Tuesday, August 21, 2018

Are your pH readings credible?


pH measurements are a basic requirement for all laboratories- from school laboratories to advanced research laboratories. The humble pH meter is often taken for granted by everyone in comparison to sophisticated instruments like Mass Spectrometers,Chromatographs and Atomic Absorption Spectrometers.We often do not give a serious thought before reporting a pH value. However, it should be realized that pH plays a significant role in reaction kinetics,completion of chemical and biochemical processes for getting the desired yields and efficacy of medical formulations.Even the pH of industrial effluents needs to be controlled before discharge to prevent harm to aquatic life.

First and foremost you should realize that the pH meter is a delicate device and due care needs to be exercised in its handling and use. You should never be tempted to use it as a stirring rod when making drop wise additions of acid or base solution for pH adjustments. Its bulb is extremely delicate and can suffer damage in the process.

The new pH meter probe needs to be rinsed with distilled water and then soaked in KCl solution. Soaking in distilled water is not recommended as it can affect the concentration of KCl inside the bulb.Ensure that sufficient quantity of KCl is always present and if evaporative losses take place the loss should be replenished.The reference junction should remain fully immersed all the time.

Calibration is equally important for pH meter as for other sophisticated instruments.Daily calibration with standard buffer solutions is strongly recommended.

 The probe should be kept cleaned after each use.This practice ensures freedom from drafting values and unexpected readings.Every time soak dry small droplets sticking to the bulb and avoid wiping as it can damage the probe membrane.

Use of pH meter outside the range 1-12 is not advised. Acid error is inherent in strong acids having pH below 1.Due to incomplete dissociation the pH reading is higher than the actual value that you would calculate from its molar strength. On the other hand in strong alkaline solutions having pH above 12 alkaline error begins to interfere.Na+ ions start replacing H+ ions in the glass membrane of the sensor bulb with the result that the observed value will be less than the actual value.

Special cleaning is essential after use in biological fluids and non-aqueous media For example,if used in oil or grease it should be soaked in a detergent or methanol, bleaching powder solution soaking when used for measuring bacterial media and in 1% pepsin in 0.5MHCl for removal of any protein deposits.In all cases final soaking should be in distilled water before reuse.

pH readings are temperature dependent. Maintain the laboratory temperature at 250 C and allow hot or cold solutions to attain laboratory temperature before noting down the readings.

It is not sufficient to keep the bulb or probe partial immersed in the solution whose pH is to be recorded. Ensure that sufficient volume is available so that the probe or bulb is completely immersed   and there is no contact with the side walls or bottom of the container.

After taking the readings do follow the prescribed cleaning and storage procedures when the pH meter is to remain idle for long time periods.This will ensure same confidence level every time it is used. A well maintained pH meter can be used for months and years and generate reliable data provided it is properly maintained and calibrated as per prescribed recommendations.

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