Alkaline Phosphatase (AP) and Horseradish Peroxidase (HRP) Colorimetric Detection Systems—A Complete Guide: Mechanisms of Soluble Colorimetric vs. Precipitating Substrates, Selection, and Troubleshooting (with Product Navigator Tables 1–3)

Colorimetric AP and Peroxidase Substrate Detection Systems: What Does Each Word Mean?

A. What does “colorimetric” mean?

“Colorimetric” means the reaction generates a colored product. You can judge color intensity by eye, or quantify it by measuring absorbance (OD, optical density) with an instrument. In general, deeper color indicates more colored product formed → a stronger target signal (within an appropriate linear range).

B. What is “alkaline phosphatase (Alkaline Phosphatase, AP/ALP)”?

AP (alkaline phosphatase; also written as ALP) is an enzyme that excels at one key task: removing phosphate groups from substrates (dephosphorylation). In classic colorimetric systems, AP converts pNPP (p-nitrophenyl phosphate) into the yellow product pNP (p-nitrophenol), enabling OD (optical density/absor

bance) measurement.

pNP has a maximal absorbance around 405 nm (nanometers; the wavelength of light), so readings are commonly taken at 405 nm.

Note: A405 / A450 refers to absorbance (dimensionless).

C. What does “peroxidase” typically refer to here?

In most lab applications, the most common peroxidase is HRP (Horseradish Peroxidase). It typically works by oxidizing certain chromogenic substrates into colored products in the presence of H₂O₂ (hydrogen peroxide). For example, TMB (3,3′,5,5′-tetramethylbenzidine): HRP/H₂O₂ oxidizes TMB to form a blue product; after adding an acidic stop solution, the color turns yellow and absorbance is usually read at 450 nm (nanometers; wavelength of light).

D. What is a “substrate”?

A substrate is simply the “raw material” that an enzyme acts on.

1. AP substrates are typically phosphate esters (e.g., pNPP, BCIP).

2. HRP substrates are typically oxidizable chromogens + H₂O₂ (e.g., TMB, DAB).

E. What does “detection system” mean?

In reagents/protocols, a “detection system” refers to the complete set of components that make the signal visible and measurable, typically including:

1. A reporter enzyme (AP or HRP) (often conjugated to an antibody/probe)

2. A substrate/chromogen (produces color or precipitate)

3. Buffers/additives (provide suitable pH and ionic environment; HRP systems often include H₂O₂)

4. A stop solution (puts the reaction “on pause” so you can read at a fixed time)

For example, the TMB system commonly uses an acidic stop solution to convert blue to yellow and measure at 450 nm.

How Are These Two Detection Systems Related?

In one sentence:

AP- and HRP-based chromogenic systems are two parallel reporter-enzyme color development strategies. They share the same purpose—turning a target event into a color signal—but use different chemical reaction routes.

More intuitively:

1. Antibody/probe recognition is like “a lock and key matching,” but you can’t see it directly.

2. AP or HRP is like a “small motor attached to the key.”

3. The substrate is the “material the motor processes, changing color after processing.”

4. The color is the final readout you can see/measure.

How Do They “Generate Color”? (Two Typical Routes)

Route 1: AP (Alkaline Phosphatase) → Dephosphorylation → Color Appears

(1) Soluble colorimetric: pNPP → Yellow (for ELISA/enzyme activity assays)

a) AP dephosphorylates pNPP to generate pNP

b) Under alkaline conditions, pNP is yellow and is commonly measured at 405 nm; strong base is also often used to stop the reaction.

c) Advantages: standardized readout, easy quantification; limitation: not suitable when a precipitating readout is required.

(2) Precipitating color development: BCIP/NBT → Purple/Blue-Black precipitate (for membrane/tissue localization)

a) BCIP/NBT refers to a classic alkaline phosphatase substrate pair: BCIP (5-bromo-4-chloro-3-indolyl phosphate) and NBT (nitro blue tetrazolium). Under AP (alkaline phosphatase) catalysis, an insoluble dark precipitate forms, allowing direct visualization of bands/localization on membranes or tissue sections.

