Phosphates in the Reef Tank: Understanding the HI713 and HI736 (Free Conversion Table)

[Hanna Instruments]

Phosphorus and Phosphates in your Reef tank: Understanding the HI713 and HI736

By: Kevin Costa, Hanna Instruments

Introduction

Measuring phosphates is a critical component of reef tank water testing practices. Phosphates are one of the many forms of phosphorus compounds found in marine waters. Excessive phosphate levels can prevent calcification, skeletal formation and growth of many coral species particularly those referred to as SPS or small-polyp stony. In addition, high phosphate levels can cause nuisance algae growth in your reef tank. This can become problematic because algae often outcompetes sessile organisms for light, nutrients and space, jeopardizing long term coral health in your reef. There are several compounds containing phosphorus in the marine aquaria, but almost every test kit used in the aquarium industry measures orthophosphates.

Orthophosphate analysis is the only type of phosphorus test which can be performed without sample preparation involving a digestion in acid. Orthophosphates are compounds containing both phosphorus and oxygen atoms, and commonly referred to as phosphate. Tests for orthophosphates are usually displayed as either PO₄³ˉ (phosphates) or PO₄-P (orthophosphate as phosphorus). The former combines the oxygen atoms into the results while the latter only considers the phosphorus atoms. Orthophosphate tests that display results as phosphorus are not to be confused with a total phosphorus test, which requires a digestion and is much more difficult to perform.

Understanding the Conversion

Converting PO₄-P to PO₄³ˉ is a simple equation where PO₄-P (orthophosphate as phosphorus) is multiplied by 3.066 to equal PO₄³ˉ (phosphate) concentration. The conversion factor of 3.066 is based off of the atomic weights for oxygen and phosphorus molecules. One phosphorus atom has the atomic weight of about 31 g/mol while an oxygen molecule has an atomic weight of 16 g/mol. Since an orthophosphate molecule has one phosphorus atom weighing 31g/mol and four oxygen molecules (64g/mol (or 16 g/mol each) the total molecular weight is 95 g/mol.

We can divide the total weight of an orthophosphate molecule by the weight of one phosphorus atom to get the conversion factor of 3.066. To convert parts-per-billion (ppb) to parts-per-million (ppm) we simply divide by 1,000. To convert the HI736 ULR Phosphorus Checker reading to ppm phosphate, multiply the results by 3.066 then divide by 1,000.


PO₄³ˉ= 16×4+31= 95 g/mol

95g/mol÷31 g/mol= 3.066 Conversion Factor


Testing your Reef Tank’s Phosphate

Our primary orthophosphate testing instruments for the hobbyists reef market are the HI713 Low Range (LR) Phosphate Checker HC and the HI736 Marine Ultra Low Range (ULR) Phosphorus Checker. The HI736 is labeled ‘Marine’ because it is specifically designed for saltwater and utilizes an absorbance curve for marine environments, while the HI713 can be used in either freshwater or saltwater. The HI736 ULR Phosphorus Checker has a range of 0 to 200 ppb PO₄-P orthophosphate as phosphorus, which correlates to 0.003ppm to 0.613ppm PO₄³ˉ. The HI713 LR Phosphate Checker has a range of 0.00 to 2.50 ppm PO₄³ˉ orthophosphate. Both Checkers use an adaptation of the Ascorbic-Acid method, with an LED light source at a wavelength of 525 nm and a silicon photocell as a light detector.

The HI713 LR Phosphate Checker is be a better fit for reef tanks housing coral species less sensitive to phosphate, such as certain soft corals. The HI713 LR Phosphate Checker is also ideal for aquarists who have freshwater aquariums or fish only saltwater tanks (FOWLR). The HI736 ULR Phosphorus Checker is designed for reef tank owners keeping SPS corals and aquariums aiming for low phosphate levels less than 0.03ppm. While the HI713 LR Phosphate Checker will work for any saltwater aquarium, the unit has an accuracy statement of ±0.04 ppm ±4% of reading. This means if your HI713 LR Phosphate Checker displays a result of 0.08ppm, the measurement uncertainty is from 0.04ppm to 0.12ppm, which can be problematic for certain reef aquarists. Displaying the results in part-per-billion phosphorus give a narrower range for detection, but increased accuracy at low range phosphate concentrations desired by many reef tank hobbyists. The HI736 Marine ULR Phosphorus Checker should not be confused with a total phosphorus test. Testing for orthophosphates is standard practice for saltwater aquariums and provides valuable insight into your reef tank’s water chemistry to help ensure long-term coral health.

