Water Testing Tips

thegrun · 10/09/2013, 03:24 PM

One of the keys to getting accurate results in our testing is precise measurements. Many of us have never taken a chemistry class or it was so long ago that we have forgotten how to measure correctly. Since the test kits we use were born in the scientific world where measurements are conducted in a uniform manner throughout the world, we should be aware of that methodology. The same method of measurement I used 30 years ago in Professor Pratt’s Chem 101 class is the same method used today in laboratories in Germany. The volumes of liquids and/or dry chemicals we use are in testing are relatively small, so improper measurements can have a significant impact on our results.

Liquid measurements made in a test tube or beaker presents a challenge as to where to read the results since the liquid usually forms a curved surface due to surface tensions. The curve seen at the top of a liquid is called a meniscus; it is the response to its container. The meniscus can be either concave or convex (curve down or up). A concave meniscus which is the most common in our water testing occurs when the molecules of the liquid are more strongly attracted to the container than to each other. A convex meniscus like mercury in glass is produced when the molecules of the liquid are more strongly attracted to each other than to the container. In some cases, the meniscus appears flat like some fluids in a plastic test tube. When we read the scale on the container we read from the center of the meniscus (the lowest or highest point found at the center of the test tube). For water and most other liquids this is the lowest point of the surface.

When we use a syringe to measure liquid you start with the plunger pushed fully to the bottom of the syringe. Place the end of the syringe (the hub) in the fluid being measured and gently pull the plunger up past the desired gradation mark on the side of the syringe. Then slowly depress the plunger until the very bottom of the plunger is even with the desired gradation mark. There will be a small bubble between the bottom of the plunger and the top of the fluid being measured. Do not worry about the bubble; it was formed by the air gap between the bottom of the plunger and the fluid in the hub that was present when we first started to draw fluid up into the barrel of the syringe and has been accounted for in the gradation marks.

For dry measurements made with a measuring spoon it is assumed that the chemical being measured will be level with the top of the spoon we are using. The best method to achieve a level surface is to initially overfill the spoon and then scrape across the top of the spoon with a spatula or other flat device.
As far as the chemicals we use it is important that they are properly stored in a dry environment at room temperature and are not past their expiration date. Should you use chemicals past their expiration date? No. If we are looking for accurate test results (if we don’t care about accurate results why bother to test in the first place?) do not use expired chemicals.
Proper cleaning of the test tubes, beakers, syringes and spoons after use is also important. Residue left over from previous tests can alter our results. Again since the quantities of chemicals we use are so small a very small amount of residue can change the test readings. Thoroughly wash all testing apparatus when done and rinse in RO/DI water after cleaning.
Finally when using color changing titration tests (the most common type in the hobby) the results are measured when the analyte has completely changed color, not at the start of the color change. This usually requires us to add one or two additional drops of the titrant to achieve full color change after the initial start of the color change.
Happy testing!

NHATRANG · 10/09/2013, 08:10 PM

Good advice, thanks.

Reef Frog · 10/10/2013, 12:08 AM

Very good advice indeed! The only other idea to add is to get some good magnification eyeglasses so you can read the teeny tiny 1/100 ml graduations on those 1ml syringes. Plus black marks on the tubes with black rubber plungers aren't exactly the easiest to read.

10/09/2013, 03:24 PM	#1
thegrun Registered Member Join Date: Dec 2008 Location: Garden Grove, Ca Posts: 17,023	Water Testing Tips One of the keys to getting accurate results in our testing is precise measurements. Many of us have never taken a chemistry class or it was so long ago that we have forgotten how to measure correctly. Since the test kits we use were born in the scientific world where measurements are conducted in a uniform manner throughout the world, we should be aware of that methodology. The same method of measurement I used 30 years ago in Professor Pratt’s Chem 101 class is the same method used today in laboratories in Germany. The volumes of liquids and/or dry chemicals we use are in testing are relatively small, so improper measurements can have a significant impact on our results. Liquid measurements made in a test tube or beaker presents a challenge as to where to read the results since the liquid usually forms a curved surface due to surface tensions. The curve seen at the top of a liquid is called a meniscus; it is the response to its container. The meniscus can be either concave or convex (curve down or up). A concave meniscus which is the most common in our water testing occurs when the molecules of the liquid are more strongly attracted to the container than to each other. A convex meniscus like mercury in glass is produced when the molecules of the liquid are more strongly attracted to each other than to the container. In some cases, the meniscus appears flat like some fluids in a plastic test tube. When we read the scale on the container we read from the center of the meniscus (the lowest or highest point found at the center of the test tube). For water and most other liquids this is the lowest point of the surface. When we use a syringe to measure liquid you start with the plunger pushed fully to the bottom of the syringe. Place the end of the syringe (the hub) in the fluid being measured and gently pull the plunger up past the desired gradation mark on the side of the syringe. Then slowly depress the plunger until the very bottom of the plunger is even with the desired gradation mark. There will be a small bubble between the bottom of the plunger and the top of the fluid being measured. Do not worry about the bubble; it was formed by the air gap between the bottom of the plunger and the fluid in the hub that was present when we first started to draw fluid up into the barrel of the syringe and has been accounted for in the gradation marks. For dry measurements made with a measuring spoon it is assumed that the chemical being measured will be level with the top of the spoon we are using. The best method to achieve a level surface is to initially overfill the spoon and then scrape across the top of the spoon with a spatula or other flat device. As far as the chemicals we use it is important that they are properly stored in a dry environment at room temperature and are not past their expiration date. Should you use chemicals past their expiration date? No. If we are looking for accurate test results (if we don’t care about accurate results why bother to test in the first place?) do not use expired chemicals. Proper cleaning of the test tubes, beakers, syringes and spoons after use is also important. Residue left over from previous tests can alter our results. Again since the quantities of chemicals we use are so small a very small amount of residue can change the test readings. Thoroughly wash all testing apparatus when done and rinse in RO/DI water after cleaning. Finally when using color changing titration tests (the most common type in the hobby) the results are measured when the analyte has completely changed color, not at the start of the color change. This usually requires us to add one or two additional drops of the titrant to achieve full color change after the initial start of the color change. Happy testing!

10/09/2013, 08:10 PM	#2
NHATRANG Registered Member Join Date: Sep 2013 Location: Milford Posts: 109	Good advice, thanks.

10/10/2013, 12:08 AM	#3
Reef Frog Registered Member Join Date: Feb 2013 Location: Maryland Posts: 3,121	Very good advice indeed! The only other idea to add is to get some good magnification eyeglasses so you can read the teeny tiny 1/100 ml graduations on those 1ml syringes. Plus black marks on the tubes with black rubber plungers aren't exactly the easiest to read.