Document ID : TE233
Published Date : 04/30/2019
Source : Hach-US HSO

How do the calibration curves vary for each of the COD reagents?
Explanation of the calibration curves for different COD ranges

The Dichromate Chemical Oxygen Demand (COD) test measures the oxygen equivalent of the amount of organic matter oxidizable by potassium dichromate in a 50% sulfuric acid solution. The end products of the COD reaction include carbon dioxide, water, and various states of the chromium ion. The reaction is always from Cr6+ to Cr3+. The different Hach reagents listed below will determine the amount of chromium in a particular valence state. Either the amount of reduced chromium (chromic ion), or the amount of unreacted dichromate, is measured colorimetrically.

The Hach Ultra Low Range COD test is the lowest range and highest sensitivity COD test available. Results are measured at a wavelength of 350 nm. The maximum sensitivity is at 345 nm, but the test measurement is made at 350 nm for instrumentation considerations. The calibration line for this test has a negative slope. The amount of hexavalent chromium remaining after digestion is measured and it decreases as the COD concentration increases.
Hach’s High Range and High Range Plus COD are the highest test ranges available from Hach Company. The chemistry and calibration data are identical for both tests. High Range Plus COD is designed to eliminate the dilution step normally required for COD samples which have concentrations from 1500 mg/L up to 15,000 mg/L. When the High Range Plus vial is manufactured, dilution water is added directly to the High Range COD Reagent to accomplish a 1 to 10 dilution when 0.20 mL of sample is added to the reagent vial. The results are measured at 620 nm, and the calibration line has a positive slope.  The amount of trivalent chromium is measured, and its concentration increases as the COD concentration increases. One observes the appearance of the green chromium ion, which is a product of the reaction between the reagent and any COD, so the scale is not reversed. The instrument is measuring the amount of green color produced.
重鉻酸鹽化學需氧量(COD)分析測量的是50%硫酸溶液中重鉻酸鉀可氧化的有機物的含氧當量。 COD反應的最終產物包括二氧化碳,水和鉻離子的各種狀態。 反應始終是從Cr6 +到Cr3 +。 以下列出的不同Hach試劑將確定特定價態的鉻含量。 比色法測量還原的鉻(鉻離子)量或未反應的重鉻酸鹽的量。

Hach的超低量測範圍COD分析試劑是目前最低量測範圍和最高靈敏度的COD分析測試。 在350nm的波長下測量結果。 最大靈敏度為345 nm,但基於儀器方面的考慮,測試測量是於350 nm波長進行。 此測試的校正曲線具有負斜率。 測量消解後的六價鉻的殘留量,並隨著COD濃度的增加而減少。

Hach的High Range和High Range Plus COD是Hach所提供的最高測試量測範圍。 兩種測試的化學和校正數據相同。 High Range Plus COD的設計目的是省略1500 mg / L到15,000 mg / L濃度的COD樣品通常所需的稀釋步驟。 製作High Range Plus樣品瓶時,將0.20 mL樣品添加到High Range COD試劑時,將稀釋水直接添加到High Range COD試劑中,以完成1至10的稀釋。 在620 nm波長測量結果,而校正曲線具有正斜率。 測量三價鉻的量,其濃度隨COD濃度的增加而增加。 觀察到綠色鉻離子的出現,這是試劑與任何COD反應後的產物,此反應不可逆,故綠色鉻離子不會受外界影響而減少。 分析儀則會測量綠色的生成量。