HI713-11 Phosphate Standard
http://hannainst.com/hi713-11-phosphate-low-range-checker-calibration-set.html

HI713-25 Phosphate Reagent
http://hannainst.com/hi713-25-phosphate-checker-reagents.html

HI736-11 ULR Phosphorus Standard
http://hannainst.com/hi736-11-phosphorus-checker-calibration-set.html

HI736-25 ULR Phosphorus Reagent
http://hannainst.com/hi736-25-phosphorus-ultra-low-range-checker-reagents.html

HI731315 Extra Cuvettes
http://hannainst.com/hi731315-glass-cuvettes-and-caps-for-checker-hc-colorimeters.html

 

[madweazl]

I sent some samples to Triton a couple weeks and took my own measurements for the sake of comparison at the same time. Triton results showed 0 P and PO4 while I measured .046 PO4 with the ULR (reagents expire in 2018 but I cant recall the month off the top of my head). I'm typically in the .03 range using the Hanna but I don't test it very often (monthly at most I imagine). I use the same cuvette for calibration and the reagent, try pretty intently to get all the reagent out of the packet, and keep the cuvettes clean between tests. Do you have any tips for aiding in accuracy? I'll order the standard solution and see how that comes out I guess.

 

[Hanna Instruments]

I sent some samples to Triton a couple weeks and took my own measurements for the sake of comparison at the same time. Triton results showed 0 P and PO4 while I measured .046 PO4 with the ULR (reagents expire in 2018 but I cant recall the month off the top of my head). I'm typically in the .03 range using the Hanna but I don't test it very often (monthly at most I imagine). I use the same cuvette for calibration and the reagent, try pretty intently to get all the reagent out of the packet, and keep the cuvettes clean between tests. Do you have any tips for aiding in accuracy? I'll order the standard solution and see how that comes out I guess.

Hello, thanks for using our products! Please follow the steps outlined in our 8 Checker Best Practices Blog also with the HI736 ULR Phosphorus model, you got a reading of 15ppb?
Does Triton measure orthophosphate and if so what units are they measuring (ppb or ppm)?

 

[madweazl]

Hello, thanks for using our products! Please follow the steps outlined in our 8 Checker Best Practices Blog also with the HI736 ULR Phosphorus model, you got a reading of 15ppb?
Does Triton measure orthophosphate and if so what units are they measuring (ppb or ppm)?

My results are typically in the 9-12 range on the ULR but at the time of taking these samples, it was 15. As for the steps, I do follow those with the exception of placing the cuvette in the checker with the same orientation, I'll make a mark on the cuvette next time I test to try and isolate that better. Expiration on the samples is actually 07/2019. I'm not sure how Triton derives P and PO4 (cant find info in English).

 

[bubbaque]

I had a pretty big difference from my triton results and the ulr checker. Triton results said po4 was .02 and the Hanna was reading .07.

 

[Reefltx]

I have the opposite. Hanna ULR 0ppb and triton .03

 

[justingraham]

I have the opposite. Hanna ULR 0ppb and triton .03

Mine as well

 

[Hanna Instruments]

My results are typically in the 9-12 range on the ULR but at the time of taking these samples, it was 15. As for the steps, I do follow those with the exception of placing the cuvette in the checker with the same orientation, I'll make a mark on the cuvette next time I test to try and isolate that better. Expiration on the samples is actually 07/2019. I'm not sure how Triton derives P and PO4 (cant find info in English).

The measures of uncertainty for both Checkers are as followed:
HI736 measure of uncertainty= ±5 ppb ±5% of reading (±5 0.015 ppm PO₄³ˉ)
HI713 measure of uncertainty= ±0.04 ppm ±4% of reading

It can never hurt to have the standards for your Checker to make sure the unit is functioning at its best. (HI736-11)

Its important to note that each parameter has a maximum recommended holding time before analysis is preformed and preservation technique: For the various phosphorus compounds is as followed:

Orthophosphate: Filter immediately, 48 hours. Cool, 4 °C
Phosphorus, total: Cool, 4 °C, H2SO4 28 days. to pH < 2

Referenced from the Hach DR700 (7-30-96-9ed Rev. 2b, 11/01) Page 68-69: (1-50 to 1-51) Please see attached document

"(pg. 68 (1-50)- Take samples as closely as possible to the source of the supply to minimize the effects of a distribution system.Generally, as little time as possible should elapse between collecting the sample and making the analysis.... Depending on the nature of the test, special precautions in handling the sample also may be necessary to prevent natural interferences such as organic growth or loss or gain of dissolved gases. Sample preservatives and storage techniques are described in each procedure for sample held for later testing...."

