Pirazol

El pirazol es un compuesto de anillo de cinco miembros con dos átomos de nitrógeno adyacentes. Se utiliza en la síntesis de productos farmacéuticos y agroquímicos, particularmente por sus propiedades antiinflamatorias y antipiréticas. En CymitQuimica, ofrecemos una variedad de pirazoles de alta calidad para apoyar sus proyectos de investigación e industriales.

100
- 20
- 50
- 200

productos por página. Hay 17117 productos en esta categoría.

Datos del producto	Pureza	Rango de precios	Entrega estimada
3-Methyl-2-phenylpyrazole-4-carboxylic acid REF: 10-F017266CAS: 91138-00-0	95.0%	A consultar	Vie 17 Ene 25
1-methyl-3-(pyridin-3-yl)-1H-pyrazole-5-carboxylic acid REF: 10-F542027CAS: 1201199-02-1	95.0%	A consultar	Vie 17 Ene 25
methyl 4-bromo-1,3-dimethyl-1H-pyrazole-5-carboxylate REF: 10-F313375CAS: 660408-08-2	95.0%	A consultar	Vie 17 Ene 25
3,4-Dimethyl-1H-pyrazole phosphate REF: 10-F239249CAS: 202842-98-6	95.0%	A consultar	Vie 17 Ene 25
1-(tert-Butyl)-1H-pyrazole-4-carbaldehyde REF: 10-F240683CAS: 127949-06-8	95.0%	A consultar	Vie 17 Ene 25
2-(3-Methoxybenzyl)-4-methyl-2H-pyrazol-3-ylamine REF: 10-F070520CAS: 1152526-41-4	>95%	A consultar	Vie 17 Ene 25
3-(4-Methoxy-phenyl)-1H-pyrazole-4-carboxylic acid REF: 10-F057832CAS: 618383-46-3	98.0%	A consultar	Vie 17 Ene 25
4-Bromo-5-ethyl-1-methyl-1H-pyrazole-3-carboxylic acid REF: 10-F620212CAS: 1248387-82-7	95%	A consultar	Vie 17 Ene 25
5-(2,6-DIMETHOXYPHENYL)-1-(4-FLUOROPHENYL)-1H-PYRAZOLE-3-CARBOXYLIC ACID REF: 10-F531007	90.0%	A consultar	Vie 17 Ene 25
2-methyl-5-[5-(trifluoromethyl)-1H-pyrazol-3-yl]-3-thiophenesulfonyl chloride REF: 10-F312926CAS: 1287752-72-0	95.0%	A consultar	Vie 17 Ene 25
1-ethyl-5-nitro-1H-pyrazole-3-carboxylic acid REF: 10-F428450CAS: 1245806-48-7	95.0%	A consultar	Vie 17 Ene 25
N,1-DIMETHYL-1H-PYRAZOL-4-AMINE2HCL REF: 10-F533614CAS: 948572-94-9	95.0%	A consultar	Vie 17 Ene 25
4-Bromo-1-ethyl-3-methyl-1H-pyrazole-5-carboxylic acid REF: 10-F316609CAS: 175276-99-0	95.0%	A consultar	Vie 17 Ene 25
3-(1H-pyrazol-1-yl)-1-butanol REF: 10-F311034CAS: 1007517-63-6	95.0%	A consultar	Vie 17 Ene 25
3-methyl-1-vinyl-1H-pyrazole-4-carbaldehyde REF: 10-F308707CAS: 120841-95-4	95.0%	A consultar	Vie 17 Ene 25
1-methyl-5-nitro-1H-pyrazole-3-carbonitrile REF: 10-F428758CAS: 1245772-35-3	95.0%	A consultar	Vie 17 Ene 25