Route 2: HRP (Peroxidase) + H₂O₂ → Oxidation of chromogenic substrate → Color Appears

(1) Soluble colorimetric: TMB → Blue (during reaction) → Yellow after acid (read at 450 nm)

a) HRP (Horseradish Peroxidase) oxidizes TMB (3,3′,5,5′-tetramethylbenzidine) in the presence of H₂O₂: a blue product forms first; after adding an acidic stop solution, it turns yellow and is typically read at 450 nm (nanometers; wavelength of light).

(2) Precipitating color development: DAB → Brown precipitate (for long-term mounting/preservation on tissue/membranes)

a) DAB (3,3′-diaminobenzidine) forms an insoluble brown precipitate under HRP catalysis and is widely used for IHC (immunohistochemistry) or membrane color development.

Why Focus Specifically on “These Two Enzyme Detection Systems”?

Because in immunodetection (ELISA, WB, IHC, nucleic acid hybridization, etc.), AP and HRP are the two most commonly used reporter-enzyme labels:

1. High catalytic efficiency and strong signal amplification (one enzyme can repeatedly turn over many substrate molecules)

2. Mature substrate systems that support both soluble quantification and precipitating localization

3. Many kits offer the same workflow but provide two final-step options: HRP-based color development or AP-based color development (essentially two alternative “translators”).

AP vs HRP: Quick Guide to Key Selection Differences

1. Speed / time window: HRP/TMB typically develops color faster; AP/pNPP is often more controllable with a longer working time window.

2. Background sources: Tissue/blood samples often contain endogenous peroxidase → HRP-IHC commonly requires H₂O₂ blocking first. Some tissues also have endogenous AP → levamisole can be used to suppress background.

3. Buffer compatibility: For HRP systems, avoid sodium azide (azide); for AP color development, avoid phosphate (PBS) as much as possible—prefer TBS/TBST.

4. Preferred readout: For high-throughput, plate-based quantification → commonly HRP/TMB (450 nm) or AP/pNPP (405 nm); for localization → choose precipitating substrates (e.g., DAB, BCIP/NBT).

5. “Permanence”: DAB is closer to a “permanent precipitate.” AEC/4-CN are often chosen because they’re “fast/visually pleasing,” but mounting and long-term storage require specific conditions.

6. One-liner: Decide “quantification vs. localization” first, then consider sample background/buffer compatibility, and only then fine-tune the substrate choice.

How Does a Typical Detection System Work? (ELISA Example)

Think of it as three stages: “binding → color development → readout”

1. Immobilize or capture the target (e.g., coat antibody/antigen on a plate)

2. Bind an enzyme-labeled probe/antibody

a) The reporter enzyme may be HRP or AP (often via secondary antibody conjugates or polymer systems)

3. Wash away unbound components (this step strongly affects background)

4. Add substrate for color development

a) HRP: add TMB/DAB, etc. (often with H₂O₂ in the system)

b) AP: add pNPP/BCIP-NBT, etc.

5. Stop (optional) and read/photograph at the designated time

a) TMB is often stopped with acid and read at 450 nm

b) pNPP is often read at 405 nm (can be stopped with base)

How to Evaluate Whether a Detection System Is “Good”?

A. Quantitative assays (ELISA/enzyme activity): common metrics

1. Blank and background: signal should be minimal when there is no target/no primary antibody

2. Signal-to-background (S/B) or signal-to-noise (S/N): OD with target should be clearly higher than background

3. Linear range: within a defined concentration range, OD should be proportional to target amount (or enzyme amount)

4. Sensitivity / limit of detection (LOD): the lowest amount that can be reliably distinguished from background

5. Reproducibility (CV%): low well-to-well variability for replicates

For the AP/pNPP system, a common approach is to infer enzyme activity from the time-dependent increase of A405; when enzyme amount is the rate-limiting factor, the change in absorbance is proportional to enzyme activity.

