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Authors
Abstract(s)
A alcalinidade, que reflete a capacidade de uma solução aquosa neutralizar ácidos,
é uma característica importante em sistemas aquáticos e industriais, sendo utilizada na
monitorização de processos de tratamento de água, ainda que, não seja um parâmetro com
valor paramétrico definido pela legislação.
Este trabalho teve como objetivo a validação do método de determinação da
alcalinidade em águas de consumo e águas de processo, através de volumetrias, para que
este se torne acreditado perante a entidade competente, o Instituto Português da
Acreditação.
O processo de validação incluiu a avaliação da precisão (repetibilidade,
reprodutibilidade e precisão intermédia) e limiares analíticos (limite de deteção e limite
de quantificação). Além disso, foram implementados procedimentos de controlo de
qualidade interno, como a utilização de padrões de controlo, ensaios em branco e
duplicados. Ao longo do desenvolvimento do trabalho, foram realizadas análises diárias
de amostras (águas de consumo e águas de processo) para a determinação da alcalinidade.
O método de determinação da alcalinidade foi avaliado quanto à sua repetibilidade
em diferentes concentrações de carbonato de cálcio (CaCO₃) (25, 125 e 625 mg/L). O
método mostrou maior precisão na concentração intermédia (125 mg/L), enquanto as
concentrações mais baixas (25 mg/L) e mais altas (625 mg/L) apresentaram maior
variabilidade. No entanto, os coeficientes de variação permaneceram dentro do limite de
10% estabelecido pela empresa, demonstrando que o método é adequado em todas as
faixas de concentração.
Para as amostras de água de consumo e de processo, os valores de limite de
repetibilidade e coeficiente de variação indicam que o método garante medições precisas
e consistentes. As cartas de controlo utilizadas confirmam que o método é estável ao
longo do tempo, sendo adequado para medir alcalinidade em diferentes matrizes e
concentrações de CaCO₃.
Os limites de repetibilidade mostram a precisão do método em diferentes
concentrações de CaCO₃. Para 25 mg/L, a diferença máxima entre medições é de 3,9
mg/L; para 125 mg/L, é de 1,0 mg/L (precisão elevada); e para 625 mg/L, é de 90,9 mg/L,
indicando maior variação em concentrações mais altas. O método apresentou coeficientes de variação baixos, variando entre 1,6 % e 2,1
%, o que reflete uma boa reprodutibilidade e precisão elevada. Os erros relativos para as
concentrações de 125 mg/L e 625 mg/L foram mínimos (0,3 %), demonstrando excelente
consistência, enquanto para 25 mg/L, o erro foi maior (5,9 %), refletindo a maior
sensibilidade a variações em baixas concentrações. Esses resultados confirmam que o
método é adequado para uma gama de concentrações (25 a 625 mg/L), garantindo a
qualidade das medições.
Relativamente aos duplicados efetuados, obtiveram-se resultados satisfatórios. A
maioria das amostras apresentaram duplicados de alcalinidade idênticos ou com
diferenças mínimas, indicando precisão nas medições.
A validação do método da alcalinidade foi concluída, confirmando conformidade
com os requisitos técnicos, para que o parâmetro da alcalinidade se torne acreditado pelo
Instituto Português da Acreditação.
