Eriochrome Black T Indicator, 25 g

$46.92

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Product Number:EB0000-25

What it is. Eriochrome Black T (EBT) is a metallochromic azo dye used as the indicator in EDTA titrations of calcium and magnesium, the standard method for total water hardness. It is supplied as a brownish-black powder in a 25 g size.
How the color change works. At pH 10, free Eriochrome Black T is blue, while its complexes with magnesium and calcium are wine red. Added to a buffered sample, the dye binds a small amount of metal and turns red; as EDTA is titrated in, it pulls the metal away from the dye, and at the endpoint the solution turns clear blue.
Why pH 10 matters. EBT is itself an acid-base indicator with pKa values near 6.3 and 11.6. Near pH 10, the free dye is blue, which gives the strongest contrast with the red metal complexes. That is why hardness titrations buffer samples with an ammonia-ammonium chloride buffer at about pH 10.
Chemical identity. Eriochrome Black T (C.I. 14645, Mordant Black 11, CAS 1787-61-7) is the sodium salt of an azo naphthol sulfonic acid, C20H12N3NaO7S, with a molecular weight of 461.38 g/mol. It dissolves in hot water to give a reddish-brown solution and was originally developed as a wool dye.
Using the powder. EBT solutions are not stable for long, so many labs grind the dye with sodium chloride to make a dry indicator mixture, or prepare small amounts of solution in alcohol or triethanolamine and replace them frequently. For a ready-made dry mixture, see Lab Pro's Eriochrome Black T Indicator, 1% (w/w) in Sodium Chloride.
Water hardness testing. For total hardness determination, buffer the sample to about pH 10, add a small amount of EBT, and titrate with standard EDTA from wine red to blue. The volume of EDTA gives combined calcium and magnesium, usually reported as mg/L calcium carbonate.
The complete titration system. Pair EBT with Lab Pro's EDTA Titrant 0.1 N / 0.05 M or EDTA Disodium Solution, 0.1 M (0.2 N), and prepare the pH 10 buffer from Ammonium Chloride, ACS Grade, and Ammonium Hydroxide 28 to 30% Solution, ACS Grade.
Other uses. EBT also indicates zinc and other metals in EDTA titrations near pH 10, and it remains in use as a dye for wool and in teaching labs that demonstrate complexometric titration.

Technical Specifications

Chemical Name	Eriochrome Black T
CAS Number	1787-61-7
EC Number	217-250-3
Colour Index	C.I. 14645 (Mordant Black 11)
Synonyms	EBT, Chrome Black T, Solochrome Black T
Formula	C20H12N3NaO7S
Molecular Weight	461.38 g/mol
Appearance	Brownish-black powder with a faint metallic sheen
pKa Values	About 6.3 and 11.6
Endpoint Color Change	Wine red (metal complex) to blue (free dye) at pH 10
Working pH	About 10 (ammonia buffer)
Solubility	Soluble in hot water (reddish-brown solution)
Odor	Odorless
Typical Use Form	Ground with NaCl or as a freshly prepared solution
Storage	Room temperature, tightly closed, protected from light and moisture
Package Size	25 g

Applications / Common Uses

Total water hardness: Indicates the endpoint in EDTA titration of calcium plus magnesium.
Magnesium determination: Signals magnesium in buffered EDTA titrations.
Zinc titrations: Indicates zinc endpoints near pH 10.
Indicator mixtures: Grinds with NaCl to make a stable dry indicator.
Teaching labs: Demonstrates complexometric titration color changes.

Storage, Handling & Shipping

Store the powder tightly closed at room temperature in a dry place, protected from light. Prepare indicator solutions in small amounts and replace them often, since EBT solutions lose sensitivity over time. Avoid creating dust when weighing.

Incompatible materials: Strong oxidizers and strong reducing agents.
Personal protection: Safety glasses, nitrile gloves, and dust control; the dye stains skin and surfaces.
Spill response: Sweep up without raising dust and wipe residue with a damp cloth.
First aid: Eyes: rinse with water for several minutes. Skin: wash with soap and water.

Related Categories & Services

Shop Buffers, Standards & Solutions for related products in this category.
Explore General Chemistry and ACS Grade Laboratory Chemicals.
Vendor Managed Inventory (VMI) Program: Lab Pro can stock, track, and replenish this item on site so it never runs out. Contact info@labproinc.com or 888-452-2776 to set up a VMI assessment.

FAQs

What is Eriochrome Black T used for?

Eriochrome Black T is the indicator for EDTA titrations of calcium and magnesium, the standard method for total water hardness. It also indicates zinc and some other metals near pH 10. The endpoint is a change from wine red to clear blue as EDTA removes the metal from the dye.

Why does EBT need a pH 10 buffer?

EBT is also an acid-base indicator. Near pH 10, the free dye is blue, giving strong contrast with the wine-red metal complexes, so the endpoint is sharp. An ammonia-ammonium chloride buffer holds the sample at that pH throughout the titration, keeping the color change consistent.

How do I make an EBT indicator from the powder?

A common approach is to grind the powder with sodium chloride to make a dry mixture, then add a small scoop to each sample. Alternatively, dissolve a small amount in alcohol or triethanolamine. Because EBT solutions deteriorate, prepare small batches and replace them frequently. Label each batch with its preparation date.

Why does my endpoint fade or look purple?

A slow or purple endpoint usually means the indicator has deteriorated, the pH has drifted from 10, or interfering metals such as copper or iron are blocking the dye. Prepare fresh indicator, check the buffer, and use a masking agent if your method calls for one.

Is Eriochrome Black T hazardous?

EBT powder is generally treated as an irritant to the eyes, skin, and respiratory tract, and it stains skin and surfaces. Wear safety glasses and gloves, avoid inhaling dust, and review the safety data sheet supplied with this product for its exact classification. Clean spills promptly to limit staining.