B. Localization assays (WB/IHC precipitating substrates): common metrics

1. Band/staining clarity (resolution)

2. Clean background (endogenous enzymes in tissues, nonspecific binding, overdevelopment)

3. Precipitate stability and suitability for downstream handling

a) For example, BCIP/NBT forms a dark precipitate; DAB forms a brown “permanent” precipitate.

Why Does Color Development Fail or Background Become High?

Common considerations for HRP (peroxidase) systems

a) Sodium azide inhibits HRP: if your antibody/diluent contains azide, the HRP signal may become weak or even disappear.

Common considerations for AP (alkaline phosphatase) systems

a) Phosphate-buffered saline (PBS) may inhibit AP: many AP color-development workflows recommend TBS rather than PBS to avoid phosphate interfering with the enzyme reaction.

b) Note: the key point is avoid phosphate in the color-development reaction system. If PBS is only used earlier for washing, and you wash thoroughly / switch to a phosphate-free substrate buffer before adding substrate, you can often still obtain normal signal.

HRP-IHC: block endogenous peroxidase first

a) Tissue sections are commonly pretreated with H₂O₂ (skipping this step often causes strong “whole-slide” background staining).

AP-IHC: use levamisole when needed to suppress endogenous AP

a) Useful as an inhibition control / for background reduction, but sensitivity can differ across AP isoenzymes; parallel controls are recommended.

For precipitating substrates: the #1 background control knob = development time/substrate concentration

a) If background is high, prioritize: shorten development time → lower substrate concentration → strengthen washing/blocking. It’s better to be “lighter but clean,” then gradually deepen the signal.

A One-Sentence Plan for Choosing the Right System

1. For plate-based quantification (ELISA): common choices are HRP/TMB (450 nm) or AP/pNPP (405 nm)—choose based on compatibility and background control (e.g., whether you can avoid azide / PBS issues).

2. To visualize position/bands/cellular localization on membranes or tissues: commonly use precipitating substrates such as AP/BCIP-NBT or HRP/DAB.

Aladdin Alkaline Phosphatase (AP) and Horseradish Peroxidase (HRP) Colorimetric/Precipitating Chromogenic Detection Systems: Selection Navigator + Representative Product Classification Tables (Tables 1–3)

Product Selection Navigator for Colorimetric/Chromogenic Detection Systems

First define the task (AP vs. HRP; quantification vs. localization), then go to the relevant table (Tables 1–3).

