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D3590-17 – Standard Test Method Technical Guide
📋 Scope and Significance
ASTM D3590-17 provides standardized test methods for measuring Total Kjeldahl Nitrogen (TKN) in water. TKN is defined as the sum of nitrogen contained in the free ammonia and other nitrogen compounds which are converted to ammonium sulfate under the specified digestion conditions. As nitrogen compounds serve as essential nutrients for photosynthetic organisms, monitoring TKN is critical for controlling discharge into the environment from sources such as fertilizers, cleaning products, and water treatment aids.
⚠️ User Advisory: The standard explicitly states that the precision and bias statements included “may not necessarily apply to the water being tested.” Laboratories must validate the method performance for their specific sample matrix.
⚙️ Test Method Comparison
The standard covers two distinct analytical procedures. Test Method A utilizes manual digestion and distillation followed by titration. Test Method B employs a semiautomated approach based on the Bertholt colorimetric reaction for higher sample throughput.
| 🟦 Feature | 📏 Test Method A | 🔬 Test Method B |
|---|---|---|
| 📐 Full Title | Manual Digestion/Distillation | Semiautomated Colorimetric Bertholt |
| ⚡ Section Reference | 8 – 14 | 15 – 23 |
| 🎯 Analysis Technique | Manual Distillation and Titration | Automated Colorimetric Measurement |
📊 Interferences and Quality Control
Analysts must carefully manage specific interferences. Nitrate creates a serious negative interference, while volatile ammonia contamination can positively bias results. Section 24 of the standard outlines the required Quality Control procedures applicable to both test methods.
| 🟦 Category | 📐 Detail |
|---|---|
| 💡 Negative Interference | Nitrate (NO₃⁻) at a concentration of 250 mg/L can cause zero recovery of added organic nitrogen. |
| ⚡ Contamination Source | Ammonia from the laboratory atmosphere, reagents, or cleaning substances like ammonium hydroxide (NH₄OH) causes severe positive interference. |
| 🎯 Required QC | Use of a Continuing Calibration Blank (CCB) to verify blank response and a Continuing Calibration Verification (CCV) to monitor instrumental drift. |
🚫 Severe Interference Alert! The standard explicitly warns that 250 mg/L NO₃⁻ can result in zero recovery of the target analyte. Furthermore, ammonium hydroxide must not be used as a reagent or cleaning substance in the same room as the analysis to prevent volatile ammonia contamination.
❓ Frequently Asked Questions
🔍 What are the two test methods in D3590-17?
Test Method A is the Manual Digestion/Distillation method (Sections 8-14). Test Method B is the Semiautomated Colorimetric Bertholt method (Sections 15-23).
💡 How does nitrate interference impact TKN results?
Nitrate causes a serious negative interference. The standard reports that a concentration of 250 mg/L NO₃⁻ results in zero recovery of some levels of added organic nitrogen.
⚡ What is the primary source of positive contamination?
Ammonia from the laboratory atmosphere or from ammonium hydroxide reagents and cleaners is the main source of positive interference. NH₄OH must be excluded from the analysis area.
📌 What is the formal definition of Total Kjeldahl Nitrogen?
TKN is the sum of the nitrogen in free ammonia plus other nitrogen compounds converted to ammonium sulfate [(NH₄)₂SO₄] under the specified digestion conditions.