1-(cyclohex-3-en-1-ylmethyl)-1H-pyrazol-5-amine REF: 10-F309384CAS: 1015845-93-8	95.0%	A consultar	Vie 17 Ene 25
4-FLUORO-1,3-DIMETHYL-1H-PYRAZOLE-5-CARBOXYLIC ACID REF: 10-F538419CAS: 1936196-92-7	95.0%	A consultar	Vie 17 Ene 25
1-(4-Bromo-phenyl)-1H-pyrazole REF: 10-F075484CAS: 13788-92-6	95.0%	A consultar	Vie 17 Ene 25
(1,5-Dimethyl-1H-pyrazol-4-yl)methanol REF: 10-F036259CAS: 103946-58-3	95.0%	A consultar	Vie 17 Ene 25
Ethyl 3-methoxy-1H-pyrazole-4-carboxylate REF: 10-F464833CAS: 478968-48-8	95.0%	A consultar	Vie 17 Ene 25
4-Isopropyl-1H-pyrazol-5-amine REF: 10-F235204CAS: 151521-49-2	95.0%	A consultar	Vie 17 Ene 25
3-(3-Fluorophenyl)-1H-pyrazole-4-carbaldehyde REF: 10-F065287CAS: 161398-15-8	97.0%	A consultar	Vie 17 Ene 25
5-Amino-1-(2,3-dihydro-benzo[1,4]dioxin-6-yl)-1H-pyrazole-4-carbonitrile REF: 10-F511307	95.0%	A consultar	Vie 17 Ene 25
4-ACETAMIDO-1,3-DIMETHYL-1H-PYRAZOLE-5-CARBOXAMIDE REF: 10-F538170	95.0%	A consultar	Vie 17 Ene 25
(1-Benzylpyrazol-3-yl)methanol REF: 10-F718973CAS: 102846-11-7	98%	A consultar	Vie 17 Ene 25
1,3-Bis(4-bromophenyl)-1H-pyrazol-5-ylamine REF: 10-F033549CAS: 618098-21-8	99.0%	A consultar	Vie 17 Ene 25
2-(1H-pyrazol-1-yl)-4-(trifluoromethyl)pyrimidine REF: 10-F518545	- - -	A consultar	Vie 17 Ene 25
3-(Chloromethyl)-1-methyl-1H-pyrazole REF: 10-F067842CAS: 84547-64-8	95.0%	A consultar	Vie 17 Ene 25
1-(2-morpholin-4-ylethyl)-1H-pyrazol-4-amine dihydrochloride REF: 10-F515297CAS: 1351658-98-4	95.0%	A consultar	Vie 17 Ene 25
4-Bromo-3-(tert-butyl)-1H-pyrazole REF: 10-F210518CAS: 60061-63-4	95.0%	A consultar	Vie 17 Ene 25
1-(2-ethylhexyl)-1H-pyrazol-3-amine REF: 10-F524931CAS: 1240564-44-6	95.0%	A consultar	Vie 17 Ene 25
1-(2-Chlorophenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid REF: 10-F721894CAS: 948293-88-7	97%	A consultar	Vie 17 Ene 25
ethyl 5-ethyl-1-methyl-1H-pyrazole-3-carboxylate REF: 10-F372991CAS: 165744-14-9	95%	A consultar	Vie 17 Ene 25
{[1-(4-fluorophenyl)-1H-pyrazol-4-yl]methyl}amine hydrochloride REF: 10-F358805CAS: 1185300-73-5	95.0%	A consultar	Vie 17 Ene 25
3-(4-bromo-1H-pyrazol-1-yl)propanenitrile REF: 10-F307333CAS: 1007518-24-2	95.0%	A consultar	Vie 17 Ene 25
5-Iodo-1-isopropyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole REF: 10-F691795CAS: 2377612-00-3	98%	A consultar	Vie 17 Ene 25
5-Amino-1-(2,5-dimethyl-phenyl)-3-methyl-1H-pyrazole-4-carbonitrile REF: 10-F521658	95.0%	A consultar	Vie 17 Ene 25