What you are doing (define the task first)	Which table to check first	What to look for in that table	Selection logic	Common pairing / next steps
AP (alkaline phosphatase) activity quantification / ELISA colorimetry (typically read at 405 nm)	Table 2 | Soluble colorimetric substrates & standards	pNPP (PNPP disodium; pNPP liquid substrate system / 10 mM aqueous solution); p-nitrophenol (pNP) standard	The classic AP “solution colorimetry” route: good linearity; ideal for microplates/kinetics	Table 1: recombinant alkaline phosphatase + DEA buffer / Tris buffer + MgCl₂ / ZnCl₂; stop/fixed-time readout: NaOH (e.g., 2 N or 3 M); A405 (reference wavelength)
AP membrane development / dot blot / histochemistry (blue–purple precipitate localization)	Table 3 | Precipitating chromogenic & histochemistry systems	BCIP (incl. disodium salt) + NBT	Precipitating development is ideal for localization/visualization; signal is “locked in place”	Table 1: recombinant alkaline phosphatase; background reduction/control: levamisole (AP inhibitor); buffer as needed (Tris/DEA)
HRP (horseradish peroxidase) microplate colorimetry / ELISA (stable readout / high throughput)	Table 2 | Soluble colorimetric substrates & standards	TMB; ABTS (incl. tablets); OPD (o-phenylenediamine, incl. tablets)	Mainstream HRP solution-substrate systems are mature and easy to use; well suited for high-throughput workflows	Table 1: HRP + hydrogen peroxide (H₂O₂); NaOH (e.g., 2 N or 3 M); A405 (reference wavelength)
HRP membrane development / WB / IHC localization staining (insoluble precipitate; clear localization)	Table 3 | Precipitating chromogenic & histochemistry systems	DAB (incl. tetrahydrochloride salt); AEC (incl. tablets); 4-chloro-1-naphthol (4-CN)	Precipitating substrates provide clearer band/section localization and higher contrast	Table 1: HRP + hydrogen peroxide (H₂O₂); optimize reaction time/substrate concentration to control background
Peroxidase activity validation/screening (not necessarily HRP-labeled; want fast visual color change)	Table 2 | Soluble colorimetric substrates & standards (priority)	Guaiacol; pyrogallol; 2,6-dimethoxyphenol (2,6-DMP); syringaldazine (indicator)	Classic “enzyme substrates/indicator substrates” for quickly assessing activity and substrate preference	Table 1: ensure hydrogen peroxide (H₂O₂) is present; compare sensitivities across substrates in parallel if needed
Phosphatase histochemistry coupling stains (red/blue precipitates, etc.; not BCIP/NBT)	Table 3 | Precipitating chromogenic & histochemistry systems	Naphthol phosphate substrates (sodium 1-naphthyl phosphate, naphthol AS-MX phosphate, naphthol AS-BI phosphate) + diazonium salts (Fast Red TR, Fast Blue BB, Fast Blue RR, Fast Garnet GBC, Fast Red Violet LB)	“Naphthol release → coupling with diazonium salt precipitate” enables selectable colors/contrast; ideal for localization staining	Table 1: choose phosphatase source/controls according to the system; use buffers to control pH and background when necessary
Issues when building the system: pH/background/stopping/washing/unstable conditions (method development problems)	Table 1 | Core enzymes & foundational reagents (check first)	Tris, DEA, NaOH, HCl, H₂SO₄, MgCl₂, ZnCl₂, hydrogen peroxide, levamisole	Many problems come from “conditions/controls not set up correctly.” Stabilize the system first; substrate selection becomes faster and more reliable	Then return to Table 2/Table 3: quantify with “soluble substrates,” localize with “precipitating substrates,” and use blanks/inhibition/positive–negative controls to identify the signal source

Table 1 | Core Enzymes and Foundational Reagents (buffers/cofactors/co-substrate/stop/inhibition)