(pg. 69: (1-51)) Preservation techniques ****** the chemical and biological changes continuing after sample is taken. These changes may change the amount of a chemical species available for analysis. As a general rule, it is best to analyze the samples as soon as possible after collection. This is especially true when the concentration is expected to be low. Analyzing immediately reduces the potential for error and minimizes labor. Preservation methods are limited generally to pH control, chemical addition, refrigeration and freezing. The recommended preservation for various constituents is given in Table 9. Other information provided in the table is the suggested type of container and the maximum recommended holding times for properly preserved samples."

It is unclear which form P (phosphorus represents in the Triton test. Generally P refers to Total Phosphorus however it could be orthophosphate as phosphorus, similar to that in the ascorbic acid method in our Checkers. We may not be comparing apples to apples here and there could be some variation from the holding time, lack of preservation techniques during shipping and what compound are being analyzed.

 

[Hanna Instruments]

I had a pretty big difference from my triton results and the ulr checker. Triton results said po4 was .02 and the Hanna was reading .07.

Referenced from the Hach DR700 (7-30-96-9ed Rev. 2b, 11/01) Page 68-69: (1-49 to 1-50)

"Taking Water Samples Collect samples for analysis carefully to make sure the most representative sample possible is obtained. In general, they should be Figure 14 General Purpose Distillation Apparatus with Heater and Support Apparatus 1-50 taken near the center of the vessel or duct and below the surface. Use only clean containers (bottles, beakers) for collecting samples. Rinse the container several times first with the water to be sampled. Take samples as closely as possible to the source of the supply to minimize the effects of a distribution system. Allow the water to run for sufficient time to flush the system, and the sample container should be filled slowly with a gentle stream to avoid turbulence and air bubbles. Collect water samples from wells after the pump has run long enough to deliver water representative of the ground water feeding the well. It is difficult to obtain a truly representative sample when collecting surface water samples. Best results are obtained by running a series of tests with samples taken from several locations and depths at different times. Results then can be used to establish patterns applicable to that particular body of water. Generally, as little time as possible should elapse between collecting the sample and making the analysis. Depending on the nature of the test, special precautions in handling the sample also may be necessary to prevent natural interferences such as organic growth or loss or gain of dissolved gases. Sample preservatives and storage techniques are described in each procedure for sample held for later testing. Acid Washing Bottles A procedure may suggest acid-washing the sample bottles to minimize the effect of interferences. This is accomplished by using a detergent to clean the glassware or plastic-ware, rinsing with tap water, rinsing with a 1:1 Hydrochloric Acid Solution or 1:1 Nitric Acid Solution, rinsing with deionized water. This may require successive rinses, up to 12-15 may be necessary if chromium is being determined. Air dry. The nitric acid rinse also is important if lead is being determined. Chromic acid or chromium-free substitutes may be used to remove organic deposits from glass containers, but rinse containers thoroughly with water to remove traces of chromium. Glassware for phosphate determinations should be washed with phosphate-free detergents and acid-washed with 1:1 HC1. This 1-51 glassware must be rinsed thoroughly with distilled water. For ammonia and Kjeldahl nitrogen, the glassware must be rinsed with ammonia-free water."

See manual attached above

 

[Hanna Instruments]

I have the opposite. Hanna ULR 0ppb and triton .03

Mine as well

Please also find an attached document from Accutest Laboratories who does water testing for a variety of industries. Hanna does not test water for customers on a service basis.

SAMPLE CONTAINER, VOLUMES, PRESERVATIVES, AND HOLDING TIMES
http://www.accutest.com/e3e0d078-7c9d-4a71-bc68-8eba5b6e2236/Vo&Pr.pdf

 

[Hanna Instruments]

One can also use our HI93703-50Cuvette Cleaning Solution (230 mL) however it is not the same as phosphate-free detergents and acid-washed with 1:1 HC1

 

[Hanna Instruments]

It is important to note that these sample collection techniques are not referencing ICP-OES analysis directly, but refer to Spectrophotometers and other methods not clearly defined. A Checker uses a similar optical test as the spectrophotometer phosphate analysis but we do not manufacture ICP-OES equipment.

Variations in sample collection techniques could potentially exist due to the different type of test ICP-OES is. Triton provides a great service to the aquarium hobby and we simply want to provide the aquarist with the most accurate, reliable and easy to use instruments possible for testing backed with a foundation of education to better understand their results.

Stay posted for future updates regarding this matter.

 

[Hanna Instruments]

PDF version of above article

 

[Bph]

Is 0.2 ppm phosphate to high? SPS dominate tank

 

[DE FISH]

Is 0.2 ppm phosphate to high? SPS dominate tank

This thread is from 2017 yes 0.2 would be deemed as high however if it isn’t broken don’t fix it is my rule if your tank is doing well then leave it be there are sps tanks out there running higher than that but also lower

 

[Bph]

Ok thank you