3-(2,4-DIMETHOXYPHENYL)-1H-PYRAZOLE REF: 10-F471072CAS: 150433-17-3	97.0%	A consultar	Vie 17 Ene 25
3-(2-Ethoxyphenyl)-1H-pyrazole-5-carboxylic acid REF: 10-F059306CAS: 890621-20-2	95.0%	A consultar	Vie 17 Ene 25
5-Amino-1-(5-fluoro-2-methyl-phenyl)-1H-pyrazole-4-carbonitrile REF: 10-F521555	95.0%	A consultar	Vie 17 Ene 25
3-(3-pyridinyl)-1H-pyrazole-5-carboxylic acid hydrochloride REF: 10-F358556CAS: 1093636-79-3	95.0%	A consultar	Vie 17 Ene 25
5-Bromomethyl-1-methyl-1H-pyrazole-3-carboxylic acid methyl ester REF: 10-F317539CAS: 878744-22-0	95.0%	A consultar	Vie 17 Ene 25
5-(chloromethyl)-1-methyl-1H-pyrazole hydrochloride REF: 10-F360713CAS: 1434128-56-9	95.0%	A consultar	Vie 17 Ene 25
1-(2-fluoroethyl)-1H-pyrazole-3-carboxylic acid REF: 10-F429180CAS: 1198437-47-6	- - -	A consultar	Vie 17 Ene 25
(E)-ETHYL 3-(1-METHYL-1H-PYRAZOL-4-YL)ACRYLATE REF: 10-F506275CAS: 192661-36-2	95.0%	A consultar	Vie 17 Ene 25
tert-butyl 4-{[4-(tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazol-1-yl]methyl}piperidine-1-carboxylate REF: 10-F519083CAS: 1092563-72-8	97.0%	A consultar	Vie 17 Ene 25
5-Amino-1-(4-fluorophenyl)-1H-pyrazole-4-carbohydrazide REF: 10-F033554CAS: 618070-67-0	99.0%	A consultar	Vie 17 Ene 25
methyl 3-(4-bromo-1H-pyrazol-1-yl)propanoate REF: 10-F307353CAS: 1007517-46-5	95.0%	A consultar	Vie 17 Ene 25
3-ethoxy-1-methyl-1H-pyrazol-4-amine REF: 10-F429081CAS: 1431970-13-6	95%	A consultar	Vie 17 Ene 25
Ethyl 2-oxo-2-(1H-pyrazol-4-yl)acetate REF: 10-F603195CAS: 869557-79-9	97%	A consultar	Vie 17 Ene 25
3-Chloro-1-methyl-1H-pyrazole REF: 10-F044766CAS: 63425-54-7	98.0%	A consultar	Vie 17 Ene 25
3-Thiophen-2-yl-1-o-tolyl-1H-pyrazole-5-carboxylic acid REF: 10-F023834CAS: 618382-85-7	- - -	A consultar	Vie 17 Ene 25
5-(4-Isopropyl-phenyl)-1H-pyrazole-3-carboxylic acid methyl ester REF: 10-F521969CAS: 36825-86-2	95.0%	A consultar	Vie 17 Ene 25
4-Bromo-3-ethyl-5-methyl-1H-pyrazole REF: 10-F063054CAS: 736073-76-0	95.0%	A consultar	Vie 17 Ene 25
1-Ethyl-3-methyl-1H-pyrazole-4-carboxylic acid REF: 10-F014579CAS: 113131-46-7	95.0%	A consultar	Vie 17 Ene 25
5-cyclopropyl-3-methyl-1H-pyrazole-4-carboxylic acid REF: 10-F312495CAS: 1287752-79-7	95.0%	A consultar	Vie 17 Ene 25
1-Phenyl-3-trifluoromethyl-1H-pyrazole-5-carboxamide REF: 10-F545693	95.0%	A consultar	Vie 17 Ene 25