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Key features or applications
Enzyme | AP (alkaline phosphatase)	9001-78-9	A755392	Alkaline Phosphatase Recombinant	Recombinant, solution (high-activity)	Catalyzes “phosphate ester substrate → dephosphorylated product”; used in pNPP colorimetry, BCIP/NBT development, naphthol phosphate precipitating stains, and related detection systems.
Enzyme | HRP (horseradish peroxidase)	9003-99-0	P755413	Peroxidase (from horseradish)	Type X, ammonium sulfate suspension	Classic HRP enzyme label: catalyzes color development with TMB/ABTS/OPD/DAB/AEC/4-CN and other substrates; used for ELISA, WB, IHC, etc.
Buffer system | Tris	77-86-1	T110602	Tris(hydroxymethyl)aminomethane (Tris base)	For cell culture, ≥99.9% (T)	Common buffer component (sample dilution/washing/reaction buffer); used to build the baseline buffering environment for AP/HRP detection systems.
Buffer system | DEA	111-42-2	D431475	Diethanolamine (DEA)	Suitable for analysis, premium grade	A commonly used substrate buffer for AP (DEA buffer; alkaline); supports reactions such as pNPP and improves S/N and reproducibility.
System reagent | Base / pH adjustment	1310-73-2	S111498	Sodium hydroxide	Reagent grade, ≥96%	Used to prepare alkaline buffers / adjust pH (AP reactions are often better under alkaline conditions); also used for cleaning and basic method development steps.
AP cofactor | Mg²⁺	7786-30-3	M113689	Magnesium chloride, anhydrous	Anhydrous, ≥99.9% metals basis, powder	Mg²⁺ is commonly used to enhance/maintain AP activity (often used together with Zn²⁺ in typical AP reaction buffers).
AP cofactor | Zn²⁺	7646-85-7	Z112526	Zinc chloride	Reagent grade, ≥98%	AP is often a Zn²⁺-containing metalloenzyme; used to supplement/maintain metal-ion conditions (activity maintenance, buffer formulation studies, control experiments).
System reagent | HRP co-substrate / oxidant	7722-84-1	H755825	Hydrogen peroxide solution	Suitable for microbiology, 3%	Essential oxidant for HRP/peroxidase reactions; used with TMB/ABTS/OPD/DAB/AEC/4-CN and related substrates.
System reagent | Stop acid / pH adjustment	7664-93-9	S485807	Sulfuric acid (precursor)	Reagent grade, suitable for analysis, ≥98%	Commonly used to stop HRP–TMB/OPD color reactions (stabilizes readout after stopping); also used for acidification and pH adjustment.
System reagent | Acid / pH adjustment / stopping	7647-01-0	H485680	Hydrochloric acid, fuming 37% (precursor)	Reagent grade, suitable for analysis, max. 0.001 ppm Hg	Used to prepare/adjust acidic conditions (e.g., stopping certain color reactions, preparing wash solutions, buffer pH adjustment); for method development and cleaning/regeneration.
Control/Inhibitor | AP inhibition control	16595-80-5	L114327	Levamisole hydrochloride	Analytical standard	Common AP inhibitor: suppresses endogenous/nonspecific AP interference or serves as an “inhibition control” to verify the signal source.

Table 2 | Soluble Colorimetric Substrates and Standards (OD readout: microplate/solution systems)

