Measurement of total Alkalinity and Carboxylic Acid and Their Relation to Scaling and Corrosion
Authors: Tomson Mason B. , Kan Amy T. , Fu Gongmin , Cong Lili 
Published: December 2007

Alkalinity is needed in many water treatment calculations, scale, corrosion, precipitation, oxidation, etc., yet the concept is often misunderstood. In natural waters, alkalinity often is not equal to bicarbonate concentration since natural waters contain base contributing anions that can significantly affect alkalinity. However, alkalinity is commonly assumed to be equal to the bicarbonate concentration in many scale and corrosion prediction algorithms. When other anions, e.g. carboxylates, are present, bicarbonate concentration in production tubing is not a conservative quantity; it varies with CO2 partial pressure, temperature and carboxylate concentrations in a complicated manner, up and down a well.

Reliable methods to accurately measure true alkalinity are scarce, especially when multiple weak acids are present and the effects of TDS on electrode and color end point are significant. Oil field brines contain aliphatic carboxylic acids of one to six carbons, e.g. acetate, up to 5000 mg/L. The highest concentrations of carboxylates tend to be in waters from reservoirs at temperatures of 80 to 100°C. In this paper, a new analytical procedure and computation routine to determine alkalinity and carboxylic acids simultaneously will be discussed. The procedure was recently debugged and simplified by the Rice University Brine Chemistry Consortium (Rice BCC). The new titration method is based upon simultaneous analysis of the titration curve determined at fixed PCO2 and emphasizes the titration shape (profile) instead of the endpoint inflection as is done presently.

A wide range of natural and synthetic waters has been tested. Excellent agreement was observed between the true and calculated carboxylic acid concentration with a correlation coefficient squared of 0.9986. Once the total alkalinity and acetic acid concentrations are determined, the theoretically correct bicarbonate concentration and/or pH at any given operation temperature, pressure can be calculated. The intricate inter-relationship of total alkalinity, carboxylic acids, and pH on scale and corrosion will be discussed by using case studies.

Introduction

Based on charge neutrality relation, Eq. 1 has to be satified for all brine solution chemistry1.Equation 1

The left hand side of Eq. 1 represents the net difference of the summation of all strong base cations in brine and the summation of all strong acid anions in brine. The right hand side of Eq. 1 represents the summation of all weak acid anion in brine and water. The [S2-] term is always small and will be neglected hereinafter. Alkalinity is defined as the strong-acid neutralizing capacity of a solution2,3. From Eq. 1, the alkalinaity can be defined as the sumation of either the terms on the left or the terms on the right. The definition on the left would be fine if the analyses of the metals and anions were sufficiently accurate. Typically, the sum of the weak acids on the right is only 0.001 to 0.010 M and the water terms, ([OH−]-[H+]), are small. The concentrations of strong base cations and strong acid anions are about 1.000 to 6.000 M. Therefore, the difference would have to be accurate to about ±0.1 to 1.0 %, which would require the individual numbers to be even more accurate which is extremely unlikely for routine sampling and analyses. In fact, some other commercial computer codes in the business have been known to calculate the solution pH from just such an analysis; in those cases the calculated pH was routinely either about 10 to 11 or 3 to 4, depending upon whether the random error in the difference was positive or negative, respectively.

This charge balance-type definition of alkalinity helps clarify one of the most important features of why the alkalinity concept is so important, that is, that the addition or removal or CO2 or H2S gas, up and down a well, has absolutely no effect on the total alkalinity. Therefore, the alkalinity is a "conservative" property of the solution. The pH does increase as CO2 and H2S gas are released from solution as the brine flows up a well. As the pH changes the relative amounts of the various weak acids on the right hand side of the equation will change, but the total alkalinity does not change at all. Similarly, adding acetic acid will lower the pH, but will not alter the alkalinity, because it does not add any strong cations nor strong anions to the left hand side of the above equation.