Ethyl 3-(4-Bromophenyl)-1-methyl1H-pyrazole-5-carboxylate REF: 10-F033599CAS: 618070-50-1	95.0%	A consultar	Vie 17 Ene 25
1-[3-(Trifluoromethyl)benzyl-1H-pyrazol-5-amine REF: 10-F070297CAS: 1052563-64-0	- - -	A consultar	Vie 17 Ene 25
4-METHYL-1-(TETRAHYDRO-2H-PYRAN-2-YL)-1H-PYRAZOLE-5-BORONIC ACID PINACOL ESTER REF: 10-F501915CAS: 1492954-33-2	95.0%	A consultar	Vie 17 Ene 25
ETHYL 5-AMINO-1-ISOPROPYL-1H-PYRAZOLE-4-CARBOXYLATE REF: 10-F332311CAS: 1759-24-6	95.0%	A consultar	Vie 17 Ene 25
3-(4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazol-1-yl)tetrahydrothiophene 1,1-dioxide REF: 10-F458077CAS: 1233526-31-2	95.0%	A consultar	Vie 17 Ene 25
5-Amino-2 H -pyrazole-3-carboxylic acid hydrochloride REF: 10-F028024CAS: 1032334-56-7	97%	A consultar	Vie 17 Ene 25
N-methyl-2-(4-methyl-1H-pyrazol-1-yl)ethanamine hydrochloride hydrate REF: 10-F359880CAS: 1177297-64-1	95.0%	A consultar	Vie 17 Ene 25
3-NITRO-1-[[2-(TRIMETHYLSILYL)ETHOXY]METHYL]-1H-PYRAZOLE REF: 10-F530622CAS: 1313520-83-0	90.0%	A consultar	Vie 17 Ene 25
TERT-BUTYL 3-AMINO-1H-PYRAZOLE-4-CARBOXYLATE REF: 10-F470589CAS: 1646543-44-3	90.0%	A consultar	Vie 17 Ene 25
3-Bromo-1-(2-chlorophenyl)-1H-pyrazole-5-carboxylic acid REF: 10-F322285CAS: 871239-17-7	95.0%	A consultar	Vie 17 Ene 25
1-(4-Methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-3-carboxylic acid REF: 10-F550119	>95%	A consultar	Vie 17 Ene 25
(1-methyl-3-(pyridin-4-yl)-1H-pyrazol-4-yl)methanamine REF: 10-F541899CAS: 1517033-12-3	95.0%	A consultar	Vie 17 Ene 25
5-hydroxy-3-methyl-1H-pyrazole-4-carbaldehyde REF: 10-F307219CAS: 470704-73-5	95.0%	A consultar	Vie 17 Ene 25
3,5-Dimethyl-1H-pyrazole-4-sulfonyl chloride hydrochloride REF: 10-F025199CAS: 1239479-56-1	97%	A consultar	Vie 17 Ene 25
5-(3,4-difluorophenyl)-1H-pyrazole REF: 10-F526022CAS: 474707-69-2	95.0%	A consultar	Vie 17 Ene 25
1-(1-ethoxyethyl)-4-iodo-3-nitro-1H-pyrazole REF: 10-F521401	- - -	A consultar	Vie 17 Ene 25
1-Methyl-3-nitro-1H-pyrazole-4-carboxylic acid REF: 10-F429248CAS: 39205-68-0	- - -	A consultar	Vie 17 Ene 25
5-Methyl-1-phenyl-1,2-dihydro-pyrazol-3-one REF: 10-F483249CAS: 14575-62-3	- - -	A consultar	Vie 17 Ene 25
methyl 4-bromo-1-(2,2,2-trifluoroethyl)-1H-pyrazole-3-carboxylate REF: 10-F429438	- - -	A consultar	Vie 17 Ene 25
3-(3-Methyl-1H-pyrazol-5-yl)pyridine REF: 10-F547825	97.0%	A consultar	Vie 17 Ene 25
1,3,5-trimethyl-1H-pyrazole-4-sulfonic acid REF: 10-F372053CAS: 201207-66-1	95%	A consultar	Vie 17 Ene 25