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Key features or applications
AP colorimetric substrate | pNPP (solid)	333338-18-4	P118470	Disodium p-nitrophenyl phosphate hexahydrate (PNPP)	For cell culture, ≥98%	Classic AP colorimetric substrate: AP hydrolysis produces pNP (yellow), commonly read at 405 nm; used for AP activity assays / ELISA substrate.
AP substrate system | pNPP (ready-to-use)	4264-83-9	P755472	p-Nitrophenyl phosphate liquid substrate system	liquid	Ready-to-use pNPP substrate system for AP colorimetric assays (microplates/routine reactions), reducing preparation error and improving consistency.
AP colorimetric substrate | pNPP (ready-to-use solution)	4264-83-9	P1498357	PNPP disodium	Moligand™, 10 mM in Water	Pre-made 10 mM working solution for AP activity assays/method comparisons; eliminates weighing and dissolution steps.
Standard/Control | pNPP product standard	100-02-7	N591132	p-Nitrophenol	Industrial grade	Product/control for pNPP reactions: can be used to establish an A405 quantitative reference or as a methodological control (for high-accuracy quantification, higher-grade standards are recommended).
HRP colorimetric substrate | TMB	54827-17-7	T100417	3,3′,5,5′-Tetramethylbenzidine (TMB)	Standard for GC, ≥99% (GC)	One of the most widely used HRP substrates: develops blue color (turns yellow after acid stop, commonly read at 450 nm); for ELISA/microplate colorimetry.
HRP colorimetric substrate | ABTS	30931-67-0	A109612	2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)	≥98%	With HRP + H₂O₂ forms a green radical cation product; highly water-soluble; widely used in enzymology and colorimetric assays. ABTS (HRP): ABTS·⁺ typical λmax ≈ 414 nm.
HRP colorimetric substrate | ABTS (tablets)	30931-67-0	A755499	ABTS diammonium salt	tablet, 10 mg substrate per tablet	Tablet-format ABTS for convenient preparation and improved reproducibility; used for HRP colorimetric readout. ABTS (HRP): ABTS·⁺ typical λmax ≈ 414 nm.
HRP colorimetric substrate | OPD	615-28-1	P100830	o-Phenylenediamine dihydrochloride	AR, ≥99%	HRP + H₂O₂ substrate (forms orange-toned products; common in microplate assays); suitable for ELISA/enzyme activity assays. OPD (HRP): often read at 492 nm (can be stopped with acid).
HRP colorimetric substrate | OPD (tablets)	615-28-1	O755378	o-Phenylenediamine dihydrochloride	tablet, 60 mg substrate per tablet	Tablet format for easier preparation and better lot-to-lot consistency; for HRP colorimetric development (microplate/routine enzymology). OPD (HRP): often read at 492 nm (can be stopped with acid).
HRP/Peroxidase substrate | DMP	91-10-1	D434573	2,6-Dimethoxyphenol	Natural, ≥96%	Hydrogen-donor substrate: oxidized in peroxidase/HRP + H₂O₂ systems to yield color; used for activity assays, kinetics, or screening.
HRP/Peroxidase substrate | Guaiacol	90-05-1	G112735	Guaiacol	≥98%	Classic peroxidase substrate: oxidation by HRP + H₂O₂ yields brown products; used for activity assays and kinetics.
HRP/Peroxidase substrate | Pyrogallol	87-66-1	P104234	Pyrogallol	AR	Hydrogen-donor substrate: oxidized under peroxidase/HRP + H₂O₂ to develop color; often used for activity assays and enzymology studies.
HRP colorimetric substrate | Dianisidine class	20325-40-0	D111032	o-Dianisidine hydrochloride	Biochemical reagent, ≥98.5%	One of the HRP + H₂O₂ chromogenic substrates; commonly used in peroxidase-related colorimetric systems (also applicable in certain coupled detection formats).
HRP coupling chromogen | 4-AAP	83-07-8	A106066	4-Aminoantipyrine	≥98%	Key component in Trinder-type colorimetry: couples with phenols/hydrogen donors under HRP + H₂O₂ to form colored dyes (used in coupled assays).
HRP/Laccase indicator substrate | Syringaldazine	14414-32-5	S755421	Syringaldazine	indicator for laccase and peroxidase activity	Indicator substrate for peroxidase/laccase activity: used for rapid color screening, activity validation, and control experiments.

Table 3 | Precipitating Chromogenic and Histochemistry Systems

(Membrane development / IHC / localization staining: substrate + chromogenic salt)

