Arsenic Colour Change Tests: What Results Can and Can’t Tell You

Arsenic colour change tests, such as the Gutzeit test or modern test strips, produce a colour change that can indicate the presence and rough amount of arsenic in a sample. A positive result means arsenic is present above the test’s detection limit, but it cannot tell you the exact concentration, the chemical form of the arsenic, or whether the level is safe for a specific use. These tests are screening tools, not definitive proof, and their results must be interpreted with care.

Formula

The calculator above gives a rough “arsenic index� based on colour intensity and test time, but it is not a real analytical formula. In actual colour change tests, the relationship between colour intensity and arsenic concentration is usually non-linear and depends on the specific test kit. For example, the Gutzeit test uses mercuric bromide paper: arsenic gas reacts with it to form a yellow-brown stain, and the stain length or colour depth is compared to a standard chart. There is no universal formula; each kit has its own calibration curve. The index here is simply (colour intensity × 60) ÷ test time, which normalises the reading to a per-minute basis—useful only for comparing two runs with the same kit.

Worked Example

Using the defaults: test time = 20 minutes, colour intensity = 5. The index = (5 × 60) / 20 = 15. This means that if you observed a moderate colour after 20 minutes, the “arsenic index� is 15. But remember: this number has no universal meaning. A real kit would tell you, for instance, that a colour of 5 corresponds to 0.5 mg/L arsenic based on its own chart. Always rely on the kit’s instructions, not on this index.

How Colour Change Tests Work

Most arsenic field tests rely on converting arsenic in a sample into arsine gas (AsH₃) using a reducing agent like zinc and acid. The gas then reacts with a reagent—often mercuric bromide or silver nitrate—on a paper strip. The resulting colour change (yellow, brown, or grey) is compared to a colour chart. The intensity or the length of the stain correlates with the arsenic concentration. Some modern kits use a resin that traps arsenic and then a colour-forming reagent. In all cases, the test is semi-quantitative: it gives a range (e.g., 0–10, 10–50, 50–100 µg/L) rather than an exact number.

What a Positive Result Can Tell You

• Arsenic is present above the test’s detection limit (often 10 µg/L or 50 µg/L).
• The concentration falls within a certain range, based on the colour chart.
• The sample likely contains inorganic arsenic (the more toxic form) if the test is specific to that form.
• Immediate action is needed: do not drink the water, and get a laboratory analysis.

What a Positive Result Cannot Tell You

• The exact concentration—only a range.
• The chemical species: arsenite (As³â�º) vs. arsenate (Asâ�µâ�º) vs. organic arsenic (like arsenobetaine, which is much less toxic).
• Whether the level is safe for a specific use (e.g., irrigation, cooking, bathing) without comparing to local guidelines.
• The source of the arsenic (natural geology, industrial pollution, pesticides).
• Whether the arsenic is bioavailable (able to enter the body) or bound in a less harmful form.

Common Problems and Misconceptions

False positives: Other substances that produce arsine-like gases (e.g., antimony, phosphorus) can cause a colour change. Some kits include a pre-treatment to suppress these interferences, but not all do.

False negatives: If the sample is not acidified properly, or if the reducing agent is old, arsenic may not convert to arsine, leading to no colour change even when arsenic is present. Also, if the arsenic concentration is below the detection limit, the test shows negative, but that does not guarantee the water is safe—it only means below the kit’s threshold.

Misreading colours: Colour charts are subjective. Two people can read the same strip differently. Lighting conditions and colour blindness affect results.

Misconception: “It’s positive, so it’s toxic.� Not necessarily. The test may detect organic arsenic (like from seafood) which is far less toxic. Conversely, a negative result does not mean zero arsenic—it means below the detection limit.

How to Choose a Test Kit

When selecting a colour change test, consider:

• Detection limit: lower is better for drinking water (e.g., 10 µg/L vs. 50 µg/L).
• Specificity: does it distinguish inorganic arsenic from organic?
• Interference: does it suppress antimony and other gases?
• Ease of use: some kits require multiple steps and hazardous reagents.
• Cost per test: cheaper kits may have higher detection limits.
• Regulatory approval: in many countries, only certain kits are approved for compliance testing.

Costs and Safety Considerations

Field test kits range from a few dollars to over a hundred dollars per test, depending on sensitivity and brand. However, they are always less accurate than laboratory analysis (e.g., ICP-MS or atomic absorption), which can cost $50–$200 per sample but gives exact concentrations and speciation. Safety is paramount: the test generates arsine gas, which is highly toxic. Always perform tests in a well-ventilated area or under a fume hood, wear gloves and eye protection, and dispose of reagents as hazardous waste. Never touch the test strip with bare hands after the reaction.

Alternatives to Colour Change Tests

If you need a definitive answer, send the sample to a certified laboratory. Methods include:

• Hydride generation atomic absorption spectrometry (HG-AAS) – sensitive and specific for inorganic arsenic.
• Inductively coupled plasma mass spectrometry (ICP-MS) – can detect very low levels and distinguish species when coupled with chromatography.
• Anodic stripping voltammetry – a field method that can be quantitative but requires training.

For home well owners, the best practice is to test with a laboratory at least once, and use a colour change kit for regular screening. If the kit shows a positive result, confirm with a lab before taking major action like installing a treatment system.

Frequently Asked Questions

Q: Can I use a colour change test to check if my well water is safe?
It can give a quick indication, but it is not a substitute for laboratory testing. If the kit shows a positive result, assume the water is not safe for drinking and get a lab test. If it shows negative, it means arsenic is below the kit’s detection limit, but that may still be above the recommended level (e.g., 10 µg/L) depending on the kit.

Q: Why did my test turn yellow but the chart says it’s negative?
Some kits use a yellow colour to indicate a positive result, others use brown or grey. Always read the specific kit’s instructions. Also, a slight yellow tint can be due to interferences, not arsenic.

Q: What does the colour intensity mean?
Generally, the darker or longer the stain, the higher the arsenic concentration. But the relationship is not linear; use the chart that comes with the kit to convert to a concentration range.

Q: Can I test food or soil with a water test kit?
No. Water test kits are designed for liquid samples. Soil and food require different extraction procedures. Using a water kit on soil will give invalid results.

Q: How long does a colour change test take?
Most take 10–30 minutes, including the reaction time. Some newer kits take as little as 5 minutes. The time is specified in the kit instructions.

Q: Is a negative result always reliable?
No. If the reagents are expired, the acid concentration is wrong, or the sample is not representative, you can get a false negative. Always run a positive control (a sample with known arsenic) to verify the kit works.

Related

• Root or Jailbreak: What It Means & How It Works
• Planet Zoo 2 PC Requirements for Stable Performance
• Did Razer Kraken Headphones Save a Gamer’s Life?

in t | Test time | min | 20 | 1
in r | Colour intensity (0-10) | - | 5 | 0
out score | Arsenic index | - | (r * 60) / t | 2
out note | Qualitative result | - | 0 | 0