Should I be measuring water quality in TDS (total dissolved solids) or EC (electrical conductivity)?

Hi there,

I would like to ask you with regards to measuring TDS. I have did some reading and searches but am still confused. Would need your help in checking my understanding and also some qns.

Im looking to get HM Digital COM-100 which has 4 modes, µS, mS, ppm 0.5 (NaCl) Scale & 0.7 (442™) Scale. Actually I am interested in using the 442 mode. Based on the specification it is calibrated by 1413 microsiemens solution supposedly KCl i think. Im thinking of getting a Eutech 442 300ppm or 50ppm solution to recalibrate the meter.

https://www.fishersci.se/shop/products/eutech-tds-442-standard-solution-1000-ppm-442-1/15197613

1.  Which mode do i set to when I'm recalibrating with say 300ppm 442 solution?
2. Another solution which is from HM digital is a 342ppm NaCl solution.

http://hmdigital.com/product/c-342/

The specification noted that 342ppm NaCl = 478ppm in 442. Does it mean that i can calibrate at 442 mode with 478ppm or just mean that when reading at NaCl mode it will be 478?

Regards

Ron

Hi Ron,

You may want to read a previous post:How are total dissolved solids (TDS), electrical conductivity (EC) and salinity different

Incidentally, you did not mention what kind of water sample you are testing as that will affect the testing mode you want to use. But I suppose you have a rationale for choosing the 442 mode.

First and foremost, I am not a fan of using TDS for water water quality testing as it is a calculated value and it's relevance rests on the assumption that you have a certain composition of substances in your water sample. I invariably measure EC for all my water quality tests because it is a measured value (this is technically not correct since voltage is the measured value but the correlation between the 2 is strong enough for my purpose) and I DO NOT know what are the substances in my water samples. To me, it only makes sense to measure TDS when you are only dealing with a rather homogeneous water sample which does not fluctuate much in its composition.

Mathematically,

TDS (ppm) = conversion factor * EC (uS/cm)

The key lies in what conversion factor (CF) to use. If you are dealing with a sample predominantly made up of sodium chloride (NaCl) e.g. seawater, then the typical CF is ~0.5.

The 442 measurement mode refers to a sample made up of 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride. Some users claim that this composition closely resembles natural waters though I believe natural waters vary too widely to follow a simple ratio of 4:4:2 throughout. Anyway, if you are using the 442 mode, the conversion factor is ~0.7.

(In actual fact, the CF is not constant throughout all concentrations of a substance. in general, as the concentration increases, the CF also increases.)

Example

342ppm of NaCl = 342 / 0.5 = 684 uS/cm in EC

and 684 uS/cm in EC = 0.7 * 684 = 479ppm in 442

Calibration

I suppose the easiest way to calibrate is to follow the supplier's instructions as they are. I have no experience with your particular model of TDS tester and do not know what kind of calibration standards are required.

But I guess all of us want to get the most bang out of our buck and go for a more economical calibration solution. The next easiest way is then to get an equivalent calibration standard that has the same EC. 

For example, if the TDS needs to be calibrated with 479ppm 442 solution in 442 mode, you can substitute with a 342ppm NaCl solution to used in the same 442 mode.

Again, the way to get around all this hassle of TDS and composition is to use the EC mode (uS or mS depending on your range) and calibrate in the same EC mode using calibration standards certified in uS or mS.

Figure: A typical TDS/EC meter calibration standard. If you try applying the formulas above, the conversion factors are higher than 0.5 for NaCl and 0.7 for 442.

Potable Water Testing in Seawater Desalination Plant

I received a request for help from a staff in a seawater desalination plant. As it was a private request, I will not paste its entire contents here. The short version of it is: he needs some good reference books to help him in his job of performing water quality testing in the plant.

To answer your questions, I have 2 boring but essential books to recommend. They have LOTS of content but are good reading for anyone in the drinking water industry.

1. WHO Drinking Water Quality Guidelines (4th ed)
   Hey, it’s free by clicking the above link to get a softcopy! You probably already know that Singapore follows the WHO Drinking Water Quality Guidelines.
   
