PH / Conductivity
Acidity and conductivity are two of the most measured parameters. From simply checking the quality of a fishpond to testing medicine, foodstuff or the composition of paint; Both parameters are these days the basics of Quality control.
pH is the abbreviation of Hydrogen potential in an aqueous solution. The pH scale for pH measurement runs practically from 0 to 14. Pure water has a pH value of 7. Everything lower is acidic and all higher values are basic. The acidity is caused by the detaching of a Hydrogen ion (H+) from its salt. Basicicity is caused by the detaching of a hydrogen oxide ion (OH-) from its salt. The pH-value is a measure of the dissolved and active amount of either H+ or OH- ions. It doesn’t directly relate to the concentration of the acid. Inhibitors and other compounds can severely influence the correlation between pH-value and acid or base concentration.
To measure pH there are many possibilities, from a simple pH- indicator to elaborate electrode systems. The most commonly used pH- electrode is the Glass calomel electrode since 1906. This type of electrode has proven its stability and reliability over many years. The electrode is most reliable between -1 and 12. For higher basicicity other electrodes should be used. A pH-electrode is basically an ion-selective electrode reacting to the H+ ion. This electrode is also sensitive to several other interferences. The matrix of a sample influences the reliability of an electrode. Glass is also sensitive to the presence of Fluoride ion in a solution. This affects the stability of the glass membrane in the electrode. Also fine pigments can cause the membrane to get blocked. For measuring in paint ISO is now working on the acceptance of ISFET electrodes for measuring the pH-value in paint products.
Where pH is an ion selective method, conductivity is a non-selective method. Any salt that dissolves into water and forms an electrolyte will result in an increase in conductivity of that sample. This conductivity is measured in technically the same way that conductivity is measured in an electric wire. The more dissolved salt a solution contains the better it conducts electricity. Conductivity measurement can be influenced by a few parameters. One of these is static electricity that is caused by measuring in small volumes.
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