Alkalinity, which reflects the ability of an aqueous solution to neutralise acids, is an important characteristic in aquatic and industrial systems and is used to monitor water treatment processes, although it is not a parameter with a parametric value defined by legislation. The aim of this work was to validate the method for determining alkalinity in drinking water and process water, so that it can be accredited by the competent authority, Portuguese Accreditation Institute. The validation process included assessing precision (repeatability, reproducibility and intermediate precision) and analytical thresholds (limit of detection and limit of quantification). In addition, internal quality control procedures were implemented, such as the use of control standards, blank tests and duplicates. Throughout the course of the work, samples (drinking water and process water) were analysed daily to determine alkalinity. The alkalinity determination method was evaluated for its repeatability at different concentrations of calcium carbonate (CaCO₃) (25, 125 and 625 mg/L). The method showed greater precision at the intermediate concentration (125 mg/L), while the lower (25 mg/L) and higher (625 mg/L) concentrations showed greater variability. However, the coefficients of variation remained within the 10 per cent limit set by the company, demonstrating that the method is suitable in all concentration ranges. For drinking water and process water samples, the repeatability limit and coefficient of variation values indicate that the method guarantees accurate and consistent measurements. The control charts used confirm that the method is stable over time and is suitable for measuring alkalinity in different matrices and CaCO₃ concentrations. The repeatability limits show the accuracy of the method at different CaCO₃ concentrations. For 25 mg/L, the maximum difference between measurements is 3.9 mg/L; for 125 mg/L, it is 1.0 mg/L (high precision); and for 625 mg/L, it is 90.9 mg/L, indicating greater variation at higher concentrations. The method showed low coefficients of variation, ranging from 1.6 % to 2.1 %, which reflects good reproducibility and indicates high precision. The relative errors for the 125 mg/L and 625 mg/L concentrations were minimal (0.3 %), demonstrating excellent consistency, while for 25 mg/L, the error was higher (5.9 %), reflecting the greater sensitivity to variations at low concentrations. These results confirmed that the method is suitable for a range of concentrations (25 to 625 mg/L), guaranteeing the quality of the measurements. Regarding the duplicates, satisfactory results were obtained. Most of the samples showed identical alkalinity duplicates or the slightest differences, indicating accuracy in the measurements. The validation of the alkalinity method was completed, confirming compliance with the technical requirements for the alkalinity parameter to become accredited by the Portuguese Accreditation Institute.
Alkalinity, which reflects the ability of an aqueous solution to neutralise acids, is an important characteristic in aquatic and industrial systems and is used to monitor water treatment processes, although it is not a parameter with a parametric value defined by legislation. The aim of this work was to validate the method for determining alkalinity in drinking water and process water, so that it can be accredited by the competent authority, Portuguese Accreditation Institute. The validation process included assessing precision (repeatability, reproducibility and intermediate precision) and analytical thresholds (limit of detection and limit of quantification). In addition, internal quality control procedures were implemented, such as the use of control standards, blank tests and duplicates. Throughout the course of the work, samples (drinking water and process water) were analysed daily to determine alkalinity. The alkalinity determination method was evaluated for its repeatability at different concentrations of calcium carbonate (CaCO₃) (25, 125 and 625 mg/L). The method showed greater precision at the intermediate concentration (125 mg/L), while the lower (25 mg/L) and higher (625 mg/L) concentrations showed greater variability. However, the coefficients of variation remained within the 10 per cent limit set by the company, demonstrating that the method is suitable in all concentration ranges. For drinking water and process water samples, the repeatability limit and coefficient of variation values indicate that the method guarantees accurate and consistent measurements. The control charts used confirm that the method is stable over time and is suitable for measuring alkalinity in different matrices and CaCO₃ concentrations. The repeatability limits show the accuracy of the method at different CaCO₃ concentrations. For 25 mg/L, the maximum difference between measurements is 3.9 mg/L; for 125 mg/L, it is 1.0 mg/L (high precision); and for 625 mg/L, it is 90.9 mg/L, indicating greater variation at higher concentrations. The method showed low coefficients of variation, ranging from 1.6 % to 2.1 %, which reflects good reproducibility and indicates high precision. The relative errors for the 125 mg/L and 625 mg/L concentrations were minimal (0.3 %), demonstrating excellent consistency, while for 25 mg/L, the error was higher (5.9 %), reflecting the greater sensitivity to variations at low concentrations. These results confirmed that the method is suitable for a range of concentrations (25 to 625 mg/L), guaranteeing the quality of the measurements. Regarding the duplicates, satisfactory results were obtained. Most of the samples showed identical alkalinity duplicates or the slightest differences, indicating accuracy in the measurements. The validation of the alkalinity method was completed, confirming compliance with the technical requirements for the alkalinity parameter to become accredited by the Portuguese Accreditation Institute.
Description
Keywords
Total alkalinity Alkalinity method validation Titration Drinking water Process water Alcalinidade total Validação do método da alcalinidade Titulação Águas de consumo Águas de processo