Hi Ebrahim;

Of course, the best instrument you use the more confident you can be with the measure, but the accuracy needed depends on the use you do of these data.

pH strips are very cheap but too coarse for much purposes, as the accuracy moves through units of pH. Better data (tenths of unit) can be obtained by using small pH-meters (e.g. those of Hanna Instruments), which are relativekly cheap (less than 100 $), but need frequent calibration that you can easily make with pH buffer standards. Then, if you need even more accuracy, you need to move to more sensitive (and more expensive) pH-meters, which can offer sensitivities of

hundredths of unit.

Best

Antonio

Your best option will depend on how and where you will use it.  Will your samples be taken from field to fixed lab?  Is pH measurement required continually (process stream) or single grab sample?  In-situ?  First determine how the unit will be used and that will narrow your search.   For instance if you are going to be measuring pH in stream, like from bridges you may want to consider a sonde type meter which can be lowered from a cable into water.  If you are bringing samples back to lab you would want a bench-top meter.  If you are required to take measurements for compliance or regulatory purposes you will need a better instrument and will need to follow specific methods, develop and maintain calibration data.  If you just need to know the approximate pH - the old pH paper might just do the trick.

You did not provide any qualifying information such as what types of solutions you will be testing, the environmental conditions at the test location, the volume of liquid available for test, the expected range of pH and if the solutions are buffered or not? This type of information is critical to provide constructive suggestions to you.

Since cost sounds like a serious concern and you imply that you are not interested in using a lab grade pH meter with calibration solutions (*Which is the best way to precisely measure pH), a general suggestion would be to use pH test paper strips. They are inexpensive, require little training, can be accurate across even narrow ranges of pH and provide quick results. *Universal pH indicator solution would be another option, though this is less precise than the paper strips, but may be applicable to your needs so I will mention it.

I have added a link to a company that sells them both so you can get an idea of what pH ranges are available for purchase (I do NOT work for this or any other company that sells pH indicators at all and would suggest you purchase these or similar items, if applicable, from whichever source you prefer).

http://www.emdmillipore.com/US/en/products/analytics-sample-prep/test-kits-and-photometric-methods/visual-tests-for-semi-quantitative-analyses/ph-tests/ph-indicator-strips/UJWb.qB.jtAAAAE_9A93.Lxj,nav?ReferrerURL=http%3A%2F%2Fwww.google.com%2Furl%3Fsa%3Dt%26rct%3Dj%26q%3D%26esrc%3Ds%26source%3Dweb%26cd%3D3%26ved%3D0ahUKEwjDxezqr6HPAhVFRSYKHcFiAoUQFghLMAI%26url%3Dhttp%253A%252F%252Fwww.emdmillipore.com%252FCA%252Fen%252Fproducts%252Fanalytics-sample-prep%252Ftest-kits-and-photometric-methods%252Fvisual-tests-for-semi-quantitative-analyses%252Fph-tests%252Fph-indicator-strips%252FUJWb.qB.jtAAAAE_9A93.Lxj%252Cnav%26usg%3DAFQjCNH02C_NLuYyA56xQ6gtupSLyW6MAQ

Dear Ibrahim,

You need to use pH strips to perform the measurments.

Those strips are low cost and precise.

http://www.sigmaaldrich.com/catalog/product/aldrich/z264768?lang=en®ion=MA

Good luck !

pH of the solution can be previously determined by digital pH meter. This is very economical, easy to operate and quick in determination. Other details are available in instruction manual. Instrument on line available in market.

I would suggest that you discuss face-to-face your problem with a member of the staff of the Chemistry Department (Analytical Chemistry) of your University.

A serviceable pH meter in New Zealand can be obtained for $100 or thereabouts.  If you require highly accurate measurements (i.e to 2 or more decimal places) you will need a higher quality meter and calibration solutions.

With a ph meter previously calibrate with a standar solution (solution of known concentration)