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Key features or applications
AP precipitating substrate | BCIP	6578-06-9	B113076	BCIP p-toluidine salt (5-bromo-4-chloro-3-indolyl phosphate)	Molecular biology grade, ≥99%	After AP hydrolysis forms an indolyl intermediate; commonly paired with NBT to generate an insoluble blue–purple precipitate for membrane development/histochemistry/dot blot.
AP precipitating substrate | BCIP (disodium salt)	102185-33-1	B165498	BCIP disodium salt (5-bromo-4-chloro-3-indolyl phosphate disodium salt)	≥98% (HPLC)	Salt form of BCIP (often easier to prepare/control solubility); typically paired with NBT for AP blue–purple precipitating development.
AP chromogenic co-reagent | NBT	298-83-9	N104910	Nitro blue tetrazolium chloride (NBT)	Molecular biology grade, ≥98% (dry basis), solvent residue ≤10%	Electron acceptor/chromogenic salt paired with BCIP: forms dark insoluble precipitate, improving visualization sensitivity for AP-based detection.
HRP precipitating substrate | DAB (salt)	7411-49-6	D693820	3,3′-Diaminobenzidine tetrahydrochloride	≥99%	Classic HRP precipitating substrate: more soluble/easier to prepare; forms brown insoluble precipitate; widely used in IHC/WB/membrane development.
HRP precipitating substrate | DAB	91-95-2	D106468	3,3′-Diaminobenzidine (DAB)	≥99%	HRP precipitating chromogen: forms brown precipitate; suitable for tissue-section localization and membrane development (high-contrast visualization).
HRP precipitating substrate | AEC	132-32-1	A105545	3-Amino-9-ethylcarbazole	≥95%	HRP chromogen forming a red insoluble/semi-insoluble precipitate; commonly used for IHC/tissue staining and membrane development. Note: not resistant to ethanol/organic solvents—use aqueous mounting; for ethanol dehydration/long-term preservation, DAB is preferred.
HRP precipitating substrate | AEC (tablets)	132-32-1	A755468	3-Amino-9-ethylcarbazole	tablet	Tablet format for easier preparation and consistency control; for HRP tissue/membrane precipitating development.
HRP precipitating substrate | 4-CN	604-44-4	C109038	4-Chloro-1-naphthol	≥99%	With HRP + H₂O₂ forms a blue–purple precipitate; suitable for rapid WB/membrane development with good color contrast and convenience for double staining. However, sensitivity is often lower than DAB/AEC, requiring more enzyme or longer development; precipitate stability/mounting conditions need attention (not the first choice for “permanent” staining).
General phosphatase substrate | Phenyl phosphate	3279-54-7	D155797	Disodium phenyl phosphate hydrate	≥98% (T)	One general phosphatase substrate: used for phosphatase activity assays/method comparisons and substrate screening (choose according to readout format).
Phosphatase precipitating substrate | Naphthol phosphate	2183-17-7	N109088	Sodium 1-naphthyl phosphate	≥98%	Hydrolyzed by phosphatases to release naphthol; couples with Fast Blue/Fast Red diazonium salts to form colored precipitates for phosphatase histochemistry/membrane development.
Phosphatase precipitating substrate | Naphthol AS-MX phosphate	96189-12-7	N331653	Naphthol AS-MX phosphate disodium salt	≥98%	After phosphatase hydrolysis, couples with diazonium salts (Fast Red/Fast Blue, etc.) to form insoluble dye precipitates; used for ALP/ACP staining and localization.
Phosphatase precipitating substrate | Naphthol AS-BI phosphate	1919-91-1	N131822	Naphthol AS-BI phosphate	≥93%	Common histochemistry phosphatase substrate: released naphthol couples with diazonium salts to form precipitates; suitable for localization staining and comparing substrate systems.
Phosphatase precipitating chromogen | Diazonium salt (Fast Red TR)	89453-69-0	F113048	Fast Red TR (zinc chloride double salt)	Dye content 15%	Used with naphthol phosphate substrates: phosphatase-released naphthol couples to form a red precipitate for ALP/ACP histochemistry staining.
Phosphatase precipitating chromogen | Diazonium salt (Fast Blue BB)	5486-84-0	F106955	Fast Blue BB salt	Dye content ≥80%	Couples with naphthol phosphate substrates to form a blue precipitate; used for phosphatase localization staining/membrane development.
Phosphatase precipitating chromogen | Diazonium salt (Fast Blue RR)	14726-29-5	F100841	Fast Blue RR salt	AR	Couples with naphthol phosphate substrates to form colored precipitates; used for phosphatase chromogenic and histochemistry applications.
Phosphatase precipitating chromogen | Diazonium salt (Fast Garnet GBC)	101-89-3	F113034	Fast Garnet GBC	≥90%	Couples with naphthol phosphate substrates to form deep red/burgundy precipitates; used for phosphatase (including ALP) precipitating development and histochemistry.
Phosphatase precipitating chromogen | Diazonium salt (Fast Red Violet LB)	32348-81-5	F113050	Fast Red Violet LB salt	≥90%	Couples with naphthol phosphate substrates to form red–violet precipitates; for phosphatase precipitating development and tissue/membrane visualization.

Note: The above items are representative Aladdin catalog products only. For additional specifications/grades, please refer to the product list table at the end of the article or search by product name/CAS on the Aladdin website.

Aladdin: https://www.aladdinsci.com/