Abstract
pH as a measure of the H+ or OH- concentration in water is an important measure to assess its potability. Several methods exist to pH including litmus tests, universal indicators, and glass electrodes. Commercially available strip tests are easily available around the world and are fairly accurate.
Strip tests work by changing color depending on the pH. Values are assigned by comparing the color developed on the strip the colors printed on the packaging. Because of the inherent variability in human color perception, there could be varying interpretations of the value.
The authors present a method of reading the color developed by the strip test - Hach pH Strips, Cat No 27456-50 - automatically using a smartphone. This method removes subjective interpretation of color as a source of error in strip tests. The apparatus and software account for variables such as varying light conditions, different phone models and time taken for color.
The results show a correlation of [TODO Correlation] with a pH electrode. Though readings taken in conditions of open sunlight and shade do show a variation [TODO Correlation], the authors consider this to be sufficient for a screening test.
Keywords
pH, Strip Test, Dipstick Test, Smartphone, Mobile Phone, Colorimetry
Authors
[TODO]
Introduction
The parameter usually first tested to evaluate drinking water is its pH. This is expected to be in the range of 6 to 9 to be considered potable[citation]. Methods to test pH include litmus paper, litmus liquid, glass electrodes, indicator liquid, and strip tests.
Test Type | Advantages | Disadvantages |
Litmus Paper | Simple to use | Poor accuracy |
Litmus Liquid | Simple to use, good accuracy | Provides semi-quantitative results |
Indicator Liquid | Good accuracy | Gives semi-quantitative results in the absence of a spectrophotometer |
Glass Electrode | Good accuracy, fairly simple to use | Requires maintenance, and is not very convenient for field use |
Strip Test | Simple to use | Semi-quantitative results |
From the table above, it can be inferred that strip tests make for a good screening test, if the disadvantage of semi-quantitative results can be alleviated. Performing a pH strip test is fairly straightforward and can be done in the field with very little training.
The authors propose a method where the human subjectivity in interpreting colour is removed in comparing the colour developed by the strip by automating the colour interpretation by the smartphone app. In screening tests a variability of +/- 0.2 is considered acceptable [cite].
Materials and Methods
Strip Tests
In this study, Hach pH strips were used directly (Cat No. 27456-50). The strips have a range 4 - 9.
[TODO: Images of strips and colour chart]
Colour Card
The colour card developed to automate reading the strip tests measures 167mm x 109mm.