5-(2,4,5-Trimethyl-phenyl)-1H-pyrazole-3-carboxylic acid methyl ester REF: 10-F521977	95.0%	A consultar	Vie 17 Ene 25
5-(2-chloroethyl)-1-methyl-1H-pyrazole hydrochloride REF: 10-F512208CAS: 1797374-16-3	95.0%	A consultar	Vie 17 Ene 25
3-(2-methoxyphenyl)-1-phenyl-1H-pyrazol-5-amine REF: 10-F351138	- - -	A consultar	Vie 17 Ene 25
3,5-Dimethyl-1H-pyrazol-4-ylamine REF: 10-F030286CAS: 5272-86-6	95.0%	A consultar	Vie 17 Ene 25
2-(3-Bromo-1H-pyrazol-1-yl)ethanol REF: 10-F785199CAS: 1233525-80-8	98%	A consultar	Vie 17 Ene 25
1-(2-Fluorobenzyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole REF: 10-F696236CAS: 1415825-05-6	98%	A consultar	Vie 17 Ene 25
1-(2-Hydroxyethyl)-1H-pyrazole-4-carboxylic acid REF: 10-F070532CAS: 1006469-47-1	95.0%	A consultar	Vie 17 Ene 25
4-Nitro-3-(trifluoromethyl)-1H-pyrazole-5-carboxylic acid REF: 10-F224967CAS: 1322805-15-1	95.0%	A consultar	Vie 17 Ene 25
1-cyclopentyl-3-nitro-1H-pyrazole REF: 10-F427497CAS: 1170953-36-2	- - -	A consultar	Vie 17 Ene 25
3-Methyl-1-(pyridin-4-ylmethyl)-1H-pyrazole-5-carboxylic acid REF: 10-F476198CAS: 1500530-79-9	- - -	A consultar	Vie 17 Ene 25
Methyl 5-methyl-1H-pyrazole-3-carboxylate REF: 10-F236966CAS: 25016-17-5	95.0%	A consultar	Vie 17 Ene 25
1-isobutyl-4-methyl-1H-pyrazol-5-amine hydrochloride REF: 10-F359318CAS: 1255718-25-2	95.0%	A consultar	Vie 17 Ene 25
3-Phenyl-1-p-tolyl-1H-pyrazol-5-ylamine REF: 10-F033537CAS: 77746-54-4	95.0%	A consultar	Vie 17 Ene 25
4-Bromo-3,5-dimethyl-1-phenyl-1H-pyrazole REF: 10-F317518CAS: 51294-75-8	95.0%	A consultar	Vie 17 Ene 25
5-Methyl-1-phenyl-1H-pyrazole-4-carbohydrazide REF: 10-F752431CAS: 204260-39-9	95%	A consultar	Vie 17 Ene 25
n,1-Dimethyl-1h-pyrazol-4-amine hydrochloride REF: 10-F674233CAS: 1333842-76-4	95%	A consultar	Vie 17 Ene 25
3,3'-(1H-pyrazole-3,5-diyl)dipyridine REF: 10-F756039CAS: 79476-36-1	98%	A consultar	Vie 17 Ene 25
ETHYL 1-(DIFLUOROMETHYL)-1H-PYRAZOLE-4-CARBOXYLATE REF: 10-F851242CAS: 129819-40-5	95%	A consultar	Vie 17 Ene 25
5-(4-ethylphenyl)-1H-pyrazole REF: 10-F526013CAS: 116999-33-8	95.0%	A consultar	Vie 17 Ene 25
ethyl 2-cyclopentyl-3-oxo-2,3-dihydro-1H-pyrazole-4-carboxylate REF: 10-F693221CAS: 1547888-86-7	97%	A consultar	Vie 17 Ene 25
N-methyl-1-(5-phenyl-1H-pyrazol-3-yl)methanamine oxalate REF: 10-F360859CAS: 373356-52-6	95.0%	A consultar	Vie 17 Ene 25

