software.

For more informationMettler-ToledoAGProcess AnalyticsIm Hackacker 15CH - 8902 UrdorfSwitzerland

© 09 / 2011

www.mt.com/pro

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PredictivemaintenanceThe failure of an analytical sensor during operation can be very inconvenient, and in certain circumstances hugely damaging. The ability to predict when a sensor will fail would be extremely valuable, and this is exactly what ISM sensor diagnostics provide. The Dynamic Lifetime Indicator (DLI) monitors process condi-tions and sensor wear, and based on sophisticated algorithms is able to calculate the remaining sensor life. The DLI is displayed on ISM compatible transmitters and the iSense Asset Suite.

The DLI and other diagnostic tools allow a pH measurement sys-tem to be optimized on an ongoing basis and for all critical situa-tions to be predicted so that operators can respond before produc-tion is interrupted.

BuffersolutionsA suitable range of pH buffers from pH 2 to 9.21 with 20 % NaCl are available for calibrating the InPro 4850i. This salt concentration has been chosen because it is close to the most commonly found concentration in chlor-alkali brine, and will in general eliminate the need for a 1-point process adjustment.

ConclusionpH measurement is essential in the membrane cell chlor-alkali process, but conditions mean maintaining reliable pH measure-ments is challenging. METTLER TOLEDO’s InPro 4850i dual-membrane pH electrode removes the barriers to repeatable, accu-rate pH measurement and extended sensor life. By combining advanced membrane glass technology with the digitized signal of Intelligent Sensor Management along with ISM’s other significant advantages, the InPro 4850i provides unequalled measurement performance, low maintenance and exceptional durability.

For more information, visit:

4www.mt.com/InPro4850i