2. Standard Methods of the Examination of Water and Wastewater (APHA, AWWA, WEF; 20th ed, 1999) Clicking the above link will get you the softcopy of the 20^th ed. However, if you want the latest edition (23^rd ed, 2017), look for it on Amazon which quoted a price of > USD200.

 

WHO Drinking Water Quality Guidelines (4th ed)

  

Standard Methods of the Examination of Water and Wastewater (APHA, AWWA, WEF; 23rd ed, 2017) - available on Amazon. See text above for the link to the softcopy for the 20th ed.

The one device you can't do without for serious water sampling

I am of course talking about the depth sampler. If you want to collect a water sample at a given depth, you simply need this standard piece of equipment. Depending on the length of your rope, it can easily go down to 100 feet and beyond.

You can suspend this guy from a surface craft (e.g. kayak, motor boat) or a bridge overlooking the water body of interest. Once you have reached the desired depth as indicated on the marking on the rope, you release the messenger (a metal weight) which rushes down along the rope towards the sampling bottle. Upon hitting the bottle's "trigger", the 2 ends snap shut like a fish trap to enclose your water sample within the bottle.

Purchase Considerations

1. Alpha (aka van Dorn) vs. beta (Kemmerer). Alpha samplers are not recommended for chemical analysis (trace metals and organics) of the water sample due to possible cross contamination (mercury, phosphorus) from the materials in its construction (seals & closure tube). For chemical analysis, one is advised to use a beta sampler.
   
2. PVC vs. acrylic. Whether alpha or beta, the bottle body can be either made of PVC or acrylic. PVC is opaque but durable while acrylic is clear and less durable. If you need to see the contents within the sampling bottle, by all means, go for acrylic.
   
3. Horizontal vs. vertical. A horizonal sampler means the sampling bottle descends into the water parallel to the bed. Water enters from both sides of the bottle. This is useful for sampling at the thermocline, other stratification levels or just above the bottom sediment. A vertical sampler on the other hand descends with its body perpendicular to the bed. Water flow is not restricted during descent, making it desirable for collecting plankton and suspended sediments.

Figure: Lowering the depth sampler into the water body of interest. In this case, we were using the alpha, horizontal, acrylic version.

Figure: After retrieving the depth sampler, the water sample was drained into a suitable container.

Worried about lead? Here's a simple test

Lead in tap water was mentioned in some of my earlier posts - 1, 2. Essentially, SG tap water is relatively safe when it comes to lead. "Unless you happen to live in a really old building which still retains its leaded pipes over the decades... and unless somehow your building falls under the radar of the extensive sampling and monitoring programme... then maybe you should be worried."

Nevertheless, I had a concerned reader contacting me about his worry of lead in tap water because he bought a water faucet over the net from Taobao. Apparently, he read (only after buying the faucet) about lead being used in fabrication of the faucet. With online shopping becoming more common now, who even knows much about the product one bought? Where are the parts from? Are they certified safe? Are the accreditation standards different from SG?

But I hear you say that it is SO convenient to buy online. Indeed it is. I too buy lots of stuff online...

Anyway, back to lead...

In another post, I recommended this product to test for lead in water painlessly.

PurTest DIY Lead Water Testing Kit. USD16.95

I chanced upon another similar product but of a different supplier on Amazon and got 2 tests for SGD33, inclusive of delivery.

Figure: Test kit for lead from USA. About as foolproof as a test kit can be.

Figure: The testing materials are all packed in a foil package. Open it only when you are ready to test your water.

Figure: A workshop participant residing in the Havelock area was concerned about lead in her tap water as her building was really old, say 40 years or older. An aliquot of her tap water was placed in the plastic container (part of the test kit) before dipping the test strip inside and waiting for 10min.

Figure: Just like a pregnancy test kit! The pattern of the 2 lines - 1 faint, 1 dark, indicate a negative result. As usual, such a test only tells you YES or NO or in this case, whether the lead level has exceeded the safety level. Do keep in mind that this test kit is designed for USA with a safety level of 15ppb (parts per billion) whereas SG follows WHO standards which stipulate a safety level of 10ppb. Nevertheless, I am not going to get a stressed up over a difference of 5ppb. If the lead level is safe enough for USA, it is good enough for me. Still, doing the test 1 or 2 more times and getting a negative will improve my confidence in the water's safety.