Short communication
Methodology for pH measurement in high alkali cementitious systems

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Abstract

A methodology for calibrating pH meters in highly alkaline solutions such as those relevant to cementitious systems is presented. This methodology uses an extended form of the Debye-Hückel equation to generate a calibration curve of pH vs. the measured electrochemical potential (mV) based on a series of aqueous alkali hydroxide solutions of known concentrations. This methodology is compared with the ‘built-in’ process of calibration based upon pH 4, 7, and 10 standard solutions. The built-in calibration process underestimates the real pH values by up to 0.3 log units, which is attributed to the alkali error. A spreadsheet for determining the calibration curve and its application to pH meter readings is provided as Supporting Information. The implications of improperly calibrated pH meters on interpreting solution chemistry in cementitious systems are discussed.

Introduction

Measurement of pH is a quick, simple and cost-effective technique that is fundamental to analytical chemistry and widely used in cement science. Fresh Portland cement concrete typically has a pH > 13 [1]. Maintaining such a high pH is essential to ensure passivation of steel in reinforced concrete, thereby preventing structural deterioration [2,3]. The pH of the activator solution in an alkali-activated material plays a critical role in precursor dissolution [4,5], and high pH solutions (>13) are typically employed for this purpose [6]. In both Portland cement and alkali-activated material systems, the formation of reaction products has also been shown to depend on pH [[7], [8], [9], [10]].

Calibration of a pH meter is necessary for accurate pH measurements. pH meters are typically calibrated using standard solutions with pH values of 4, 7, and 10 – a process we refer to here as the ‘built-in’ pH meter calibration. Saturated aqueous Ca(OH)2 solution may be used as a pH 12.45 standard (at 25 °C) [11]. However, these pH values are below the pH range of most cementitious systems; therefore, using the built-in calibration in cementitious systems will likely lead to systematic pH measurement errors. Through appropriate selection of solutions of known concentration, pH meters can be accurately calibrated to higher pH. Although versions of this methodology have been used for years in analyses of cementitious systems [1,[12], [13], [14], [15], [16], [17]], it has been poorly explicitly disseminated and there has thus been apparently relatively limited uptake of it among the broader cement science community. This communication is intended to clarify this methodology to the cement science community at large. As such, a pH calculator for NaOH and KOH solutions as a function of temperature and concentration is included in the Supporting Information and the relevant physical chemistry concepts underpinning these calculations are discussed here.

Section snippets

Activity of non-ideal solutions

We begin the description of our methodology to calibrate pH meters by expressing the acidity of a solution using pH values (Table 1; Eq. (1)).pH=log10aH+=log10γH+bH+b0where aH+ is the activity of aqueous H+, bH+ is the molality of aqueous H+ (mol kg1, i.e., mol of aqueous H+ per kg of solvent), b0 is the standard molality which is defined as 1 mol kg1 (included to make the activity dimensionless), and γH+ is the activity coefficient of aqueous H+. Activity is a measure of the effective

NaOH and KOH standards, and LiOH solutions

NaOH and KOH standards were prepared with the following concentrations: 0.0001 M (mol L1), 0.0005 M, 0.001 M, 0.005 M, 0.01 M, 0.05 M, 0.1 M, 0.2 M, 0.5 M, 1 M, 2 M, and 3 M. NaOH and KOH standards were made by diluting commercial solutions of 3 (±0.005) M NaOH (BDH Chemicals) and 8 (±0.005) M KOH (Ricca), respectively. Dilutions were performed by transferring quantities of Na(K)OH commercial solution into volumetric flasks and filling to their marks with high purity water (18.2 MΩ cm,

pH meter calibration

The linear calibration curve fittings for prepared aqueous NaOH and KOH solutions are respectively shown in Fig. 1(A) and (B). The alkali error associated with measuring the pH of LiOH solutions is also demonstrated (Fig. 1(C)). The pH is calculated using the H-DH equation (Eq. (3)) and methodology presented here and in the Supporting Information. The measured built-in pH is based on an extrapolated calibration curve using pH 4, 7, and 10 standard solutions. The divergence at high pH (Fig. 1A,

Application to cementitious systems

To demonstrate the utility of the methodology presented, pH values of filtrates taken from a series of alkali-activated biomass ash samples were measured. In each case, we use the NaOH fitted calibration curve (Fig. 1A) to convert the measured electrochemical potential to pH. Details on how these samples were made are given in Section 3.4. The results of the pH measurements and calculations using the methodology presented here are shown in Table 2.

Differences between the calculated and measured

Conclusions

A methodology for calibrating pH meters in solutions of high alkali content (pH >13) has been presented. The Helgeson extension to the Debye-Hückel (H-DH) equation was used to calculate the pH of aqueous ionic solutions of known molarity. The H-DH equation was chosen because of its practicality and accuracy at high ionic strengths (approaching 4.5 mol kg1). We provide a spreadsheet for the determination of this calibration curve and application to pH meter readings of sample solutions as

CRediT authorship contribution statement

Brian Traynor:Investigation, Writing - original draft, Writing - review & editing, Visualization. Hugo Uvegi:Writing - review & editing. Elsa Olivetti:Writing - review & editing, Resources, Supervision. Barbara Lothenbach:Conceptualization, Methodology.Rupert J. Myers:Conceptualization, Methodology, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to acknowledge the financial support for this research through the Environmental Solutions Initiative at Massachusetts Institute of Technology (MIT), Cambridge. We also acknowledge support from NSF CAREER #1751925. Funding provided by the Scottish Research Partnership in Engineering Grant #PECRE1718/02 is gratefully acknowledged. The research leading to this publication benefitted from EPSRC funding under grant No. EP/R010161/1 and from support from the UKCRIC Coordination Node,

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