1g		A consultar
100mg		A consultar
250mg		A consultar
500mg		A consultar

1g		A consultar
5g		782,00 €
10g		1.227,00 €
25g		2.519,00 €
100mg		45,00 €
250mg		75,00 €
500mg		A consultar

1g		49,00 €
5g		175,00 €
10g		287,00 €
25g		507,00 €
50g		936,00 €
100g		1.634,00 €
100mg		A consultar
250mg		34,00 €
500mg		46,00 €

1g		A consultar
2.5g		A consultar
250mg		A consultar
500mg		A consultar

1g		249,00 €
5g		672,00 €
10g		A consultar
100mg		71,00 €
250mg		85,00 €

1g		54,00 €
5g		155,00 €
10g		284,00 €
25g		480,00 €
250mg		23,00 €

Pirazol

3-Methyl-2-phenylpyrazole-4-carboxylic acid

Ref: 10-F017266

1-methyl-3-(pyridin-3-yl)-1H-pyrazole-5-carboxylic acid

Ref: 10-F542027

methyl 4-bromo-1,3-dimethyl-1H-pyrazole-5-carboxylate

Ref: 10-F313375

3,4-Dimethyl-1H-pyrazole phosphate

Ref: 10-F239249

1-(tert-Butyl)-1H-pyrazole-4-carbaldehyde

Ref: 10-F240683

2-(3-Methoxybenzyl)-4-methyl-2H-pyrazol-3-ylamine

Ref: 10-F070520

3-(4-Methoxy-phenyl)-1H-pyrazole-4-carboxylic acid

Ref: 10-F057832

4-Bromo-5-ethyl-1-methyl-1H-pyrazole-3-carboxylic acid

Ref: 10-F620212

5-(2,6-DIMETHOXYPHENYL)-1-(4-FLUOROPHENYL)-1H-PYRAZOLE-3-CARBOXYLIC ACID

Ref: 10-F531007

2-methyl-5-[5-(trifluoromethyl)-1H-pyrazol-3-yl]-3-thiophenesulfonyl chloride

Ref: 10-F312926

1-ethyl-5-nitro-1H-pyrazole-3-carboxylic acid

Ref: 10-F428450

N,1-DIMETHYL-1H-PYRAZOL-4-AMINE2HCL

Ref: 10-F533614

4-Bromo-1-ethyl-3-methyl-1H-pyrazole-5-carboxylic acid

Ref: 10-F316609

3-(1H-pyrazol-1-yl)-1-butanol

Ref: 10-F311034

3-methyl-1-vinyl-1H-pyrazole-4-carbaldehyde

Ref: 10-F308707

1-methyl-5-nitro-1H-pyrazole-3-carbonitrile

Ref: 10-F428758

1-(cyclohex-3-en-1-ylmethyl)-1H-pyrazol-5-amine

Ref: 10-F309384

4-FLUORO-1,3-DIMETHYL-1H-PYRAZOLE-5-CARBOXYLIC ACID

Ref: 10-F538419

1-(4-Bromo-phenyl)-1H-pyrazole

Ref: 10-F075484

(1,5-Dimethyl-1H-pyrazol-4-yl)methanol

Ref: 10-F036259

Ethyl 3-methoxy-1H-pyrazole-4-carboxylate

Ref: 10-F464833

4-Isopropyl-1H-pyrazol-5-amine

Ref: 10-F235204

3-(3-Fluorophenyl)-1H-pyrazole-4-carbaldehyde

Ref: 10-F065287

5-Amino-1-(2,3-dihydro-benzo[1,4]dioxin-6-yl)-1H-pyrazole-4-carbonitrile

Ref: 10-F511307

4-ACETAMIDO-1,3-DIMETHYL-1H-PYRAZOLE-5-CARBOXAMIDE

Ref: 10-F538170

(1-Benzylpyrazol-3-yl)methanol

Ref: 10-F718973

1,3-Bis(4-bromophenyl)-1H-pyrazol-5-ylamine

Ref: 10-F033549

2-(1H-pyrazol-1-yl)-4-(trifluoromethyl)pyrimidine

Ref: 10-F518545

3-(Chloromethyl)-1-methyl-1H-pyrazole

Ref: 10-F067842

1-(2-morpholin-4-ylethyl)-1H-pyrazol-4-amine dihydrochloride

Ref: 10-F515297

4-Bromo-3-(tert-butyl)-1H-pyrazole

Ref: 10-F210518

1-(2-ethylhexyl)-1H-pyrazol-3-amine

Ref: 10-F524931

1-(2-Chlorophenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid

Ref: 10-F721894

ethyl 5-ethyl-1-methyl-1H-pyrazole-3-carboxylate

Ref: 10-F372991

{[1-(4-fluorophenyl)-1H-pyrazol-4-yl]methyl}amine hydrochloride

Ref: 10-F358805

3-(4-bromo-1H-pyrazol-1-yl)propanenitrile

Ref: 10-F307333

5-Iodo-1-isopropyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole

Ref: 10-F691795

5-Amino-1-(2,5-dimethyl-phenyl)-3-methyl-1H-pyrazole-4-carbonitrile

Ref: 10-F521658

3-(2,4-DIMETHOXYPHENYL)-1H-PYRAZOLE

Ref: 10-F471072

3-(2-Ethoxyphenyl)-1H-pyrazole-5-carboxylic acid